linux-kernel - Re: [PATCH 00/26] Fixing wait, exit, ptrace, exec, and CLONE

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Message-ID: <87ink8vxkf.fsf@xmission.com>
Date:   Wed, 07 Jun 2017 06:36:48 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Aleksa Sarai <asarai@...e.de>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Roland McGrath <roland@...k.frob.com>,
        linux-api@...r.kernel.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...IV.linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>
Subject: Re: [PATCH 00/26] Fixing wait, exit, ptrace, exec, and CLONE_THREAD

Aleksa Sarai <asarai@...e.de> 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