linux-kernel - Re: kernel: signal: NULL ptr deref when killing process

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Message-ID: <20140821171726.GB27140@redhat.com>
Date:	Thu, 21 Aug 2014 19:17:26 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, richard@....at,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: kernel: signal: NULL ptr deref when killing process

On 08/21, David Howells wrote:
>
> Oleg Nesterov <oleg@...hat.com> wrote:
>
> > David, any other reason why ->real_cred can be NULL ? (assuming that I
> > interpret this asm correctly).
>
> It should only be possible to see ->real_cred as being NULL after exit_creds()
> has been called from __put_task_struct() for a task that has finished
> construction.

Yes, thanks, this was my understanding...

> Is there a race between kill() and exit() brought on by the kill path only
> using the RCU read lock?  This doesn't prevent ->real_cred from being
> modified, but it looks like this should, in combination with
> delayed_put_task_struct(), prevent it from being cleared.

Yes, rcu should protect us from both delayed_put_pid() and delayed_put_task().
Everything looks correct... And there are a lot of other similar users of
find_vpid/find_task_by_vpid/pid_task/etc under rcu, I can't recall any bug
in this area.

I am puzzled. Note also that ->signal == NULL. Will try to think more,
but so far I have no any idea.

Oleg.

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