CVE-2025-21696

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

mm: clear uffd-wp PTE/PMD state on mremap()

When mremap()ing a memory region previously registered with userfaultfd as

write-protected but without UFFD_FEATURE_EVENT_REMAP, an inconsistency in

flag clearing leads to a mismatch between the vma flags (which have

uffd-wp cleared) and the pte/pmd flags (which do not have uffd-wp

cleared). This mismatch causes a subsequent mprotect(PROT_WRITE) to

trigger a warning in page_table_check_pte_flags() due to setting the pte

to writable while uffd-wp is still set.

Fix this by always explicitly clearing the uffd-wp pte/pmd flags on any

such mremap() so that the values are consistent with the existing clearing

of VM_UFFD_WP. Be careful to clear the logical flag regardless of its

physical form; a PTE bit, a swap PTE bit, or a PTE marker. Cover PTE,

huge PMD and hugetlb paths.