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

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Message-ID: <6168b181-c57d-babb-e700-bdc30186a22a@suse.de>
Date:   Wed, 7 Jun 2017 22:21:52 +1000
From:   Aleksa Sarai <asarai@...e.de>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
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

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/