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

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Message-ID: <3200.1408638173@warthog.procyon.org.uk>
Date:	Thu, 21 Aug 2014 17:22:53 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, 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

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.

It shouldn't be possible to introduce a NULL pointer through commit_creds() or
override_creds() since both of those should crash immediately if given one,
but it's possible revert_creds() could be so used.

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.

David
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