In case the tbb team hasn't seen this, I'm forwarding this email over.
---------- Forwarded message ---------- From: Szabolcs Nagy nsz@port70.net Date: 17 February 2016 at 16:19 Subject: [oss-security] Address Sanitizer local root To: oss-security@lists.openwall.com
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-poten... https://blog.hboeck.de/archives/879-Safer-use-of-C-code-running-Gentoo-with-... (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.