Operating Systems 2015F Lecture 22
Video
The video for the lecture given on November 27, 2015 is now available.
Notes
Lecture 22
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Project
- you need to pass the oral to get credit for it
- Due December 4th by midnight, late penalty is 20%/day
Patching
Malware detection & operating systems
* Most Linux systems don't run "antivirus"
* Most Macs don't run antivirus
* iOS devices don't
* some Android devices do
What does this tell us about those operating systems?
- not that much
- it is more about what exists today rather than what could exist
Ideally your OS should only allow you to run "good" code.
Antivirus is what you get when you are likely to run "bad" code.
How do operating systems keep out bad code?
* user authentication (accounts and passwords)
* otherwise, mostly isolation
* user permissions (not all users can access all
resources)
("users" are now often applications)
* firewalls (isolate network connections to apps)
* supervisor versus user mode on the CPU
(kernel vs userspace)
* memory protection (virtual memory)
* CPU & I/O scheduling to prevent programs from
monopolizing resources
* Desktop OSs assume installed applications can be trusted
- they are allowed to do arbitrary damage
* Mobile OSs do not trust most applications, or greatly limit trust
Android
- applications can be trusted in different ways
- user can decide what apps are trustworthy for different
things
- users will look at an app's permissions and decide
whether it is worthy of the trust level requested
- regular users are not sufficiently knowledgeable to make such trust decision
- every app runs as a different user. Each app can only access its own files
- but, then we have processes that accept requests from apps to access just about anything else
* iOS sandboxing
- OS-level virtualization: every app gets its own userspace
- then iOS puts things in that "sandbox" that it deems
safe to access
On Android, you've always been able to install keyboards
On iOS, only recently have keyboards been added
- and they can't be used for passwords
on iOS, Apple-supplied apps have always had more access
What is antivirus?
- downloader's remorse
- or, I really shouldn't have installed that
How does antivirus work?
- tech equivalent to criminal profiling
- their history
- their behavior
- their friends
- profiling is fundamentally hard and limited
- profiling is especially hard because of lies
How do you catch the lies of programs?
- surveillance
all antivirus need kernel-level help to work
why can't malware have kernel-level help?
- that's a kernel-level rootkit
Nexus (border control) for operating systems is...
- code signing
Code signing in any sort of "open context" is suspect
App store signing is not bad
Traditional OSs are for running "trusted" code
We have the technology for running untrusted code.
It is called a web browser.