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Date: Thu, 21 Aug 2014 19:17:26 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: David Howells <dhowells@redhat.com>
Cc: Sasha Levin <sasha.levin@oracle.com>,
        Andrew Morton <akpm@linux-foundation.org>, richard@nod.at,
        Dave Jones <davej@redhat.com>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: kernel: signal: NULL ptr deref when killing process
Message-ID: <20140821171726.GB27140@redhat.com>
References: <20140820150619.GA12706@redhat.com> <53F48402.4080302@oracle.com> <20140820141252.GA27301@redhat.com> <3200.1408638173@warthog.procyon.org.uk>
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On 08/21, David Howells wrote:
>
> Oleg Nesterov <oleg@redhat.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.