OS VERIFICATION- A SURVEY AS A SOURCE OF FUTURE CHALLENGES

International Journal of Computer Science & Engineering Survey (IJCSES) Vol.6, No.4, August 2015
DOI:10.5121/ijcses.2015.6401 1
OS VERIFICATION- A SURVEY AS A SOURCE OF
FUTURE CHALLENGES
Kushal Anjaria and Arun Mishra
Department of Computer Science &Engineering, DIAT, Pune, India
ABSTRACT
Formal verification of an operating system kernel manifests absence of errors in the kernel and establishes
trust in it. This paper evaluates various projects on operating system kernel verification and presents in-
depth survey of them. The methodologies and contributions of operating system verification projects have
been discussed in the present work. At the end, few unattended and interesting future challenges in
operating system verification area have been discussed and possible directions towards the challenge
solution have been described in brief.
KEYWORDS
Formal verification, operating system, kernel.
1. INTRODUCTION
The security and reliability of computer system is dependent on the underlying operating system
kernel; kernel is the core of operating system. The kernel provide mechanism for user level
applications to access hardware, scheduling and inter-process communication. Therefore, if
anything goes wrong in the kernel while programming or implementation, it will affect the
operation of entire system. To ensure the correct working of a kernel, testing and/or verification
techniques have been used. Testing reduces frequency of failures, while verification detects errors
and eliminates failures. Consider the fragment of C code shown in figure-1
Figure1. Fragment of C code
Here r being an integer, if testing has been performed with p=8, q=4 and p=16, q=32, full
coverage of code can be achieved. Full coverage implies each line of code is executed at least
once and every condition has been tested once. But here still two divide by zero errors are
remaining. To identify these errors, human tester will immediately suggest that p=0, q=-1 and p=-
1, q=0 should be tested. But for bigger and non-trivial program, it is difficult to find these cases
and tester can never be sure that all such cases are covered.
int div(int p, int q)
{
int r;
if (p<q)
r=p/q;
else
r=q/p;
return r;
}

International Journal of Computer Science & Engineering Survey (IJCSES)
From the example similar to above, G. Klein[19] has concluded that humans are good at
creativity, they are not so good at r
system verification is complex and needs many repetitive tasks. That’s why formal verification of
operating system is feasible compared to normal testing. Formal verification of an operating
system kernel produces mathematical proofs of correctness of an operating system. Formal
verification may differ from the user’s view of correctness, this difference is called semantic
gap[51]. Bridging this semantic gap is called formalisation[52]. The figure
entire verification procedure of any system with formalisation. The specification block in the
figure-2 describes the collection of mathematical entities; these entities will be in the form which
can be analysed by mathematical methods later
representation of the system at some chosen level of abstraction. The verification tool will take
mathematical model and mathematical entities as input and it will verify that the model is correct
for all the entities or not. After that it will generate the verification result.
Figure
After brief overview of the verification procedure, some of the formal verification projects of
operating system have been surveyed. Before moving on to the operating system verification
survey, some basic terminologies related to OS verification have been
a. Model checking: Model checking is formal verification method for finite state concurrent
system. Large systems, like OS can reside in many states. That means they have large state space.
Model checking can be applied to abstract model of r
the conclusion which can be drawn from model checking for operating system[27, 55, 56].
b. Proof-carrying code: Proof carrying code is an approach in which the kernel accepts only
those extensions which are accompanied with valid proof for particular security policy. It tackles
the problem of untrusted code execution in kernel mode[27, 57, 58].
c. Static source-code checking:
source code. The static source-
about the absence of errors, while testing runs the system for code analysis. In this way it is
different from testing[27, 59].
d. Functional correctness: The functional correctness in O
implementation always strictly follows high
Functional correctness makes it feasible to prove security properties at the code level. It does not
necessarily imply the security. Functional correctness provides reasons about an implementation
and a specification. Functional correctness proof has been considered as the right first step and
basis for proving high level properties[27, 60, 61] .
creativity, they are not so good at repetitive, complex and high detailed task. But operating
ernel produces mathematical proofs of correctness of an operating system. Formal
gap[51]. Bridging this semantic gap is called formalisation[52]. The figure-2 below shows the
2 describes the collection of mathematical entities; these entities will be in the form which
can be analysed by mathematical methods later. The model block describes mathematical
es or not. After that it will generate the verification result.
Figure 2. Verification procedure [52]
survey, some basic terminologies related to OS verification have been explained:
Model checking is formal verification method for finite state concurrent
Model checking can be applied to abstract model of real systems only. This diminution restricts
Proof carrying code is an approach in which the kernel accepts only
ompanied with valid proof for particular security policy. It tackles
the problem of untrusted code execution in kernel mode[27, 57, 58].
code checking: Static source-code checking statically performs the analysis of
-code checking analyzes the source code and gives guarantees
The functional correctness in OS kernel verification means that the
implementation always strictly follows high-level abstract specification of kernel behaviour[43].
e security. Functional correctness provides reasons about an implementation
basis for proving high level properties[27, 60, 61] .
Vol.6, No.4, August 2015
2
epetitive, complex and high detailed task. But operating
ernel produces mathematical proofs of correctness of an operating system. Formal
elow shows the
2 describes the collection of mathematical entities; these entities will be in the form which
. The model block describes mathematical
Model checking is formal verification method for finite state concurrent
eal systems only. This diminution restricts
Proof carrying code is an approach in which the kernel accepts only
ompanied with valid proof for particular security policy. It tackles
code checking statically performs the analysis of
code checking analyzes the source code and gives guarantees
S kernel verification means that the
level abstract specification of kernel behaviour[43].
e security. Functional correctness provides reasons about an implementation

