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CVE-2025-21696 is a Linux kernel memory-management flaw in the mremap() handling of mappings registered with userfaultfd write protection. When a write-protected mapping is remapped without UFFD_FEATURE_EVENT_REMAP enabled, the kernel clears the VM_UFFD_WP state in the VMA but can leave the logical userfaultfd write-protect state set in the corresponding page-table entry or PMD. The stale state may be represented by a PTE bit, swap PTE bit, or PTE marker. The affected logic includes regular PTE, huge-PMD, and hugetlb paths. A later mprotect(PROT_WRITE) then attempts to make an entry writable while its stale userfaultfd write-protect state remains set, violating page-table state consistency and triggering page_table_check_pte_flags().
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Linux kernel memory-management UFFD write-protect state flaw.
A Linux kernel memory-management flaw in mremap() handling of userfaultfd write-protected mappings. Failure to clear UFFD-WP state from PTE/PMD entries after clearing the VMA flag can cause an inconsistent page-table state and a page_table_check_pte_flags() warning when mprotect(PROT_WRITE) subsequently makes a page writable. The fix explicitly clears UFFD-WP logical state across standard PTE, swap-PTE, PTE-marker, huge-PMD, and hugetlb paths.
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