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

From: Konstantin Khlebnikov
Date: Wed Nov 19 2014 - 09:36:50 EST

On Wed, Nov 19, 2014 at 2:50 AM, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> On 11/19/2014 12:02 AM, Konstantin Khlebnikov wrote:
>> On Wed, Nov 19, 2014 at 1:15 AM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>>> On Tue, Nov 18, 2014 at 11:19 PM, Andrew Morton
>>> <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>> On Mon, 17 Nov 2014 21:41:57 -0500 Rik van Riel <riel@xxxxxxxxxx> 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.

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.

> 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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