linux-kernel - Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid sometimes doesn't)

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Date:	Sun, 29 Mar 2009 05:10:22 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Hugh Dickins <hugh@...itas.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Malicki <jmalicki@...acarta.com>,
	Michael Itz <mitz@...acarta.com>,
	Kenneth Baker <bakerk@...acarta.com>,
	Chris Wright <chrisw@...s-sol.org>,
	David Howells <dhowells@...hat.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Q: check_unsafe_exec() races (Was: [PATCH 2/4] fix setuid
	sometimes doesn't)

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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