Secure Operating Systems

Secure Operating Systems Lesson 9: Multics

Where are we? • We now know all the background… so it’s time to figure out why Dr. Ford likes Multics so very much • Multics is pretty much the poster child for “proper” system design • And we did it years ago

A Little History: 1963 • Cuba transactions made illegal • Debut of Iron Man!!! • Beatles release their first album… • First James Bond movie • UCF founded • ZIP codes introduced • IEEE founded, ASCII introduced • Kevin Mitnick born ;)

In the Midst… • The Multics project begins • The move from batch systems to timesharing • Released as a commercial project in 1973… that’s a 10 year development cycle

Processes • We’re very comfortable with this idea, but it was newer then • Processes are the things that execute stuff in Multics • All the things the process accesses are stored as “segments” • The “protection domain” determines the segments a process can access

Segments • These are created hierarchically • This became the roadmap to things like the Unix file system • The process has a descriptor segment which contains a set of segment descriptor words (SDWs) that refer to all the segments the process can access directly

Security • Three primary parts: the supervisor, protection rings, and SDWs • The supervisor is the ultimate arbiter – it decides if a process can have a SDW • This is isolated from other processes by protection rings (64 possible) • The basic idea was to protect the supervisor from unauthorized changes

Segment Access Control • Simple ACL • Segments: read, write, execute • Directories: status, modify, append • However. The SDW also includes rings and brackets – this can be a little tricky • To grant access, the ACL and Access brackets must both allow…

Rings/Brackets • Imagine we have code running in Ring r. • Access brackets define access – range of rings is r1, r2 where r1 < r2 • If r < r1, the process has full access (r/w) • If r1 ≤ r ≤ r2, the process can read the segment only • If r2 < r, then the process has no access

Call Brackets • Imagine we have code running in Ring r, trying to invoke a code segment • Call brackets define access – range of rings is(r2,r3)where r2 ≤ r3 • If r < r1, the process can execute, but there is a loss of privilege, where r changes to r_prime • If r1 ≤ r ≤ r2, the process executes with its current privilege • If r2 ≤ r ≤ r3, the process executes with higher privilege IF the location is authorized by the gates • If r3 < r, then the process has no access

MLS • Multilevel Security was pioneered by Multics – the policy prevents a subject from reading data that is “more secret” than itself, or writing to objects that are “less secret” • This is part and parcel of the way the Multics protection system worked • MLS is MAC, ACL and Ring Brackets are DAC • Think about performance for a minute…

The Gatekeeper • Multics tries hard to prevent the confused deputy problem… • The gatekeeper carefully (!) checks the parameters passed when privilege increases • The gatekeeper sometimes copies code to avoid giving a whole segment of the caller to the callee • The kernel is split between Ring 0 and 1 – the gatekeeper is Ring 0

Security Eval • Need: complete mediation, tamper proofing, and verifiability • How does Multics do?

Discussion • How does the reference monitor interface ensure that all security-sensitive operations are mediated correctly? • Does the reference monitor interface mediate security-sensitive operations on all system resources? • How do we verify that the reference monitor interface provides complete mediation?

Discussion • How does the system protect the reference monitor from modification? • Does the protection system protect all of the TCB? • What is the basis for the correctness of the system’s TCB? • Does the protection system enforce the system’s goals?

Multics Vulnerabilities • Karger and Schell’s analysis is very interesting • Primarily looks at implementation errors in the system • Actually included a hardware error that allowed instructions to bypass the SDW

Master Mode • To me, this is a classic • For performance, it’s ugly to have all traps dealt with by Ring 0 • However, to handle that, we need a user level trap handler… which requires access to some privileged instructions • And the trap handler used a register to determine where to go… and thus, disaster

Lots to do! • 2 weeks, large project • Write an essay that compares Multics with the modern OS of your choice: Linux, Windows or iOS. Look at the trajectory of your chosen OS, not just how it is today, but how it was • How does the modern OS handle the things that Multics already had? • You’re aiming at 10-20 pages

Resources • You should read “Protection and the Control of Information Sharing in Multics” and “Thirty Years Later: Lessons from the Multics Security Evaluation” • Resource (long): Final Report of the Multics Kernel Design Project • We will discuss these papers a week Thursday, be ready to share your ideas

Questions & Comments • What do you want to know?

Secure Operating Systems