3
2. AN OVERVIEW OF OPERATING SYSTEM VERIFICATION PROJECTS
To capture the wide domain of operating system verification, following projects have been
surveyed. In the present survey UCLA, KIT, PSOS, VFiasco, EROS and seL4 projects have been
discussed. These projects are among the prominent OS verification projects and it has been
believed in the present work that survey of these projects is sufficient to find new challenges in
OS verification field.
2.1. UCLA Project
The verification of UCLA system is based on data security model provided by G. Popek and D.
Farber[1]. The data security model can be used to verify many of those properties in an operating
system which are necessary to ensure reliable security enforcement. They haven’t tried to prove
that an operating system is entirely correct; instead they centralized all the operations which
affect the security into nucleus. This nucleus is called the security kernel. If the operations of this
kernel are correct then this implies that entire system is secure. Besides UCLA, the application of
this approach has been used in other areas also, e.g. to design security model for military message
systems[2] and to decide dependability of trusted bases[3].
UCLA secure unix[4] and Provably secure operating system (PSOS)[15, 16] have been
considered as first serious attempts to verify an OS kernel. These projects were attempted 35
years ago. The UCLA secure unix had been developed as an operating system for the DEC PDP-
11/45 computer. The project tried formal modelling and verification of Unix kernel, which was
written in simplified pascal.
Project Implementation
The project was divided into two parts: first, a four level specification, ranging from Pascal code
at bottom to top level specification was developed. Then in verification, it needs to be proved that
different levels of abstractions were consistent with each other. The UCLA project managed to
finish 90% of their specification and 20% of their proofs in 5 person-year.
Figure-3 shows the detail of implementation of UCLA secure unix project. Left hand side figure
shows the specification layer, used in this project. All the specifications in the specification layers
are called state machine. Here instead of one specification layer, multiple specification layers
were designed because proof consistency can be handled easily with the multiple layers. The
Pascal code block is actual Pascal code of kernel. The low level specification block in figure-3
describes data structures of implementation, in which some of the details are omitted. The
abstract level contains specific objects like process, pages and devices. The top level specification
described in figure-3 actually contains data security notion. This data security notion has been
discussed by G. Popek and D. Farber[1]. The figure in right shows the consistency proofs
between the levels. These proofs show that the specifications are consistent with each other. The
proofs define functions. The function maps program state of concrete level to the program state of
abstract level. The figure in right describes that for each operation in concrete system,
corresponding operation in abstract system transforms the mapped concrete state accordingly [8].
Now the points below show some important findings that can be derived from UCLA project,
which will connect us with the current formal verification scenario:
• Prior efforts to this project, to make operating system secure merely found the flaws in
the system, so it became clear that piecemeal alterations were unlikely ever to succeed
[6]. So, this UCLA project has been seen as a systematic formal approach that control
OS’s design and implementation.

• In UCLA they first generate the nucleus using the approach described in [1]. This nucleus
was very close to modern microkernels[27], although at that time microkernels hadn’t
been invented[5].
Figure 3. UCLA specification layers and consistency proofs [8]
• It has been discussed by the authors that the error has been discovered which justifies the
need of formal specification. The error was related to boundary condition of kernel ca
Generally boundary condition of kernel call maps a page into user
been properly handled. Mischievous process can read and read or modify the memory
pages adjacent to its own.
• When the project took place, formal refinement hadn’t ca
technique used in this project is formal refinement, defined by Morgan[7]. In fact, it is
data refinement technique[8].
• At the end of this project, it has been observed that performance of the system was poor,
it was slower than the standard unix in some cases. This was because of the nature of the
pascal compiler and high context switch cost. Modern microkernels have overcome this
limitation. For example SeL4 kernel. The Sel4 project has been described later in this
paper.
• Authors have observed that the approach of program verification and proof development
before or during the software development is not practical. They have also said that if
system needs to be verified then it should be developed with verification in mind.
similar conclusion has been drawn from other projects as well for example
SeL4[43,44,45] project.
2.2. KIT Project
KIT(Kernel for Isolated Task)[9] was the first operating system which had been verified at the
assembly level. Main target of multitasking operating system is to implement the processes. The
purpose of KIT project was to verify whether all these processes
stands for ‘Kernel for Isolated Task’, that means task can communicate only in specified way.
KIT is written in the machine language of uniprocessor von
provides exception handling, access to async
passing. KIT kernel had been implemented in artificial, yet realistic assembly language. Its code
size was 620 lines, which was very small. Out of these 620 lines only 300 lines were of actual
instructions [5].
In UCLA they first generate the nucleus using the approach described in [1]. This nucleus
3. UCLA specification layers and consistency proofs [8]
It has been discussed by the authors that the error has been discovered which justifies the
need of formal specification. The error was related to boundary condition of kernel ca
Generally boundary condition of kernel call maps a page into user-address space hadn’t
pages adjacent to its own.
When the project took place, formal refinement hadn’t came into existence, though the
data refinement technique[8].
At the end of this project, it has been observed that performance of the system was poor,
than the standard unix in some cases. This was because of the nature of the
uthors have observed that the approach of program verification and proof development
system needs to be verified then it should be developed with verification in mind.
SeL4[43,44,45] project.
purpose of KIT project was to verify whether all these processes are isolated or not. Name KIT
KIT is written in the machine language of uniprocessor von-Neumann computer. KIT kernel
provides exception handling, access to asynchronous I/O devices and a single word message
4
In UCLA they first generate the nucleus using the approach described in [1]. This nucleus
It has been discussed by the authors that the error has been discovered which justifies the
need of formal specification. The error was related to boundary condition of kernel call.
address space hadn’t
me into existence, though the
At the end of this project, it has been observed that performance of the system was poor,
than the standard unix in some cases. This was because of the nature of the
uthors have observed that the approach of program verification and proof development
system needs to be verified then it should be developed with verification in mind. The
are isolated or not. Name KIT
Neumann computer. KIT kernel
hronous I/O devices and a single word message

