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

Hugh Dickins
2021-07-20T20:45:04Z

On Tue, Jul 20, 2021 at 8:52 AM Peter Xu <peterx@redhat.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@gooddata.com> and
> debugged by Hugh Dickins <hughd@google.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@vger.kernel.org to
cherry-pick the two commits
5fc7a5f6fd04bc18f309d9f979b32ef7d1d0a997
8f34f1eac3820fc2722e5159acceb22545b30b0d

Hugh

(I'd usually reply with alpine rather than gmail, but I see extra
blank lines on these 0/2s that way; but the patches themselves are
good.)

>
> 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
>
>