linux-kernel - Re: [PATCH] wait_for_helper: SIGCHLD from user-space can lead to use-after-free

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Date:	Wed, 10 Mar 2010 22:19:23 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	David Howells <dhowells@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] wait_for_helper: SIGCHLD from user-space can lead to
	use-after-free

On 03/10, Roland McGrath wrote:
>
> SIGCHLD being blocked doesn't affect reaping, so SIG_IGN or sa_flags &
> SA_NOCLDWAIT is the only thing that would do this.  How does that come
> about here in this kthread?  Is it inherited from the instigating user
> process?  If so, then SA_NOCLDWAIT is as much a problem as SIG_IGN.
> Or I guess maybe it's from ignore_signals() in kthreadd()?
> In that case SIG_IGN is indeed all that matters.  (I don't really
> know all the kthread/kmod/userhelper code organization.)

Yes. kthreads run with all signal ignored, this is inherited from
kthreadd() which does ignore_signal().

> Perhaps it would be cleaner to do:
>
> 	flush_signal_handlers(current, 1);
>
> in wait_for_helper

I don't think this can work. SIG_DFL for SIGCHLD is OK because it is
sig_kernel_ignore(). But, say, SIGHUP and other signals still should
be ignored, otherwise we have the same problems with the unwanted
signal_pending() this patch tries to avoid.

But even if we could do this,

> That should make it redundant in ____call_usermodehelper,
> so it could be removed from there.

Please note that __call_usermodehelper() forks ____call_usermodehelper() too.

Oleg.

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