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

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Message-ID: <20090330014040.GA4807@redhat.com>
Date:	Mon, 30 Mar 2009 03:40:40 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
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 03/30, Oleg Nesterov wrote:
>
> On 03/30, Al Viro wrote:
> >
> > On Mon, Mar 30, 2009 at 02:08:43AM +0100, Al Viro wrote:
> >
> > > So...
> > > 	* check_unsafe_exec() locks current->fs shared, goes through all
> > > threads comparing their ->fs with our, if the number doesn't match - bail
> > > out.  Otherwise we mark it "under exec".
>
> Unless I missed something again, check_unsafe_exec() should check "under exec"
> after it takes fs->lock. If set - we are racing with sub-thread, return -EAGAIN.
>
> We can't proceed. If that another exec() fails, it will clear "under exec" at
> the end of do_execve(), before we kill other threads.

Or we need a counter to mark/unmark.

Oleg.

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