linux-kernel - Re: [PATCH 5/7] sched: Use rq->rd in sched

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Message-ID: <20140920191843.GX2832@worktop.localdomain>
Date:	Sat, 20 Sep 2014 21:18:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Kirill Tkhai <ktkhai@...allels.com>
Subject: Re: [PATCH 5/7] sched: Use rq->rd in sched_setaffinity() under RCU
 read lock

On Sat, Sep 20, 2014 at 11:05:46PM +0400, Kirill Tkhai wrote:
> On 20.09.2014 22:59, Peter Zijlstra wrote:
> > On Sat, Sep 20, 2014 at 08:51:40PM +0400, Kirill Tkhai wrote:
> >> From: Kirill Tkhai <ktkhai@...allels.com>
> >>
> >> task_rq(p)->rd and task_rq(p)->rd->span may be used-after-free here.
> >> Probability of NULL pointer derefference isn't zero in this place.
> > 
> > I don't see NULL derefs, just use-after-free.
> > 
> 
> It's very paranod case :). Two pointers are here:
> 
> task_rq(p)->rd (somebody zeroed it "rd") ->span

What you're saying is: due to the reuse someone might have put a NULL
in there. Which is fair, but I'd still call it use-after-free because
that is the first order problem. Dereferencing 'unknown' memory can of
course cause all kinds of 'fun' problems :-)
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