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

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Date:	Fri, 20 Feb 2009 17:01:57 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Vegard Nossum <vegard.nossum@...il.com>
Cc:	stable@...nel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: fix lazy vmap purging (use-after-free error)


* Vegard Nossum <vegard.nossum@...il.com> wrote:

> 2009/2/20 Ingo Molnar <mingo@...e.hu>:
> >
> > * 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.
> 
> Well, it's not the particular line that you posted, in any case.
> That's &va->list, but the traversed list is &va->purge_list.
> 
> I thought it would be the line:
> 
>         call_rcu(&va->rcu_head, rcu_free_va);
> 
> (which does kfree() in the callback) that was the problem.
> 
> >
> > 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.
> 
> I added some printks to __free_vmap_area() and rcu_free_va(), and it
> shows that the kfree() is being called immediately (inside the list
> traversal). So the call_rcu() is happening immediately (or almost
> immediately).
> 
> If I've understood correctly, the RCU processing can happen inside a
> spinlock, as long as interrupts are enabled. (Won't the timer IRQ
> trigger softirq processing, which triggers RCU callback processing,
> for example?)
> 
> And interrupts are enabled when this happens: EFLAGS: 00000292
> 
> Please correct me if I am wrong!

The timer irq will do RCU garbage-collection - but only of 
entries where a grace period has passed.

Otherwise there would be no point in using RCU at all, if the 
kfree() can happen immediately. RCU is about delaying action, 
and doing it at a point in time when we sure are in a quiescent 
state. (we have done a context-switch or scheduled to idle)

The question is, is this piece of loop traversal code 
preemptible? I dont think it is since it's embedded in a 
spinlock:

                spin_lock(&vmap_area_lock);
-               list_for_each_entry(va, &valist, purge_list)
+               list_for_each_entry_safe(va, n_va, &valist, purge_list)
                       	__free_vmap_area(va);
                spin_unlock(&vmap_area_lock);

[ on -rt this could be preemptible and this would be a real fix 
  there. I just dont see how the kfree() can execute on 
  mainline. Obviously it did, since you got the kmemcheck 
  assert. ]

	Ingo
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