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Date: Thu, 18 Feb 2016 16:46:48 +0100
From: Robert Święcki <>
Subject: Re: Address Sanitizer local root

2016-02-18 11:37 GMT+01:00 Gynvael Coldwind <>:
> 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:

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

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 <> 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:
>> 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:
>> The Tor project distributes ASan "hardened" binaries:
>> And there are various projects for full Linux distro instrumentation:
>> (the later was presented at FOSDEM 2016:
>> )
>> 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

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