linux-kernel - Re: [PATCH stable 5.10.y 0/2] mm/thp: Fix uffd-wp with fork(); crash on pmd migration entry on fork

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Date:   Tue, 20 Jul 2021 13:38:53 -0700
From:   Hugh Dickins <hughd@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     stable <stable@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>, Igor Raits <igor@...ddata.com>,
        Hillf Danton <hdanton@...a.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH stable 5.10.y 0/2] mm/thp: Fix uffd-wp with fork(); crash
 on pmd migration entry on fork

On Tue, Jul 20, 2021 at 8:57 AM Peter Xu <peterx@...hat.com> wrote:
>
> In summary, this series should be needed for 5.10/5.12/5.13. This is the 5.10.y
> backport of the series.  Patch 1 is a dependency of patch 2, while patch 2
> should be the real fix.
>
> There's a minor conflict on patch 2 when cherry pick due to not having the new
> helper called page_needs_cow_for_dma().  It's also mentioned at the entry of
> patch 2.
>
> 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.

And these two for 5.10.y look good to me also: I'm glad you decided in
the end to keep 5.10's support for uffd-wp-fork.
The first is just a straight cherry-pick of
5fc7a5f6fd04bc18f309d9f979b32ef7d1d0a997, but as you noted above,
8f34f1eac3820fc2722e5159acceb22545b30b0d needed one line of fixup for
that tree.

Thank you Peter,
Hugh


>
> Peter Xu (2):
>   mm/thp: simplify copying of huge zero page pmd when fork
>   mm/userfaultfd: fix uffd-wp special cases for fork()
>
>  include/linux/huge_mm.h |  2 +-
>  include/linux/swapops.h |  2 ++
>  mm/huge_memory.c        | 36 +++++++++++++++++-------------------
>  mm/memory.c             | 25 +++++++++++++------------
>  4 files changed, 33 insertions(+), 32 deletions(-)
>
> --
> 2.31.1
>
>