On 11/19/2014 03:36 PM, Konstantin Khlebnikov wrote: > 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. 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. 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. 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 >> >>>> >>>>> >> > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>