Project Implementation
The KIT kernel has been formalized in Boyer
checked mechanically by Boyer
proving correspondence between the two finite stat
state machine and target machine finite state machine. To describe finite state machines,
description of the set of machine states and a definition of each transition on a machine state are
required.
Now the points below show some important findings that can be derived from KIT project:
• There is a fiction in KIT OS that each process owns the processor. The processor state
maintains this fiction. The verification of KIT proves that the processor state has bee
saved correctly.
• An interpreter equivalence theorem establishes implementation relation. This relation is
similar to the Milner’s weak simulation
• KIT described process isolation properties down to object code level, but it was simpler
and had less general abstraction than modern
verification, KIT had same order of complexity as modern
• KIT was different from current microkernels because, it doesn’t provide dynamic process
creation, virtual memory in modern sense and no support for shared
2.3. PSOS
PSOS (Provably Secure Operating System)[15,16] project was based on Robinson
paper[14], which had introduced the concept of formal mappings between different level of
functional specifications that represented abstract implementations of each layer as a function of
the lower layers. This layered system architecture can be seen in figure
between each layer serves as a high level functional specification for lower layer and at the same
time it serves as a machine model for the higher layer.
Figure
The KIT kernel has been formalized in Boyer-Moore logic[10] and all the proofs have been
checked mechanically by Boyer-Moore theorem prover[10]. KIT verification has been done by
proving correspondence between the two finite state machines’ behaviour: abstract kernel finite
oints below show some important findings that can be derived from KIT project:
There is a fiction in KIT OS that each process owns the processor. The processor state
maintains this fiction. The verification of KIT proves that the processor state has bee
An interpreter equivalence theorem establishes implementation relation. This relation is
similar to the Milner’s weak simulation relation [11].
KIT described process isolation properties down to object code level, but it was simpler
and had less general abstraction than modern microkernel [12]. But in terms of
verification, KIT had same order of complexity as modern microkernel [13].
rent from current microkernels because, it doesn’t provide dynamic process
creation, virtual memory in modern sense and no support for shared memory [
PSOS (Provably Secure Operating System)[15,16] project was based on Robinson
14], which had introduced the concept of formal mappings between different level of
the lower layers. This layered system architecture can be seen in figure-4 below. The interface
time it serves as a machine model for the higher layer.
Figure 4. PSOS layered system architecture [8]
5
Moore logic[10] and all the proofs have been
Moore theorem prover[10]. KIT verification has been done by
e machines’ behaviour: abstract kernel finite
oints below show some important findings that can be derived from KIT project:
There is a fiction in KIT OS that each process owns the processor. The processor state
maintains this fiction. The verification of KIT proves that the processor state has been
An interpreter equivalence theorem establishes implementation relation. This relation is
KIT described process isolation properties down to object code level, but it was simpler
12]. But in terms of
13].
rent from current microkernels because, it doesn’t provide dynamic process
memory [5].
PSOS (Provably Secure Operating System)[15,16] project was based on Robinson–Levitt
14], which had introduced the concept of formal mappings between different level of
4 below. The interface

6
Project implementation
As shown in the figure-4, PSOS has 17 layers in its layered system architecture. Out of these 17
layers, bottom six layers are implemented using hardware and layers seven to seventeen are
implemented using software. As shown in the figure, layer 0 is tagged. Tagged means it has
hardware enforced capabilities; hardware supports a bit that indicates whether a word in memory
stores a capability or not. This layered architecture is similar to the TCP/IP network stack, that’s
why in the figure it has been shown that the top layer contains application. Similar to the TCP/IP
network stack, the top layer contains all the applications which have been needed by the layers
below it.
It had been believed in the project that layering would make the formal verification easier. PSOS
was a capability based system in which incorporated hardware implemented capability. Now the
points below show some important findings that can be derived from PSOS project:
• Hardware implemented capability in PSOS had weakened the primitive similar in some
respects to the diminish operator; existing access rights can be selectively retained. It is
unclear from the literature whether the application of this operation was imposed by an
access right or was discretionary [17].
• PSOS also implemented concept named “store permissions”. This mechanism could
selectively control which capabilities can be stored to which capability segments. This
feature can be used to enforce write down permissions [17].
• The properties of application specific object type could enforce with the hardware
assistance provided with the capability based access control. The design allowed
application layers to efficiently execute instructions, with object-oriented capability-
based addressing directly to the hardware, although it appeared at a much higher level of
abstraction in design specification [18].
2.4. VFiasco project
The main challenge in VFiasco project [27] is to entitle high level reasoning in terms of typed
objects during verification, yet assume only low level hardware properties. The VFiasco project
aims at mechanical verification of security relevant properties of Fiasco microkernel. Fiasco
microkernel is L4-compatible Fiasco microkernel [28]. The aim of the project is an OS kernel
which provides security guarantees which has been verified.
Fiasco kernel has been implemented in C++. The language C++ is not the language with precise
semantics. For this purpose, M. Hohmuth et al.[27] developed a language which had precise
semantics called ‘safe C++’. After converting code to safe C++, the verification will be carried
out in the theorem prover Isabelle/HOL [29]. In this project, conversion from safe C++ to HOL
semantics has been done automatically by the logic compiler. For verification, authors have
abstraction level of virtual machine that provides a type-safe object store which is a memory that
supports reading and writing of typed values.

