On Wed, 22 Aug 2012 21:50:43 +0200 Andrea Arcangeli <aarca...@redhat.com> wrote:
> Hi Andrew, > > On Wed, Aug 22, 2012 at 12:15:35PM -0700, Andrew Morton wrote: > > On Wed, 22 Aug 2012 18:29:55 +0200 > > Andrea Arcangeli <aarca...@redhat.com> wrote: > > > > > On Wed, Aug 22, 2012 at 02:03:41PM +0800, Xiao Guangrong wrote: > > > > On 08/21/2012 11:06 PM, Andrea Arcangeli wrote: > > > > > CPU0 CPU1 > > > > > oldpage[1] == 0 (both guest & host) > > > > > oldpage[0] = 1 > > > > > trigger do_wp_page > > > > > > > > We always do ptep_clear_flush before set_pte_at_notify(), > > > > at this point, we have done: > > > > pte = 0 and flush all tlbs > > > > > mmu_notifier_change_pte > > > > > spte = newpage + writable > > > > > guest does newpage[1] = 1 > > > > > vmexit > > > > > host read oldpage[1] == 0 > > > > > > > > It can not happen, at this point pte = 0, host can not > > > > access oldpage anymore, host read can generate #PF, it > > > > will be blocked on page table lock until CPU 0 > > > > release the lock. > > > > > > Agreed, this is why your fix is safe. > > > > > > ... > > > > > > Thanks a lot for fixing this subtle race! > > > > I'll take that as an ack. > > Yes thanks! > > I'd also like a comment that explains why in that case the order is > reversed. The reverse order immediately rings an alarm bell otherwise > ;). But the comment can be added with an incremental patch. If you can suggest some text I'll type it in right now. > > Unfortunately we weren't told the user-visible effects of the bug, > > which often makes it hard to determine which kernel versions should be > > patched. Please do always provide this information when fixing a bug. > > This is best answered by Xiao who said it's a testcase triggering > this. > > It requires the guest reading memory on CPU0 while the host writes to > the same memory on CPU1, while CPU2 triggers the copy on write fault > on another part of the same page (slightly before CPU1 writes). The > host writes of CPU1 would need to happen in a microsecond window, and > they wouldn't be immediately propagated to the guest in CPU0. They > would still appear in the guest but with a microsecond delay (the > guest has the spte mapped readonly when this happens so it's only a > guest "microsecond delayed reading" problem as far as I can tell). I > guess most of the time it would fall into the undefined by timing > scenario so it's hard to tell how the side effect could escalate. OK, thanks. For this sort of thing I am dependent upon KVM people to suggest whether the fix should be in 3.6 and whether -stable needs it. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
