Lecture 15
Designing Trusted
Operating Systems
Thierry Sans
15-349: Introduction to Computer and Network Security
Anatomy of an operating system
Concept of Kernel
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Definition
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Component that provides an communication layer
between the hardware and the software
The kernel is in charge of
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Managing the memory
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Managing processes (allocation and synchronization)
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Managing data resources (filesystem, I/O devices)
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Managing communication
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... and so in charge of enforcing security mechanisms
Two design philosophies
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Monolithic Kernels
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Like the Linux kernel
Microkernels
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Like the Windows NT or BSD kernels
(even though considered as hybrid kernels)
Discussion between L. Torvalds and A. Tanenbaum
Monolithic kernels
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Philosophy
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All OS services run along with the main kernel
thread in the same memory area
Pros and Cons
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Easier to design
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Dependencies between components
Microkernels
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Philosophy
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Implement minimal OS services for memory and
process management
Other services (I/O, networking ...) are
implemented as servers in the user-space memory
The first general-purpose microkernel was Mach
(Carnegie Mellon University)
Pros and Cons
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Easy to maintain
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Many system calls that can slow down the system
Where the security should be ...
Open Design principle
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Open Design
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A protection mechanism must not depend on the
fact that its design is secret
Kerckhoffs' principle
Unfortunately wrong designs, that violates this
principle, exist in practice
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See lecture 17 on Digital Rights Management (DRM)
Design principles to restrict privileges
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Least Privilege
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Separation of privileges
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Each user (understand each program) must have the
smallest privilege set needed to operate
A business process must be split in different
elementary tasks with minimum privileges
Least Common Mechanism
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Reduce and control the exchange of information
between shared objects and resources
(potential channels for information leakage)
Access Control design principles
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Permission based
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Complete mediation
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Identifies what can be permitted and any
unidentified access (“close world” hypothesis)
Every access attempt must be checked and cannot
be circumvent
Trusted path
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Access control mechanisms cannot be spoofed or
intercepted by a malicious user program
The “keep it simple and usable” principles
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Economy of mechanism
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The design of a security mechanism must be small
and easy to analyze
Increasing the reliability in security mechanisms
Ease of use
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A security mechanism must be easy to use
Avoiding users and/or administrators to disable
security mechanisms
Security features for Operating Systems
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Identification and Authentication of users
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Protection of the execution context
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Focus: protecting the processes
Protection of general objects (access control)
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Focus: the reference monitor
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Focus: the object reuse attack
Protection of administrative data and processes
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Focus: managing the logs
Protection of the execution context
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Protection of the memory
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Already seen in lecture 12
In a concurrent context, a process needs to
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Access to some resources
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Synchronize with other processes
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Be executed
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All of these must be controlled by the operating
system
Focus: Protecting processes
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Enforced Sharing
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Interprocess communication and Synchronization
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Must have access to resources as appropriate
Must have access to synchronization mechanisms
Guaranteed Fair Service
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Must get a fair CPU allocation time to run
(preventing starvation)
Protection of general objects
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Two kind of objects to consider
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Static objects
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Dynamic objects
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File and I/O devices
Mainly used for synchronization and sharing between
concurrent programs
The “Reference Monitor”
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In charge of enforcing the access control policy
(DAC and/or MAC)
Focus: the concept of “Reference Monitor”
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Objective
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Controlling access to objects
Not necessarily a single piece of code
but rather a collection of protection mechanisms
The reference Monitor must be
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Tamperproof
- impossible to weaken or disable
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Unbypassable
- always invoked on every access
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Analyzable
- small enough to be easily validated
Focus: The Object Reuse attack
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Reusable objects
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Possible counter-measure
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“Free” disk or memory space can contain old (and
sensitive) data that have been previously disallocated
“Clear” the portion of memory by rewriting it with
garbage code
A more general problem
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Magnetic remanence
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How to discard old magnetic devices?
Security features for administration
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Protect security configuration data and
processes
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Definition of system (and/or security) administrators
Configuring (even implementing) an administration
model for access control
Setup and protect accountability mechanisms
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Useful to detect a misconfiguration or an attack
(remember an attack is not necessarily disruptive)
Useful to recover from attack and take countermeasures
Focus: Managing and analyzing logs
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Problem
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Logs are difficult to manage and analyze in practice
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For instance, a program can cause hundreds of access
creating a huge volume of data making it hard to analyze
Solution
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Classify the logs according to their sensitivity level
Analyzed the logs using specialized audit programs
(or intrusion detection programs)
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Passive (off-line or on-line) raise an alert
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Pro-active (on-line) block the access (IPS)
Strengthening the security of an OS
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Trust Computing Base Operating Systems make
a distinction between TCB and non-TCB
components of the kernel
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TCB components that handles the security of the
system and that must not be tampered by users
(nor even administrators)
Non-TCB components that will not jeopardize the
security of the system if tampered by the users (or
administrators)
Example of Trusted-OS: SELinux
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Security-Enhanced Linux (SELinux)
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Developed by the NSA to implements the multilevel
military security policy proposed by the US DoD
First released as a Linux patch, SELinux is now fully
integrated into the Linux Kernel (version 2.6)
Virtualization
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Objective
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Provide a constraint execution environment by
simulating a collection resources
Examples
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The Java virtual machine
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Virtual Memory Space
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Virtual Machines
Virtual Machines
Conclusion
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The best way to learn more ....
... is to take a closer look at your OS
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Play with your OS and learn how it has been built
Identify the security mechanims and understand
how they contribute to secure the system
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Play with other OS and compare them
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Learn about the details and found the breach
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Create a proof of concept attack (exploit) and
become an “ethical” hacker
Open question ...
Policy
Administrates
How can I be sure that Bob will not
tamper its system to bypass access
control mechanisms and have a full
access to my data?
Administrates