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

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Message-Id: <1235139492.29813.53.camel@penberg-laptop>
Date:	Fri, 20 Feb 2009 16:18:12 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	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,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)

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.

			Pekka

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