Re: [PATCH] mm: mmu_notifier: fix inconsistent memory between secondary MMU and host

Andrew Morton
2012-08-22T19:58:05Z

On Wed, 22 Aug 2012 21:50:43 +0200
Andrea Arcangeli <aarcange@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 <aarcange@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.