oss-sec mailing list archives
Re: Address Sanitizer local root
From: Robert Święcki <robert () swiecki net>
Date: Thu, 18 Feb 2016 16:46:48 +0100
2016-02-18 11:37 GMT+01:00 Gynvael Coldwind <gynvael () coldwind pl>:
Just a random fun addition to the topic - there were exploitation challenges on CTFs with ASANafied binaries in the past, and they in fact were exploitable. One example: http://int3pids.blogspot.ch/2015/04/confidence-2015-teaser-quarantine-write.html
Also, a more general observation on the example (a set-uid bin) mentioned in the original post. Writing an always-correctly-behaving suid binary is a truly non-trivial task (a true mine trap), and I would go even to such claim, that the task should not be attempted (at least in cases, where such binary could be potentially widely distributed as a part of a popular OS distro) by anybody who is not a seasoned system developer with good understanding of the OS kernel and user-land libraries. Or, at least, if there's no chance that such developer (or even a few) can security-review the code. Potential attacker controls a lot when running set-uid apps - signals - resource limits - file-descriptors - certain timers - environment variables - system-specific features (bpf filters, dumpability flags and many more...) And each and every of those cases (not sure about system-specific features) has been successfully used to attack set-uid binaries in the past. Linking-in any kind of library beyond the standard libc (incl. ASAN) with a set-uid should generally trigger a complete security review of such library for any kind of misbehavior that can be exploited by a potential attacker. Just because such attacker would have almost unrestricted power over an execution environment of set-uid binaries.
On Wed, Feb 17, 2016 at 11:23 PM Szabolcs Nagy <nsz () port70 net> wrote:There is an alarming trend that Address Sanitizer and related compiler instrumentations from compiler-rt are used as a hardening solution and run in production. Even though these are debugging and testing tools, there is no clear warning against production use in their documentation: http://clang.llvm.org/docs/ And it's obvious how a tool that catches UB can be misunderstood as a hardening tool: This analysis concluded that ASan can be used for protection to stop certain attacks: http://scarybeastsecurity.blogspot.dk/2014/09/using-asan-as-protection.html The Tor project distributes ASan "hardened" binaries: https://blog.torproject.org/blog/tor-browser-55a4-hardened-released And there are various projects for full Linux distro instrumentation: http://balintreczey.hu/blog/progress-report-on-hardened1-linux-amd64-a-potential-debian-port-with-pie-asan-ubsan-and-more/ https://blog.hboeck.de/archives/879-Safer-use-of-C-code-running-Gentoo-with-Address-Sanitizer.html (the later was presented at FOSDEM 2016: https://fosdem.org/2016/schedule/event/csafecode/ ) While these are interesting projects, ASan should not be used for hardening in production systems in its current form, so at least the language ("hardening", "protection", "safe") should be fixed. My simple local root exploit is that ASan uses a lot of environment variables without checking for secure execution of setuid binaries: ASAN_OPTIONS='verbosity=2 log_path=foo' ./suid.exe will write to foo.$PID using escalated priviledge, so a normal user may be able to clobber arbitrary root owned files (by creating foo.{1,2,3,..} symlinks to it) which can lead to local root on an "ASan hardened" Linux distribution: ASAN_OPTIONS='suppressions="/foo root:passwdhash:12345:0::::: bar" log_path=foo' ./suid.exe can easily clobber /etc/shadow with AddressSanitizer: failed to read suppressions file '/foo root:passwdhash:12345:0::::: bar' if there is any setuid root executable built with ASan. (This is not a problem for testing where the env var based configuration is convenient and I haven't checked if any of the current ASan distro efforts have setuid executables with instrumentation, but I still find it a security bug given the improper advertisment of the sanitizer tools: this can lead to problems if the documentation is not fixed.) Beyond this trivial issue there are plenty reliability problems in the sanitizer runtimes that i think deserve at least a warning. It can crash conforming applications because - the shadow map overlaps with something - ulimit -v - overcommit is turned off - it allocates memory but aborts on failure - it interposes __tls_get_addr with non-as-safe code. - it uses initial-exec TLS. - it handles "deadly" signals like SIGBUS (often used by applications using mmaped files). - the c runtime is updated and incompatible (with the various interposition hacks) - does not handle c11 thread creation some of the features reduce security: - heuristic introspective unwind - nice diagnositc messages at undefined behaviour - interpositions in general (UB according to POSIX) other limitations: - static linking is not supported (This is for ASan only, I briefly looked at thread sanitizer, which seemed even worse for reliability and safe stack that is in fact advertised for hardening but it has plenty reliability problems, needs further analysis.) I believe some of the problems can be fixed by implementing the runtimes in the libc instead of second guessing libc behaviour with fragile heuristics from a compiler runtime. This would solve most of the runtime aborts. I can see an easy way to do this with musl libc (because a non-host musl is easy to distribute and link against), but non-trivial with glibc. In either case I don't see a solution to the shadow map commit charge unless the kernel is modified. So I cannot recommend even a careful reimplementation in libc for production use for reliable systems.
-- Robert Święcki
Current thread:
- Re: Address Sanitizer local root, (continued)
- Re: Address Sanitizer local root Daniel Micay (Feb 17)
- Re: Address Sanitizer local root Daniel Micay (Feb 17)
- Re: Address Sanitizer local root Konstantin Serebryany (Feb 17)
- Re: Address Sanitizer local root Daniel Micay (Feb 17)
- Re: Address Sanitizer local root Rich Felker (Feb 19)
- Re: Address Sanitizer local root Daniel Micay (Feb 19)
- Re: Address Sanitizer local root Daniel Micay (Feb 17)
- Re: Address Sanitizer local root Daniel Micay (Feb 17)
- Re: Address Sanitizer local root Hanno Böck (Feb 18)
- Re: Address Sanitizer local root Balint Reczey (Feb 18)
- Re: Address Sanitizer local root Daniel Micay (Feb 18)
- Re: Address Sanitizer local root Gynvael Coldwind (Feb 18)
- Re: Address Sanitizer local root Robert Święcki (Feb 18)
- Re: Address Sanitizer local root Darren Martyn (Feb 18)
- Re: Re: Address Sanitizer local root Rich Felker (Feb 18)
- Re: Re: Address Sanitizer local root Gynvael Coldwind (Feb 18)