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H A D | hugetlb.c | 0f230bc2 Mon Apr 17 14:53:13 CDT 2023 Peter Xu <peterx@redhat.com> mm/hugetlb: fix uffd-wp bit lost when unsharing happens
When we try to unshare a pinned page for a private hugetlb, uffd-wp bit can get lost during unsharing.
When above condition met, one can lose uffd-wp bit on the privately mapped hugetlb page. It allows the page to be writable even if it should still be wr-protected. I assume it can mean data loss.
This should be very rare, only if an unsharing happened on a private hugetlb page with uffd-wp protected (e.g. in a child which shares the same page with parent with UFFD_FEATURE_EVENT_FORK enabled).
When I wrote the reproducer (provided in the last patch) I needed to use the newest gup_test cmd introduced by David to trigger it because I don't even know another way to do a proper RO longerm pin.
Besides that, it needs a bunch of other conditions all met:
(1) hugetlb being mapped privately, (2) userfaultfd registered with WP and EVENT_FORK, (3) the user app fork()s, then, (4) RO longterm pin onto a wr-protected anonymous page.
If it's not impossible to hit in production I'd say extremely rare.
Link: https://lkml.kernel.org/r/20230417195317.898696-3-peterx@redhat.com Fixes: 166f3ecc0daf ("mm/hugetlb: hook page faults for uffd write protection") Signed-off-by: Peter Xu <peterx@redhat.com> Reported-by: Mike Kravetz <mike.kravetz@oracle.com> Reviewed-by: David Hildenbrand <david@redhat.com> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: Axel Rasmussen <axelrasmussen@google.com> Cc: Mika Penttilä <mpenttil@redhat.com> Cc: Nadav Amit <nadav.amit@gmail.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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