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

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Message-Id: <20100310203209.54168BCCD@magilla.sf.frob.com>
Date:	Wed, 10 Mar 2010 12:32:09 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Oleg Nesterov <oleg@...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

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.)

Perhaps it would be cleaner to do:

	flush_signal_handlers(current, 1);

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

Thanks,
Roland
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