Re: [PATCH] Repeated fork() causes SLAB to grow without bound

[Konstantin Khlebnikov <koct9i@gmail.com>]

On Wed, Nov 19, 2014 at 7:09 PM, Vlastimil Babka <vbabka@suse.cz> wrote:
> On 11/19/2014 03:36 PM, Konstantin Khlebnikov wrote:
>> On Wed, Nov 19, 2014 at 2:50 AM, Vlastimil Babka <vbabka@suse.cz> wrote:
>>> On 11/19/2014 12:02 AM, Konstantin Khlebnikov wrote:
>>>> On Wed, Nov 19, 2014 at 1:15 AM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
>>>>> On Tue, Nov 18, 2014 at 11:19 PM, Andrew Morton
>>>>> <akpm@linux-foundation.org> wrote:
>>>>>> On Mon, 17 Nov 2014 21:41:57 -0500 Rik van Riel <riel@redhat.com> wrote:
>>>>>>
>>>>>>> > Because of the serial forking there does indeed end up being an
>>>>>>> > infinite number of vmas.  The initial vma can never be deleted
>>>>>>> > (even though the initial parent process has long since terminated)
>>>>>>> > because the initial vma is referenced by the children.
>>>>>>>
>>>>>>> There is a finite number of VMAs, but an infite number of
>>>>>>> anon_vmas.
>>>>>>>
>>>>>>> Subtle, yet deadly...
>>>>>>
>>>>>> Well, we clearly have the data structures screwed up.  I've forgotten
>>>>>> enough about this code for me to be unable to work out what the fixed
>>>>>> up data structures would look like :( But surely there is some proper
>>>>>> solution here.  Help?
>>>>>
>>>>> Not sure if it's right but probably we could reuse on fork an old anon_vma
>>>>> from the chain if it's already lost all vmas which points to it.
>>>>> For endlessly forking exploit this should work mostly like proposed patch
>>>>> which stops branching after some depth but without magic constant.
>>>>
>>>> Something like this. I leave proper comment for tomorrow.
>>>
>>> Hmm I'm not sure that will work as it is. If I understand it correctly, your
>>> patch can detect if the parent's anon_vma has no own references at the fork()
>>> time. But at the fork time, the parent is still alive, it only exits after the
>>> fork, right? So I guess it still has own references and the child will still
>>> allocate its new anon_vma, and the problem is not solved.
>>
>> But it could reuse anon_vma from grandparent or older.
>> Count of anon_vmas in chain will be limited with count of alive processes.
>
> Ah I missed that it can reuse older anon_vma, sorry.
>
>> I think it's better to describe this in terms of sets of anon_vma
>> instead hierarchy:
>> at clone vma inherits pages from parent together with set of anon_vma
>> which they belong.
>> For new pages it might allocate new anon_vma or reuse existing. After
>> my patch vma
>> will try to reuse anon_vma from that set which has no vmas which points to it.
>> As a result there will be no parent-child relation between anon_vma and
>> multiple pages might have equal (anon_vma, index) pair but I see no
>> problems here.
>
> Hmm I wonder if root anon_vma should be excluded from this reusal. For
> performance reasons, exclusive pages go to non-root anon_vma (see
> __page_set_anon_rmap()) and reusing root anon_vma would change this.

This is simple, in my patch this can be reached by bumping its nr_vmas
by one and it'll never be reused.

> Also from reading http://lwn.net/Articles/383162/ I understand that correctness
> also depends on the hierarchy and I wonder if there's a danger of reintroducing
> a bug like the one described there.

If I remember right that was fixed by linking non-exclusively mapped pages to
root anon_vma instead of anon_vma from vma where fault has happened.
After my patch this still works. Topology hierarchy actually isn't used.
Here just one selected "root' anon_vma which dies last. That's all.

>
> Vlastimil
>
>>>
>>> So maybe we could detect that the own references dropped to zero when the parent
>>> does exit, and then change mapping of all relevant pages to the root anon_vma,
>>> destroy avc's of children and the anon_vma itself. But that sounds quite
>>> heavyweight :/
>>>
>>> Vlastimil
>>>
>>>>>
>>>>>>
>>>
>>
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