On 08/23/2012 03:50 AM, Andrea Arcangeli 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 <aarcange@xxxxxxxxxx> 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! > Andrew, Andrea, Thanks for your time to review the patch. > 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. > >> 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. Okay, i will pay more attention to this. > > 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. Yes, i agree. :) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