Figure 5. The VFiasco project verification overview [40]
In this project the existence of an object store layer with strong properties is a proof not an
assumption. The approach used in this project to a semantics of C++ is
in LOOP project[30] of Java. Figure
First semantic compiler will translate C++ code into semantics, formulated in higher order logic.
Then hardware model and C++ semantic
After that theorem prover verifies the semantic specification against security properties. Finally
verification results in proof.
Now the points below show some important findings that can be derive
• The VFiasco project uses coalgebraic[31] methods. It describes coalgebraic class
specification language called CCSL. Coalgebraic proof methods are not only charac
capturing formalism for non
system[32].
• In VFiasco project source code verification has been directly applied to the unmodified
source of Fiasco microkernel operating system written in C+
jump across the function boundaries. So formal reconstruction of goto loop, which has
been described in [33] needs to be applied. For complete C++ semantics one needs the
semantics for data-types that can deal with the type
verification, state transformation approach has been used to get relatively simple semantics
to statements like break, continue and even goto[27]. The state transformation has been
explained by M. Huisman and B.
• The VFiasco project contains single layer, i.e. object store layer. This layer provides
functions for typed objects, in such a way that typed objects can be safely manipulated.
Because of this single layer, The VFiasco project re
category (ShengWen Gong [
categories. These four categories are: (i) the component
approach[36], (iii) the single
• The VFiasco project stopped at the source
map results down to lower systems or language layers. This project doesn’t involve any
compiler verification [38].
5. The VFiasco project verification overview [40]
assumption. The approach used in this project to a semantics of C++ is very close to the one used
in LOOP project[30] of Java. Figure-5 below gives an overview of VFiasco verification process.
Then hardware model and C++ semantic library provide functions to express program semantics.
Now the points below show some important findings that can be derived from VFiasco project,
The VFiasco project uses coalgebraic[31] methods. It describes coalgebraic class
specification language called CCSL. Coalgebraic proof methods are not only charac
capturing formalism for non-terminating program, but also used for labelled transition
In VFiasco project source code verification has been directly applied to the unmodified
source of Fiasco microkernel operating system written in C++. In C++ it is not possible to
types that can deal with the type cast of data and of pointers. During
explained by M. Huisman and B. Jacob [30].
The VFiasco project contains single layer, i.e. object store layer. This layer provides
Because of this single layer, The VFiasco project resides in ‘single-layer
Gong [34] has classified verification efforts/projects into four
categories. These four categories are: (i) the component approach [35], (ii) the parallel
approach[36], (iii) the single-layer approach[27], (iv) the pervasive approach[37]).
The VFiasco project stopped at the source-code level. That means, there are no attempts to
38].
7
very close to the one used
5 below gives an overview of VFiasco verification process.
library provide functions to express program semantics.
d from VFiasco project,
The VFiasco project uses coalgebraic[31] methods. It describes coalgebraic class
specification language called CCSL. Coalgebraic proof methods are not only characteristic
terminating program, but also used for labelled transition
In VFiasco project source code verification has been directly applied to the unmodified
+. In C++ it is not possible to
cast of data and of pointers. During
The VFiasco project contains single layer, i.e. object store layer. This layer provides
layer approach’
34] has classified verification efforts/projects into four
35], (ii) the parallel
pproach[37]).
code level. That means, there are no attempts to

8
2.5. EROS project
The EROS (Extremely Reliable Operating System)[47,48] is a capability based operating system.
It is for commodity processors which uses a single level storage model. J. Shapiro et al. have
formalized and analyzed the security model of EROS using the pen and paper approach. The
security model was implemented based on take-grant model[42] of capability distribution. The
security model of the EROS was not formally connected to the implementation.
The EROS project actually started by verifying confinement mechanism. J. S. Shapiro and S.
Weber [48] provided formal definition of confinement policy in their work. After that they have
explained operations and security requirements of real operating system. In their work,
methodology and proof structures have been developed for confinement policy in capability
based structure. They have claimed that their methodology can be generalized to solve
information flow problems in many capability based architecture.
Now the points below show some important findings that can be derived from EROS project:
• One thing about EROS is, when machine starts, it loads a fully pre-initialised state. This
task makes it possible to inspect the initialised state offline[49].
• S. Maffeis et al.[50] provide the object capability based model that provides approach for
isolating untrusted components in web applications. Their work is close to EROS
confinement mechanism. While there are some other similarities between their framework
and our general setup, one substantial difference is that instead of defining authority as an
over-approximation of heap actions that can be performed by a single object; they define
authority for the whole system[50].
• The Coyotos kernel[64, 65] was successor to the EROS kernel. From the security model of
EROS kernel, Shapiro et al.[54] concluded that there should be a formal connection
between security model of the kernel and the implementation. They have tried to establish
this formal connection in Coyotos kernel[8].
• The EROS system currently runs on Pentium hardware. Future details about the project can
be obtained from its website[62].
• The CapROS (Capability based Reliable Operating System)[63] project is the continuation
project of EROS. It is using same EROS code base. The CapROS project is being led by
Charles Landau.
2.6. seL4 verification project
The seL4 (Secure Embedded L4) kernel is an evolution of the L4 microkernel. It is third
generation microkernel of L4 provenance. It targets embedded devices. The seL4 implements
capability based protection system, capabilities in seL4 are immutable. Similar to seL4 micro
kernel, seL4 provides address spaces, inter-process communication and threads. In seL4 all
system calls are invocations of capabilities. The seL4 comprises of 8,700 lines of C code and 600
lines of assembler.
The seL4 verification project[43,44,45] was the project which provided the proof of functional
correctness of a complete general purpose operating system for the first time. In this project,
formal machine-checked verification of seL4 microkernel has been performed from an abstract
specification down to the C implementation. In this project, first the access control model has

