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

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Message-ID: <20090329055513.GH28946@ZenIV.linux.org.uk>
Date:	Sun, 29 Mar 2009 06:55:13 +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 06:52:06AM +0200, Oleg Nesterov wrote:

> Let's suppose that check_unsafe_exec() does not set LSM_UNSAFE_SHARE and
> drops ->siglock. After that, another sub-thread does clone(CLONE_FS) without
> CLONE_THREAD.

Lovely.  And yes, AFAICS that's a hole.

> Unless we killed other threads, I can't see how we can check ->fs is not
> shared with another process, we can fool ->bprm_set_creds() anyway.

We can't do that, until we are past the point of no return.  Charming...
In principle, we can mark these threads as "-EAGAIN on such clone()" and
clean that on exec failure.
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