In the Linux kernel, the following vulnerability has been resolved:

userfaultfd: gate must_wait writability check on pte_present()

userfaultfd_must_wait() and userfaultfd_huge_must_wait() read the PTE
without taking the page table lock and then apply pte_write() /
huge_pte_write() to it.  Those accessors decode bits from the present
encoding only; on a swap or migration entry they read the offset bits that
happen to share the same position and return an undefined result.

The intent of the check is "is this fault still WP-blocked?".  A
non-marker swap entry means the page is in transit -- the userfault
context the original fault delivered against is no longer the same, and
the swap-in or migration completion path will re-deliver a fresh fault if
userspace still needs to handle it.  Worst case under the current code the
garbage write bit says "wait", and the thread stays asleep until a
UFFDIO_WAKE that may never arrive.

Gate the writability check on pte_present() so the lockless re-check only
inspects present-PTE bits when the entry is actually present.  The
non-present, non-marker case returns "don't wait" and lets the fault path
retry.

https://git.kernel.org/stable/c/29e6f952c5fb7dc1d6b90fe5b7f36063d44ddae0

https://git.kernel.org/stable/c/5f4dbdb0a87596214076b20b94f2b71b522170b3

https://git.kernel.org/stable/c/60d696a037eeeedfb57756dfe7ec08a1587c8631

https://git.kernel.org/stable/c/710183888174639a15fcec16cd1af766b8480bb7

https://git.kernel.org/stable/c/8e80af52db652fbc41320eee45a4f73bc029faf2

https://git.kernel.org/stable/c/a5700a4c1c9099ac2ac73fe8a09cf059972e0d2f

https://git.kernel.org/stable/c/a6e9a4939e359599701b1da351e84f72e021443a

https://git.kernel.org/stable/c/d4026417e8184d13850a0bad4d96ceb6ed9f7152

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