been verified and then the actual functional verification of kernel had been started. Before the
project, correctness of compiler, assembl
has been assumed.
An access control model of seL4 has been verified by D. Elkaduwe et al.[41] in the theorem
prover isabelle/HOL[29]. In their work the take
rule and with more realistic create rule that is explicitly authorized by capability. In their
formalization, remove rule has also been modified. In seL4, remove
capability instead of removing few parts. D. Elkaduwe et al.[4
mechanisms are sufficient to enforce mandatory isolation between subsystems. They have shown
that it is possible to build fully spatially separated system on top of seL4. So here spatial memory
separation has been guaranteed but authors have said that with current stock hardware preventing
all covert timing channels is not possible.
The seL4 kernel design process has been shown in the figure
formal artefacts. These artefacts have direc
figure represent implementation effort; the single arrows represent design influence of artefacts
on other artefacts. The central artefact is the actual Haskell prototype of the kernel. The prototype
requires the design and implementation of algorithms that manage the low
The figure-6 shows that hardware and Haskell prototype has design influence on formal
executable specification.
After the design process, verification process too
interactive, machine assisted and machine checked proof. Figure
which were used in verification of seL4. Abstract specification layer shows the details to specify
outer interface of the kernel. It doesn’t describe in detail how the effects or interfaces are
implemented in kernel; in short it describes what the system does without saying how it is done.
The executable specification layer has been generated from Haskell into the t
contains all data structure and implementation details which have been expected in the final C
implementation. The high performance C implementation layer deals with the formally
semantics.
Fig
project, correctness of compiler, assembly code, boot code, management of caches and hardware
prover isabelle/HOL[29]. In their work the take-grant[42] model has been used, but with
formalization, remove rule has also been modified. In seL4, remove-rule will remove whole
capability instead of removing few parts. D. Elkaduwe et al.[41] have proved that kernel provided
teed but authors have said that with current stock hardware preventing
all covert timing channels is not possible.
The seL4 kernel design process has been shown in the figure-6 below. The square boxes show the
formal artefacts. These artefacts have direct role in the proof. The double arrows shown in the
ires the design and implementation of algorithms that manage the low-level hardware details.
6 shows that hardware and Haskell prototype has design influence on formal
After the design process, verification process took place. In this project, the verification was
interactive, machine assisted and machine checked proof. Figure-7 shows the specification layers
ce of the kernel. It doesn’t describe in detail how the effects or interfaces are
The executable specification layer has been generated from Haskell into the theorem Prover. It
implementation. The high performance C implementation layer deals with the formally
Figure 6. The seL4 design process [43]
9
y code, boot code, management of caches and hardware
grant[42] model has been used, but without take
rule will remove whole
1] have proved that kernel provided
teed but authors have said that with current stock hardware preventing
6 below. The square boxes show the
t role in the proof. The double arrows shown in the
level hardware details.
6 shows that hardware and Haskell prototype has design influence on formal
k place. In this project, the verification was
7 shows the specification layers
ce of the kernel. It doesn’t describe in detail how the effects or interfaces are
heorem Prover. It
implementation. The high performance C implementation layer deals with the formally-defined

Figure 7. The refinement layers in the verification of seL4 [43]
Now the points below show some important findings that can be derived from seL4 project,
• There are many techniques for formal verification like model checking, static analysis or
kernel implementations in type safe language. But in this project it has been believed that
functional correctness is stronger and more precise technique compared to the techniques
mentioned above[43].
• In this project, fusion of traditional operating system and formal method technique had
been used i.e. rapid kernel design and implementation had been used together. Because of
this implementation, the verification focus was improved
with the better performance[43].
• In UCLA it had been observed that simplification of kernel to make verification feasible
made the kernel little bit slower. But in this project it has been shown that with modern
tools and technique, this is not the case these days.
The summary of the aforementioned projects has been shown in the table
column, the question mark in parenthesis shows that the year is not known and year in parenthesis
shows estimated completion date. Further, many verification projects like bias variance
tradeoffs[66], the Xtratum[67], L4Android[68], ORIENTAIS[69], VTOS[70] and CHERI[71]
have been studied. They are evolved from the aforementioned projects and follow same path as
described in above projects for verification of OS.
Project Highest
level
Lowest
level
UCLA Security
model
Pascal
KIT Isolated
task
Assembly
PSOS Applicatio
n level
Secure code
VFiasco Doesn’t
crash
C++
7. The refinement layers in the verification of seL4 [43]
chniques for formal verification like model checking, static analysis or
In this project, fusion of traditional operating system and formal method technique had
this implementation, the verification focus was improved and design was not conflicting
with the better performance[43].
In UCLA it had been observed that simplification of kernel to make verification feasible
echnique, this is not the case these days.
The summary of the aforementioned projects has been shown in the table-1 below. In the year
etion date. Further, many verification projects like bias variance
in above projects for verification of OS.
Table 1. OS verification projects [8]
Lowest Specs Proofs Prover Approach
90% 20% XIVUS Alphard
Assembly 100% 100% Boyer
Moore
Interpreter
equivalence
Secure code 17
layers
0% SPECI
AL
HDM
70% 0% PVS Semantic
compiler
10
chniques for formal verification like model checking, static analysis or
In this project, fusion of traditional operating system and formal method technique had
and design was not conflicting
In UCLA it had been observed that simplification of kernel to make verification feasible
1 below. In the year
etion date. Further, many verification projects like bias variance
Year
(?)-1980
(?)-1987
1973-
1983
2001-
2008

