[PATCH 00/26] Fixing wait, exit, ptrace, exec, and CLONE_THREAD

[Aleksa Sarai]

On 06/07/2017 09:36 PM, Eric W. Biederman wrote:
>>> Another easy entry point is to see that a multi-threaded setuid won't
>>> change the credentials on a zombie thread group leader.  Which can allow
>>> sending signals to a process that the credential change should forbid.
>>> This is in violation of posix and the semantics we attempt to enforce in
>>> linux.
>>
>> I might be completely wrong on this point (and I haven't looked at the patches),
>> but I was under the impression that multi-threaded set[ug]id was implemented in
>> userspace (by glibc's nptl(7) library that uses RT signals internally to get
>> each thread to update their credentials). And given that, I wouldn't be
>> surprised (as a user) that zombie threads will have stale credentials (glibc
>> isn't running in those threads anymore).
>>
>> Am I mistaken in that belief?
> 
> Would you be surprised if you learned that if your first thread
> exits, it will become a zombie and persist for the lifetime of your
> process?
> 
> Furthermore all non-thread specific signals will permission check
> against that first zombie thread.

Ah okay, so it really is a matter of Linux's threadgroup semantics just 
not being "right" on a more fundamental level than nptl.

> Which I think makes this surprising even if you know that setuid is
> implemented in userspace.

Quite surprising, thanks for the explanation.

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/