Re: mm: NULL ptr deref in anon_vma_interval_tree_verify

[Sasha Levin]

On 11/06/2012 10:54 PM, Michel Lespinasse wrote:
> On Tue, Nov 6, 2012 at 12:24 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>> On Mon, Nov 5, 2012 at 5:41 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>>> On Sun, Nov 4, 2012 at 8:44 PM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
>>>> On Sun, Nov 4, 2012 at 8:14 PM, Bob Liu <lliubbo@xxxxxxxxx> wrote:
>>>>> Hmm, I attached a simple fix patch.
>>>>
>>>> Reviewed-by: Michel Lespinasse <walken@xxxxxxxxxx>
>>>> (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() ???
> 
> Looking at it a bit more:
> 
> the same_vma linked list is *generally* protected by *exclusive*
> mmap_sem ownership. However, in expand_stack() we only have *shared*
> mmap_sem ownership, so that two concurrent expand_stack() calls
> (possibly on different vmas that have a different anon_vma lock) could
> race with each other. For this reason we do need the validate_mm()
> taking each vma's anon_vma lock (if any) before calling
> anon_vma_interval_tree_verify().
> 
> While this justifies Bob's patch, this does not explain Sasha's
> reports - in both of them the backtrace did not involve
> expand_stack(), and there should be exclusive mmap_sem ownership, so
> I'm still unclear as to what could be causing Sasha's issue.
> 
> Sasha, how reproduceable is this ?

This is pretty hard to reproduce, I've seen this only twice so far.

> 
> Also, would the following change print something when the issue triggers ?

I'll run it with your patch, but as I've mentioned above - it's a PITA
to reproduce.


Thanks,
Sasha
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