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

[ebiederm-aS9lmoZGLiVWk0Htik3J/w@public.gmane.org (Eric W. Biederman)]

Aleksa Sarai <asarai-l3A5Bk7waGM@public.gmane.org> writes:

>> 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.

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

Eric