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 > >>> >>>> > -- 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>