linux-kernel - Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

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Date:	Fri, 20 Feb 2009 07:41:09 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Vegard Nossum <vegard.nossum@...il.com>, stable@...nel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

On Fri, Feb 20, 2009 at 04:18:12PM +0200, Pekka Enberg wrote:
> On Fri, 2009-02-20 at 15:01 +0100, Ingo Molnar wrote:
> > * Ingo Molnar <mingo@...e.hu> wrote:
> > 
> > > ah, indeed:
> > > 
> > >         list_del_rcu(&va->list);
> > > 
> > > i suspect it could be hit big time in a workload that opens 
> > > more than 512 files, as expand_files() uses a 
> > > vmalloc()+vfree() pair in that case.
> > 
> > hm, perhaps it's not a problem after all. The freeing is done 
> > via rcu, and list_del_rcu() leaves the forward pointer intact.
> > 
> > So how did it happen that the entry got kfree()d before the loop 
> > was done? We are in a spinlocked section so the CPU should not 
> > have entered rcu processing.
> 
> RCU. Lets CC Paul.
> 
> Looking at it, Documentation/RCU/checklist.txt states that:
> 
> 7.      If the updater uses call_rcu(), then the corresponding readers
>         must use rcu_read_lock() and rcu_read_unlock().
> 
> which we don't do for this loop. I fail to see how it could be a
> kmemcheck false positive, so it's probably a real bug.

And checklist.txt rule #7 is even more important in preemptable kernels,
especially if CONFIG_PREEMPT_RT.

							Thanx, Paul
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