11
EROS Security
model
BitC Security
model
0% ACL2(
?)
Language
based
2004-(?)
L4
verified
Security
model
C/assembly 100% 70% Isabell
e
Performance
production
code
2005-
(2008)
3. FUTURE CHALLENGES
Further, challenges out of the above projects and inherited projects have been discussed. In the
present survey it has not been claimed that the challenges described in this section haven’t been
implemented at all. But to the best of our knowledge, the areas described in this section haven’t
been covered in detail in literatures.
From all the above projects, especially from the SeL4 project it can be concluded that functional
correctness is one of the strongest properties that can be proven about the system. The functional
correctness can help to make precise formal prediction of how the kernel behaves in all possible
situations for all possible inputs. G. Klein[19] has suggested that, if any specific property is
needed to be checked then it can be expressed in Hoare’s logic, it is enough to work with this
formal prediction, with the specification. But functional correctness property of any system can’t
prove that the system is secure, it just says that system is functionally correct [19]. The word
‘secure’ in secure system requires formal definition, but this depends on what you want to use the
kernel for. So verification of all security properties or secure system which has been built on top
of the OS kernel is one of the recent challenges in formal verification field.
In SeL4 project specific security properties hadn’t been proven, but G. Klein[19] believed that if
the functional correctness property of the operating system can be proven, then below
assumptions can be made easily about that OS: (1) No code injection attacks (2) No buffer
overflow attack (3) No NULL pointer access (4) No ill-typed pointer access (5) No memory leaks
(6) No non-termination (7) No arithmetic or other exceptions (8) No unchecked user arguments.
He has explained these assumptions with reasons. He has also explained that the functional
correctness can tell following properties about the code: (1) Aligned object (2) Well formed data
structures (3) Algorithmic invariants and (4) Correct book-keeping. So research challenges here
are: can functional correctness property be used to verify other security properties or can
framework be made that covers verification of most of the security properties.
G. Heiser et al.[20] have provided some research challenges in different way, they have told that
the users of the fully verified kernel have trustworthy foundation for the entire system. For
example, the kernel seL4 is fully verified and it can be used as trustworthy foundation. Now the
challenge is how the trustworthy foundation of the system can be used further. They have
suggested three uses of the trustworthy foundation: (1) Secure web browsing (2) Increase the
usefulness of TPM (3) Cost-reduced database. But still there are many challenges related to the
kernel used as trustworthy foundation.
Let’s start with the secure web browsing topic. One security policy of the browser is Same Origin
Policy (SOP). SOP means web pages from different sources cannot observe or alter each other’s
state and behaviour and script running inside the web page must denied unauthorized access to
OS resources. Recent browsers like Chrome[22] address this problem by encapsulating security
policy in separate module, the browser kernel. So, this approach is based on the OS, specifically
dependent on browser’s TCB. So one challenge here is can TCB of the browser be reduced using
microkernel approach. IBOS[21] project has actually shown that TCB of browser can be reduced
by using the microkernel approach. The authors have proposed architecture for secure browsing

which contains two trusted parts: microkernel and user level security process which are shown in
figure-8.
Figure 8. Secure web browser architecture. Components that belong to the TCB of the system are
Figure-8 shows that monitor is the only part in the TCB that needs to be verified. The monitor
instantiates browser process with permissions t
stack processes and the monitor itself[20]. Once this security monitor is verified one can have
completely verified TCB for secure browsing. So, research challenge here is can the
aforementioned architecture be extended to include complete OS stack and that too running inside
a VM (Virtual Machine).
Next challenge is to increase usefulness of TPM. Trusted Computing Group[23] has introduced
the Trusted Platform Module (TPM). The TPM provides remote attestation facility which
provides evidence that the trusted software stack has been loaded by the remot
booting time. Now let’s take classical example of bank transaction. The bank can reject the
transaction request from the remote system if the trusted software stack hasn’t been loaded by the
remote machine. Bank clients want to use the sm
smart phones and computers have many apps and software for the bank to manage. Many of these
apps and software are not trusted. So, for successful remote attestation it has become necessary to
kept these out of TCB and therefore the system. To solve this problem, the concept of Dynamic
Root of Trust Measurement (DRTM)[24] has been introduced. DRTM allows user to switch
between trusted and untrusted environment. But to achieve this trusted code should be small a
it can run only for a fraction of seconds. But bank transaction needs more time than few seconds.
So above approach needs user to suspend OS while he is doing the bank transaction. But this is
not a practical solution. So to solve this issue TCB is need
and change in controlled fashion. After concluding this G. Heiser et al. believed that formally
verified kernel do not change or changes very rarely because it won’t require any bug fixes. They
have given example of seL4 kernel also. The challenge here is to minimize TCB and isolate TCB
from rest of the general purpose OS components using the trusted system which contains fully
verified kernel.
Final challenge that G. Heiser et al.[20] have mentioned is about the databa
databases can have disk failures, power failures and OS crash[20]. These days RAID protects
database from disk failure and UPS protects from power failure. This leaves protection against
OS crash as one of the open research problem. Here re
properties of kernel be verified in such a way that it can make kernel crash
be directly implemented on verified kernel and if it is possible then what kind of changes required
web browser architecture. Components that belong to the TCB of the system are
highlighted using grey shading[20]
8 shows that monitor is the only part in the TCB that needs to be verified. The monitor
instantiates browser process with permissions to directly communicate with their private network,
ure be extended to include complete OS stack and that too running inside
provides evidence that the trusted software stack has been loaded by the remote system from its
remote machine. Bank clients want to use the smart phone or computer for the transaction. The
f TCB and therefore the system. To solve this problem, the concept of Dynamic
between trusted and untrusted environment. But to achieve this trusted code should be small a
not a practical solution. So to solve this issue TCB is needed that do not change, change rarely
4 kernel also. The challenge here is to minimize TCB and isolate TCB
Final challenge that G. Heiser et al.[20] have mentioned is about the database systems. The
OS crash as one of the open research problem. Here research questions can be formed: can
properties of kernel be verified in such a way that it can make kernel crash-proof, Can database
12
web browser architecture. Components that belong to the TCB of the system are
8 shows that monitor is the only part in the TCB that needs to be verified. The monitor
o directly communicate with their private network,
ure be extended to include complete OS stack and that too running inside
e system from its
art phone or computer for the transaction. The
f TCB and therefore the system. To solve this problem, the concept of Dynamic
between trusted and untrusted environment. But to achieve this trusted code should be small and
ed that do not change, change rarely
4 kernel also. The challenge here is to minimize TCB and isolate TCB
se systems. The
search questions can be formed: can
proof, Can database

