linux-kernel - Re: [PATCH] signals: check_kill_permission: don't check creds if same_thread

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Date:	Thu, 20 May 2010 12:42:14 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Tridgell <tridge@...ba.org>,
	David Howells <dhowells@...hat.com>,
	Eric Paris <eparis@...isplace.org>,
	Jakub Jelinek <jakub@...hat.com>,
	James Morris <jmorris@...ei.org>,
	Roland McGrath <roland@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] signals: check_kill_permission: don't check creds if
 same_thread_group()

On Mon, 17 May 2010 21:54:14 +0200
Oleg Nesterov <oleg@...hat.com> wrote:

> Andrew Tridgell reports that aio_read(SIGEV_SIGNAL) can fail if the
> the notification from the helper thread races with setresuid(), see
> http://samba.org/~tridge/junkcode/aio_uid.c
> 
> This happens because check_kill_permission() doesn't allow to send
> a signal to the task with the different cred->xids. But there is no
> any security reason to check ->cred's when the task sends a signal
> (private or group-wide) to its sub-thread. Whatever we do, any thread
> can bypass all security checks and send SIGKILL to all threads, or
> it can block a signal SIG and do kill(gettid(), SIG) to deliver this
> signal to another sub-thread. Not to mention that CLONE_THREAD implies
> CLONE_VM.
> 
> Change check_kill_permission() to avoid the credentials check when
> the sender and the target are from the same thread group.
> 
> Also, move "cred = current_cred()" down to avoid calling get_current()
> twice.
> 
> Note: David Howells pointed out we could relax this even more, the
> CLONE_SIGHAND (without CLONE_THREAD) case probably does not need
> these checks too.

So... which kernel(s) do we think this fix should be merged into?
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