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Paper by Engler, Kaashoek, O’Toole
Presentation by Charles Haiber
 Exokernels
• Overview vs. traditional kernels
• Library Operating Systems
• Design Principals
 Secure Bindings
 Visible Resource Revocation
 Abort Protocol
 Testing
• Hypothesizes
• Results/Conclusion
 Is it secure?
 The main goal of an exokernel:
• The separation of protection from management
 Instead
of emulating hardware resources,
it exports them directly to the
 In
addition to the exokernel, a “Library
OS” can act as an abstraction layer
between hardware and application
 Fixed, high-level
abstractions, provided
by kernels to enable as many types of
applications to run on the system, tend to
have a very high cost in system resources
• Hurts the performance of applications
• Hides information from applications
• Limits the functionality of applications
 Exokernels
exist as a thin layer on top of
the hardware that multiplexes and
exports physical resources securely
 The
idea is that an application will better
know how it wants to manage it’s
resources than a monolithic kernel or
 Multiple
Library Operating Systems can
 Exist to provide specialized abstractions
• One OS might cater specifically to networking
 Library OS are not trusted by the exokernel
• They are free to trust the applications
• They run on the application level themselves
 Allow
applications to be ran on any
 Securely
expose hardware
• Avoid hardware management, except when
required for the protection of the system
 Expose
allocation to Library OS
 Expose names of physical resources
 Expose revocation protocol
 Secure
• Allows applications to securely bind themselves
to resources
 Visible
Resource Revocation
• Applications participate in a resource revocation
 Abort
• The exokernel can forcibly break secure
bindings of uncooperative applications
 Protection
mechanism that separates
authorization from the use of a resource
• Authorization only checked at bind time
• Applications responsible for resources with complex
hardware semantics (networks, file systems, etc.)
• This frees up the exokernel to perform access
checks at access time
 Allows
the kernel to protect hardware
resources without needing to understand
 Traditionally, resource
revocation is
invisible to applications in monolithic
and micro-kernels
 By
exposing resource revocation, the
exokernel allows applications and
Library OSs to monitor resource usage
and act accordingly
 Allows
the exokernel to retrieve
resources from a Library OS that is not
responding to revocation requests
• Revocation request – “Please return a memory
• Revocation imperative – “Return a memory page
within 50 microseconds”
 Secure
binding is broken, and the Library
OS sent a repossession exception
 Aegis
– an experimental exokernel
• Exports the processor, physical memory,
exceptions, interrupts, and network resources
 ExOS
– an experimental Library OS
• Provides processes, virtual memory, user-level
exceptions, interprocess abstractions, and
several network protocols
 Ultrix
is a mature monolithic Unix-based
 Exokernels
are very efficient
 Low-level, secure
multiplexing can be
 Traditional
OS abstractions can be
implemented efficiently at application level
 Applications
can create special-purpose
implementations of these abstractions
• Both Aegis and Ultrix ran on the same hardware
 No
overhead added to procedure calls
 Exception
dispatch about 100x faster in
 ExOS’s
implementation of pipes about
100x faster
 All
four hypothesizes were proven to be
correct when compared to Ultrix
 Results
show that the exokernel design is
well suited as a high-performance,
extensible OS. Additionally, previous
research found that applications benefit
greatly from specialized abstractions,
and ExOS backed those conclusions up
as well
 Total
mediation: NO
• OS does very little resource management and
only authorizes resource usage at bind time
 Trustworthy: YES
• Only the exokernel is part of the TCB, with the
Library OSs being ran at application level
 Verifiable: YES
• Kernel is extremely small, with only two goals:
 Present hardware resources
 Do so securely
Exokernel: An Operating System Architecture for
Application-Level Resource Management
• Dawson R. Engler, M. Frans Kaashoek, and James O’Toole Jr.,
M.I.T. Laboratory for Computer Science
The Operating System Kernel as a Secure
Programmable Machine
• Dawson R. Engler, M. Frans Kaashoek, and James O'Toole Jr.,
M.I.T. Laboratory for Computer Science
The case for application-specific operating
• Thomas E. Anderson, Division of Computer Science, University
of California at Berkeley