13
in the lower layers of DBMS. Can database be implemented on verified kernel in such a way that
the changes required in the lower layer of database become minimal and practical to achieve.
J. Andronick et al.[25] have tried to solve a research problem related to scalability of the formal
verification. They have dealt with the large and complex system which uses seL4 for which
security guarantees can be given. They have proposed a framework to build such large and
complex systems. Authors’ vision here is that not all the software in a large system necessarily
contributes to a security property of interest. From the vision, the methodology has been
developed: (1) isolate the software parts which are not critical to a targeted property and prove
that for specific property nothing more is needed to be proven about them (2) formally verify and
prove that remaining part satisfy the targeted security property. In this paper the case study of
Secure Access Controller (SAC) has been taken. Access control-based security policy has been
taken as a property of interest here. Authors have explained that verifying such a property for
large system is far beyond the abilities of current verification methods. To verify this property
large code base has been divided into trusted code and untrusted code. The authors have used here
seL4[26] kernel to get the code isolation in this system which is already verified. So question
arises that if the kernel which hasn’t been verified is used for the large and complex system, can
we get security for such system and get the code separation.
In 2001, during VFiasco project[27] M. Hohmuth et al. Observed that huge bug affected
monolithic kernels are outside the scope of verification technology which were available at that
time. Although the microkernels are smart choice for constructing verified secure system, but is it
possible to have verification technology now which can accommodate verification of monolithic
kernel. Verification of monolithic kernel can be costly in terms of time and efforts, but it can be
taken as a recent research challenge.
The formal verification community has nice security properties, high level formal model and
ways of architecting secure systems, but still no signs of implementation level proofs. Even
recently implemented seL4 microkernel doesn’t have these implementation level proofs. G. Klein
et al.[39] believes that this needs to be changed. It is obvious that if the system is large then it can
be secured by reducing the Trusted Computing Base (TCB). For large systems, microkernels
provide good foundation, though with reduced TCB of large systems, nobody has proved security
down to the implementation level. With type 1 hypervisors, it has been assumed that they will
perform the role of separation kernel, but there are no implementation level proofs for these
hypervisors either[39]. For a moment let’s assume that in next few years, kernels are available
with fully formally verified functional correctness down to the implementation level. But then
what next? Such kernels do not automatically imply security [19].
After the completion of sel4 verification, G. Klein et al.[39] have mentioned in their work that the
mandatory access control can be implemented on OSes for example in Linux, but it is not
possible to get provable or assurable security for such OSes at least for next few years. Assume
that all security properties of kernel have been proved in next few years (Sorry, this will never
happen), still we as a formal verification community are not done yet. Our ultimate goal should
be to achieve proof that whole systems enforce their security goals; we can get much stronger
assurance for much larger systems than what is thought feasible today. It should be remembered
that proofs say that the code will follow specification; it doesn’t say that specification enforces
specific security property.
From the projects like VFiasco and seL4, it can be observed that there is a gap between idealized
security properties and properties that hold real kernels. G. Klein et al.[39] have mentioned in
their work that there is no good formal handle available on timing and time based covert channels
for practical implementations, so it is a research challenge. The authors are not expecting that the
same level formal proofs will be obtained in near future as it is available for storage channel.

K Elphinstone[46] has described challenge that takes place while using verified microkernel like
seL4 in Digital Rights Management (DRM), and based on this he has provided some new issues
which can be addressed by research communities. One can think solutions of this problem
direction, not necessarily in formal verification point of view. The DRM is the concept of
specifying, enforcing and limiting rights associated with digital content. Before identifying issues,
the architecture of DRM has been explained. The DRM arc
9[46]. First user posses the device upon which he wishes to view content. Then the user must be
authenticated by the content provider.
Figure
The content must be securely transferred to user device, for this encryption has been used. The
user player should be able to decrypt the content when user wishes to view it.
Here the content-use policy can be violated by end
engineering, modifying players or running the player on modified operating system. This is the
research challenge. One solution is to provide assurance that a trusted player on a trusted
operating system is the only software that has access to the content, other solutions
of in the context of hardware or networking.
Next, the challenge is about the gap between formal model of OS and hardware implementation.
A. Cohn[51] has described that the physical hardware is a realisation of a model, and correct
hardware operations are beyond the scope of the formal verification. One example of this is:
manufacturers can’t prove absence of manufacturing defects. So, even if the verified processor or
kernel is available, the gap between formal model of such kernel or proc
will always exist[51]. The research challenge is how to minimize this gap.
Tuch H. et al.[52] observed UCLA, KIT and VFiasco closely. They have described in their work
that in kernel verification, challenges related to performance
still available. They have mentioned that features like direct hardware access, pointer arithmetic
and embedded assembly code haven’t been the subjects of mainstream verification research, so
research scope is available in these areas.
The KIT was multitasking OS, which gave direction to the present survey to find verification
challenges in concurrent OS. S. Rajamani et al.[53] have discussed challenges of an OS which
support concurrent execution of the program. Sup
execution of program has normal page size. This OS supports third party plug
OS, there are many research challenges: (1) Is it possible to guarantee isolation in this OS where
third party plug-ins can be ill-behaved[53]? (2) With the isolation guarantee in OS, is it possible
to manage properties like safety and permissiveness? (3) Is it possible to achieve isolation without
the problems like deadlocks, livelocks[53], memory fragmentation and conditio
handling? (4) Various operating systems provide various types of memory protections. How to
handle these variations if memory protection is used for isolation?
s described challenge that takes place while using verified microkernel like
which can be addressed by research communities. One can think solutions of this problem
the architecture of DRM has been explained. The DRM architecture has been shown in the figure
provider.
Figure 9. General DRM architecture [46]
transferred to user device, for this encryption has been used. The
user player should be able to decrypt the content when user wishes to view it.
use policy can be violated by end-users in many ways, ranging from reverse
fying players or running the player on modified operating system. This is the
operating system is the only software that has access to the content, other solutions can be thought
of in the context of hardware or networking.
e operations are beyond the scope of the formal verification. One example of this is:
kernel is available, the gap between formal model of such kernel or processor and implementation
will always exist[51]. The research challenge is how to minimize this gap.
that in kernel verification, challenges related to performance, size and the level of abstraction is
le in these areas.
support concurrent execution of the program. Suppose an OS which supports concurrent
execution of program has normal page size. This OS supports third party plug-ins. In this type of
behaved[53]? (2) With the isolation guarantee in OS, is it possible
the problems like deadlocks, livelocks[53], memory fragmentation and conditio
handle these variations if memory protection is used for isolation?
14
s described challenge that takes place while using verified microkernel like
which can be addressed by research communities. One can think solutions of this problem in any
hitecture has been shown in the figure-
transferred to user device, for this encryption has been used. The
users in many ways, ranging from reverse
fying players or running the player on modified operating system. This is the
can be thought
e operations are beyond the scope of the formal verification. One example of this is:
essor and implementation
, size and the level of abstraction is
pose an OS which supports concurrent
ins. In this type of
behaved[53]? (2) With the isolation guarantee in OS, is it possible
the problems like deadlocks, livelocks[53], memory fragmentation and condition variable

