CVE-2026-64514: userfaultfd: gate must_wait writability check on pte_present()

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

userfaultfd: gate mustwait writability check on ptepresent()

userfaultfdmustwait() and userfaultfdhugemustwait() read the PTE without taking the page table lock and then apply ptewrite() / hugeptewrite() 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 UFFDIOWAKE that may never arrive.

Gate the writability check on ptepresent() 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.