Outline

• linking and unlinking
• distributed computing overview
• security overview
• multics security

Minix Linking and Unliking

• linking adds a new link to an existing file
• unlinking removes an existing link to a file or directory
• rmdir is a slightly safer (more error checking) version of unlink
• when unliking a directory, must remove the "." and ".." entries and update the parent's link count
• renaming is almost like link followed by unlink, but slightly optimized to still work even when the disk is full
• renaming also allows renaming directories, whereas linking directories is only allowed to the superuser
• linking directories is dangerous because it introduces the risk of having loops in the file system hierarchy -- this could lead to infinite loops in the file system, which might be bad, e.g. line 27197 on p. 1007

Minix FS miscellaneous calls

• gen_io (p 1027) tries to send a message to a task, and is prepared to receive a completely unrelated response
• fetch_name ( utility.c, not in book) takes an argument name either from the message (if it is short), or by copying bytes from user space (for a long argument) -- the library must of course set up the message accordingly
• conv2 and conv4 (utility.c) do byte-swapping if the byte ordering on the disk is not the same as the byte ordering of the machine -- this may also be needed for networking

Distributed Computing: Concepts

• Do we really care where our computer lives?
  • no -- as long as we can access all our files and run all our programs
  • yes -- for security
  • sometimes -- for performance
• benefits of distributed computing: the right number of computers at the right time
• challenges of distributed computing:
  • providing fast access to computing power
  • providing fast access to data (files)
  • providing access to I/O devices
  • providing secure access and computing
• the computing environment may be as important as the computing itself -- fortunately, Unix-like systems have supported distributed environments well, e.g. remote window clients via X windows (1980s), secure logins such as SSH (late 1990s)
• Windows environments have supported a different subset of distributed access, mostly access to data (e.g. SMB)

Distributed Computing: Goals and Strategies

• use a collection of computers as a single computer
• sometimes, designed to enhance reliability (if one computer is down, can still use other computers)
• process migration for faster performance or for access to specialized devices (fork can make programs faster, e.g. in make)
• remote procedure call to execute code that must be executed on a given machine
• paging across a network (to another system's memory), may be faster than paging to disk
• a distributed file system is usually included with any kind of distributed computer, so all processors "see" the same file system

Architectures

• closely coupled processors with a single shared memory (but separate caches) and devices shared among all CPUs: MP, multiprocessor, often now handled by traditional operating systems (Linux, Windows)
• Network of Workstations (NoW), general-purpose workstations that agree to cooperate to get work done
• Beowulf cluster, similar to an NoW but processors not intended for general-purpose use (maybe no graphics card, maybe no local disk), interconnected for high performance, and often in larger numbers than a typical NoW
• grid computing: workstations, scattered across an internet, that agree to perform requested work while idle

Security Overview

• a policy determines who has access to what data and resources
• a mechanism allows the specification of a policy and implements it
• e.g. file ownership and permissions are a mechanism, assignment of specific permissions (and/or ownership) to a file are policy
• an audit trail keeps track of who read or modified what data or code. It is best if audit trails are write-once
• users execute programs, but it is sometimes hard for a user to know what a program will do (even if the user has authored the program!), so programs should not trusted more than necessary

Multics overview

• an early time-sharing system (inspired Unix)
• designed for interactive use
• much of it programmed in a high-level language (PL/1)
• potentially multiprocessor

Multics security

• didn't want to repeat the insecurities of previous time-sharing systems
• hardware segmentation (with paging), with each segment having access protection bits
• segments are persistent and are stored in directories which have Access Control Lists (ACLs)
• hardware rings -- 8 concentric rings of protection, implemented as a separate descriptor table for each ring
• rings were designed to be implemented in hardware, actually implemented by having "master mode" be ring 0 and providing facilities to cross from one ring to another. When a program tries to add a segment to its space, ring 0 checks the ACL.
• process table entries contain a user ID and are only accessible from ring 0
• passwords are stored encrypted, but the passwords file is also inaccessible to users
• users are told when they last tried to log in
• all system code is reloaded from tape when rebooting

Multics attack

• classic paper, e.g. at http://csrc.nist.gov/publications/history/karg74.pdf by Karger and Schell
• conclusion: not secure enough to have untrusted users share the computer with classified documents
• attacks designed to break hardware, software, and procedures, to the point of being able to access or modify "protected" data
• attackers had access to software and to hardware design (no "security by obscurity")
• attacks designed on a computer on an Air Force Base, then reproduced on a similar computer at MIT
• attacks should be undetectable, e.g. not cause anomalous crashes
• random tests of illegal operations or parameters to try and find unexpected behavior
• random tests uncovered a hardware bug that allowed some accesses to go unchecked (the bug had been introduced after the original design by an "upgrade")
• gate 0 code uses a routine to determine if argument lists are valid, but an autoincrement feature could lead the check to succeed but the actually accessed word to not be permitted
• a procedure (the "signaller", to deliver signals to user processes) was for efficiency placed in user space ("user ring") but ran in master mode, allowing a careful user to transfer to arbitrary code while still in master mode
• a change in usage of the stack segment register allowed user programs to store the registers in an arbitrary segment on a context switch, e.g. allowing the installation of a trap door into user code
• procedural vulnerabilities included physical access, the ability to use the software attacks to masquerade as a different user or change arbitrary memory or files, modifying the software at the development facility or enroute
• trap doors include modifying the TTY driver to recognize a special string, and modifying the compiler to recognize itself (and produce a trapdoor-generating compiler) and a given piece of code
• overall result: securing a system takes much more work than breaking in to a system!
• suggestion: a small (understandable) security kernel might do the trick

Multics 30 years later

• new paper, e.g. at http://www.acsac.org/2002/papers/classic-multics.pdf, also by Karger and Schell
• Multics had security as its goal from the start, other systems don't (and therefore are weaker)
• modifying other systems to be more secure may interfere with "normal" usage, so the code base forks
• Multics did not suffer from buffer overflows, since PL/1 is safer than C and the segments prevented memory misuse
• Multics stacks grow upwards, so an overflow does not overwrite the return address
• Multics is simpler than most current systems, e.g. Multics ring 0 is much smaller than the SELinux security module
• intrusion detection often does not deal with the possibility of professional attackers who refine the attack on their system before attacking the target
• the GEMSOS OS certified to "Class A1" security standards, so the technology exists
• why aren't we doing better?
• chicken and egg problem: makers don't think customers want it, customers don't know makers can do it