Re: [PATCH stable 5.13.y/5.12.y 0/2] mm/thp: Fix uffd-wp with fork(); crash on pmd migration entry on fork

[Greg KH]

On Tue, Jul 20, 2021 at 01:32:19PM -0700, Hugh Dickins wrote:
> On Tue, Jul 20, 2021 at 8:52 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > In summary: this series should be needed for 5.10/5.12/5.13. This is the
> > 5.13.y/5.12.y backport of the series, and it should be able to be applied on
> > both of the branches.  Patch 1 is a dependency of patch 2, while patch 2 should
> > be the real fix.
> >
> > This series should be able to fix a rare race that mentioned in thread:
> >
> > https://lore.kernel.org/linux-mm/796cbb7-5a1c-1ba0-dde5-479aba8224f2@xxxxxxxxxx/
> >
> > This fact wasn't discovered when the fix got proposed and merged, because the
> > fix was originally about uffd-wp and its fork event.  However it turns out that
> > the problematic commit b569a1760782f3d is also causing crashing on fork() of
> > pmd migration entries which is even more severe than the original uffd-wp
> > problem.
> >
> > Stable kernels at least on 5.12.y has the crash reproduced, and it's possible
> > 5.13.y and 5.10.y could hit it due to having the problematic commit
> > b569a1760782f3d but lacking of the uffd-wp fix patch (8f34f1eac382, which is
> > also patch 2 of this series).
> >
> > The pmd entry crash problem was reported by Igor Raits <igor@xxxxxxxxxxxx> and
> > debugged by Hugh Dickins <hughd@xxxxxxxxxx>.
> >
> > Please review, thanks.
> 
> These two 5.13.y patches look just right to me, thank you Peter (and
> 5.12.19 announced EOL overnight, so nothing more wanted for that).
> 
> But these do just amount to asking stable@xxxxxxxxxxxxxxx to
> cherry-pick the two commits
> 5fc7a5f6fd04bc18f309d9f979b32ef7d1d0a997
> 8f34f1eac3820fc2722e5159acceb22545b30b0d

Thanks for the review, both now queued up to 5.13.y.

greg k-h