Date: Sun, 29 Mar 2009 05:10:22 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Joe Malicki <jmalicki@metacarta.com>, Michael Itz <mitz@metacarta.com>,
       Kenneth Baker <bakerk@metacarta.com>,
       Chris Wright <chrisw@sous-sol.org>, David Howells <dhowells@redhat.com>,
       Alexey Dobriyan <adobriyan@gmail.com>,
       Greg Kroah-Hartman <gregkh@suse.de>, linux-fsdevel@vger.kernel.org,
       linux-kernel@vger.kernel.org
Subject: Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid
	sometimes doesn't)
Message-ID: <20090329041022.GF28946@ZenIV.linux.org.uk>
References: <Pine.LNX.4.64.0903282307050.14892@blonde.anvils> <Pine.LNX.4.64.0903282319270.15432@blonde.anvils> <20090329005343.GA12139@redhat.com>
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On Sun, Mar 29, 2009 at 01:53:43AM +0100, Oleg Nesterov wrote:

> Can't find the patch which introduced check_unsafe_exec(), so
> I am asking here.
> 
> How it is supposed to work?
> 
> Let's suppose we have two threads T1 and T2. T1 exits, and calls
> exit_fs().
> 
> 	exit_fs:
> 
> 		tsk->fs = NULL;
> 		// WINDOW
> 		put_fs_struct(fs);
> 
> Now, if T2 does exec() and check_unsafe_exec() happens in the WINDOW
> above, we set LSM_UNSAFE_SHARE.
> 
> Or we can race with sub-thread doing clone(CLONE_FS|CLONE_THREAD),
> the new thread is not visible in ->thread_group, buy copy_fs()
> can already increment fs->count.
 
Frankly, I don't think we really care.  Note that having several sub-threads
and doing execve() in one of them will kill the rest, so you really want
to do some kind of synchronization to get something similar to reasonable
behaviour anyway.

> Another question. Why do we check sighand->count? We always unshare
> ->sighand on exec, see de_thread().

Correct.  That check can and should go.
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