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Date:   Thu, 22 Jul 2021 16:02:18 +0200
From:   Greg KH <greg@...ah.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Peter Xu <peterx@...hat.com>, stable <stable@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hillf Danton <hdanton@...a.com>, Igor Raits <igor@...ddata.com>
Subject: 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

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@...hat.com> 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@google.com/
> >
> > 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@...ddata.com> and
> > debugged by Hugh Dickins <hughd@...gle.com>.
> >
> > 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@...r.kernel.org to
> cherry-pick the two commits
> 5fc7a5f6fd04bc18f309d9f979b32ef7d1d0a997
> 8f34f1eac3820fc2722e5159acceb22545b30b0d

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

greg k-h

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