15
The challenges are summarized in a table below:
Table 2. Challenges in formal verification
Challenge Explanation Source Suggestion
Functional
correctness
Although functional
correctness is strong
property, it can’t prove that
system is secure. To verify
more number of security
properties together with or
without the functional
correctness is a challenge.
seL4 project R. Akella and Bruce M.[72]
have verified security
properties like information
flow and non deducibility of
cyber-physical system. They
have described that
information flow security
can’t be checked using
functional correctness. So,
they have provided different
framework for verification
using process algebra. Same
methodologies can be
applied to OS verification
also.
Kernel as a
trustworthy
foundation
Three uses of the
trustworthy foundation: (1)
Secure web browsing (2)
Increase the usefulness of
TPM (3) Cost-reduced
database. The challenge is
how the usage of
trustworthy foundation in
different areas can be
increased.
seL4 project G. Heiser et al.[73] have
provided prototype system
RapiLog which is based on
verified seL4 hypervisors.
This system is used to
reduce system complexity by
leveraging verification
instead of using special
hardware. Similarly more
ways can be found where
kernel can be used as
trustworthy foundation.
Security in large
system
Not all software on the large
system contributes to the
security of system. So to
verify the large system,
system should be divided in
trusted and untrusted code.
The division of large system
in trusted and untrusted
code is a challenge.
VFiasco, seL4 Stefan et al[79]. have
verified cyber-physical
system. Their work provides
hint to verify larger and
more complex systems.
Implementation
level proof
No implementation level
proofs so far for any kernel
verification. This is a
challenge.
Recent
projects like
seL4, VFiasco
Nana S. et al.[74] have
provided framework to
verify hardware and software
co-verification. IEEE
802.11ac WLAN system has
been selected by them for
case study. Their work might
provide a hint to solve this
research challenge.

16
Time-based
covert channel
To create formal handle for
time-based covert channel is
a challenge.
Recent
projects like
seL4, VFiasco
The problem with covert
channel is it is difficult to
verify it on case-by-case
bases. So, first unique model
to unified approach is
needed and then verification
can be done. So P. L.
Shrestha et al[78] have
provided this unique model
which can be used to verify
time-based covert channel.
Monolithic
kernel
Huge, bug affected
monolithic kernel
verification is a challenge.
VFiasco
project
M. Lange et al[77]. have
provided hint to solve this
research challenge. Their
work focuses on L4Android
which is monolithic
architecture. Although the
project is not entirely on the
monolithic kernel
verification, but it can
provide help to solve this
research problem.
Challenges
related to
unexplored
areas
Pointer arithmetic and
embedded assembly code
haven’t been the subjects of
mainstream verification
research so far. These fields
can be explored more.
UCLA,
PSOS, KIT,
VFiasco,
EROS
The work of Thomas S. et
al[75]. gives hint in the
direction of pointer
arithmetic. J Kobashi et
al[76] have provided
directions to verify
embedded assembly code.
Their work can be explored
further to solve these
challenges.
Challenges
related to
Concurrency
(1) To achieve and verify
isolation when third party
plug-ins are ill-behaved. (2)
With isolation, manage the
properties like safety and
permissiveness (3) Avoid
problems like deadlocks,
livelocks, memory
fragmentation and condition
variable handling (4)
Handle variations in
memory protection if it has
been used to achieve
isolation.
KIT is
multitasking
kernel. It has
provided base
to explore
concurrency
related areas.
Solutions of some of the
problem have been discussed
by S. Rajamani et al[53].
They have tried to solve this
challenge for concurrent
programs. These solutions
can be expanded for the
large, concurrent operating
system kernel and the
isolation properties can be
verified.
4. CONCLUSIONS
In the present paper, an overview of kernel verification projects has been provided, in specific
UCLA, KIT, PSOS, VFiasco, EROS and seL4 projects have been surveyed. In the present survey,

17
highest level specification, lowest level of specification, model checker and approach of
verification have been described for each project. During the survey it has been observed that
proofs of automation, memory models, proof libraries, and program logics have been developed
significantly, that’s why OS verification is not as hard as it was before 35 years.
The present article also describes challenges in kernel verification area. They are related to
monolithic kernel verification, functional correctness, implementation level proof, time-based
covert channel, direct hardware access, pointer arithmetic, concurrency and use of kernel as
trustworthy foundation areas. Future efforts in the direction of solving these challenges will make
verification faster and precise.
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Authors
Kushal Anjaria is a PhD scholar at the department of computer science and engineering
of Defence Institute of Advanced Technology, Pune-India. He received M Tech in
computer science and engineering from Manipal Institute of Technology, Manipal in
2012. His work is currently focused on operating system security and formal verification.
Arun Mishra is an assistant professor at the department of computer science and
engineering of Defence Institute of Advanced Technology, Pune-India. He got his PhD
in computer science from Motilal Nehru National Institute of Technology, Allahabad
(India). His research activity is based on Automated Systems, Trusted Computing,
Secure Software Engineering, Formal Modelling and Component based Software
Engineering. More information is available at:
http://www.diat.ac.in/index.php?option=com_content&view=article&id=206&Itemid=3
59&Itemid=267

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