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

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Date:	Fri, 20 Feb 2009 15:51:28 +0100
From:	Vegard Nossum <vegard.nossum@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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)

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!

Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036
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