Transcript Chapter 6
Chapter 6
Security Kernels
Chapter Overview
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Description
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Secure Communications Processor (Scomp)
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Architecture
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Hardware
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Trusted Operating Program
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Kernel Interface Package
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Applications
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Evaluation
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Gemini Secure Operating System
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Summary
Security Kernels
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Efforts from the 70s and early 80's”
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SCOMP (Honeywell)
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Gemini Secure OS (GEMSOS)
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Based on Provably secure OS design:
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Secure Ada Target (SAT) (Honeywell)
LOCK (Secure Computing)
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Kernelized Secure OS (KSOS) (Ford
Aerospace and Communications)
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Secure LAN (Boeing)
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etc.
The Security Kernel
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MITRE, 1974, 20 subroutines, 1000 SLOC
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Showed the what and the how.
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Focus became verification
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Three core principles:
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Implement a specific security policy
Define a verifiable protection behavior of the
system as a whole
The implementation must be shown to be
faithful to the security model's design
Secure Communications
Processor (Scomp)
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Kernel-based system
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Designed to implement Multic's MLS
requirements.
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Original idea was to build an emulator to allow
execution on an ordinary OS (UNIX).
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Ended up with new API that provided the
necessary security.
Problems with the emulation
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Incompatible representations between the two
systems:
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UNIX I/O copies data directly to application's
address space,
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SCOMP maintains data in individually
managed segments to which access must be
authorized.
There are Unix mechanisms that are inherently
insecure: for example fork and exec share file
descriptors, thereby leaking data and authorization
problems.
Scomp Architecture
SCOMP Architecture notes
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Accesses to protected resources are mediated
using an MLS policy:
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App requests hardware descriptor sufficient to
access resource.
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If granted, security kernel builds the descriptor
(object+permissions) and returns a reference
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Isolation/tamperproofing provided by ring
mechanism. Rings and transitions are implemented
in hardware.
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Verification was part of the process.
SCOMP Hardware 1
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Based on Multics design with two key changes:
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Only four rings, all in hardware.
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Argument addressing mode prevents confused
deputy problem.
Hardware includes a security protection module
(SPM).
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It mediates the main system bus (peripherals and
memory).
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Virtual memory interface unit uses SPM to
convert between virtual and physical addresses
SCOMP Hardware 2
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Each process includes a descriptor base root:
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References memory and I/O descriptors
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Used for mediating memory and I/O
references.
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DMA is authorized on a per-transaction
basis.
• I/O descriptors are built by kernel.
• Hardware does all authorizations.
• Drivers are not part of kernel! (more
efficient and secure)
SCOMP Security Protection
Module
Scomp Trusted Operating
Program (STOP)
Three components:
– A security kernel. (ring 0)
– A set of trusted software
– A kernel interface package for user
applications.
SCOMP Trusted Operating
Program Security Kernel
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Memory management, process
scheduling, interrupt management, audit
and reference monitor. 10K SLOC mostly
in Pascal.
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Objects consist of processes, segments,
devices, identified by a unique 64 bit id.
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Access control similar to Multics, but ring
brackets allow for owner/group/others
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38 gates to enter ring 0
SCOMP Trusted Software 1
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Two types:
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Trusted not to violate system or integrity
goals: e.g. secure loader is trusted to load a
process for any subject that ensure correct
enforcement of information flows.
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Trusted to maintain security policy correctly:
e.g. user authentication.
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23 processes implement trusted functions:
11K SLOC in C
SCOMP Trusted Software 2
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Three kinds of user processes:
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Trusted user processes: login, dac
management, mandatory level selection,
process management.
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Trusted operation services: system
management, logging, startup, shutdown,
set time, etc.
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Trusted maintenance services: modify
system data, install new program versions,
etc.
Invoked through a secure communications
path directly by the user.
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Scomp Kernel Interface Package
(SKIP) 1
Uniform interface for user applications to
access trusted functions.
Two parts
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SKIP functions
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SKIP libraries
Scomp Kernel Interface Package
(SKIP) 2
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SKIP functions do trusted operations on user
level objects
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Files via a hierarchical file system
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Process management
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Concurrent I/O through an event mechanism
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Allowed to manipulate system state, so trusted
not to violate MLS requirements, like trusted
software.
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In ring 2, invoked via gates
SCOMP Kernel Interface Package
(SKIP) 3
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SKIP Library runs in level 3, provides
interface to SKIP functions.
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There are applications to access files,
modify file contents, manage file hierarchy.
File operations are authorized based on
requester's sensitivity level and ring
number, thus sensitivity level is
nondecreasing from the root.
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Library also provides I/O, and the device
drivers are part of the library. Handlers are
also run in the library
Scomp Applications
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Unix??
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Mail guard
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Secure Office Management System
Scomp Evaluation 1: Complete
Mediation
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How does the reference monitor interface ensure
that all security operations are mediated correctly?
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Does the reference monitor interface mediate
security-sensitive operations on all system
resources?
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All mediation done in hardware
Initial access to file data depends on access to I/O
How do we verify that the reference monitor provides
complete mediation?
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Hardware.
Scomp Evaluation 2: Tamperproof
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How does the system protect the reference
monitor, including its protection system,
from modification?
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Protection rings, but not complete, due to
need.
Does the protection system protect the
trusted computing base programs?
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Also protection rings
Scomp Verification: Verifiable
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What is the basis for the correctness of the
system's TCB?
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Verified with Formal analysis tools
Does the protection system enforce the
system's security goals?
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Also verified for correctness.
Gemini Secure Operating System
GEMSOS Security Kernel Layers
Summary