linux-kernel - Re: mm: NULL ptr deref in anon_vma_interval_tree

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Message-ID: <CANN689F6=mkJmgLFbALRPeYKG4RwTef+_r2TsHOLuobAxXbtPg@mail.gmail.com>
Date:	Tue, 6 Nov 2012 00:24:49 -0800
From:	Michel Lespinasse <walken@...gle.com>
To:	Bob Liu <lliubbo@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Sasha Levin <levinsasha928@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>, hughd@...gle.com,
	linux-mm <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>
Subject: Re: mm: NULL ptr deref in anon_vma_interval_tree_verify

On Mon, Nov 5, 2012 at 5:41 AM, Michel Lespinasse <walken@...gle.com> wrote:
> On Sun, Nov 4, 2012 at 8:44 PM, Michel Lespinasse <walken@...gle.com> wrote:
>> On Sun, Nov 4, 2012 at 8:14 PM, Bob Liu <lliubbo@...il.com> wrote:
>>> Hmm, I attached a simple fix patch.
>>
>> Reviewed-by: Michel Lespinasse <walken@...gle.com>
>> (also ran some tests with it, but I could never reproduce the original
>> issue anyway).
>
> Wait a minute, this is actually wrong. You need to call
> vma_lock_anon_vma() / vma_unlock_anon_vma() to avoid the issue with
> vma->anon_vma == NULL.
>
> I'll fix it and integrate it into my next patch series, which I intend
> to send later today. (I am adding new code into validate_mm(), so that
> it's easier to have it in the same patch series to avoid merge
> conflicts)

Hmmm, now I'm getting confused about anon_vma locking again :/

As Hugh privately remarked to me, the same_vma linked list is supposed
to be protected by exclusive mmap_sem ownership, not by anon_vma lock.
So now looking at it a bit more, I'm not sure what race we're
preventing by taking the anon_vma lock in validate_mm() ???

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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