x86/mm: Fix user-space data loss with MADV_FREE and THP

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Summary

CVECVE-2026-97945
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-25 11:17:21 UTC
Updated2026-09-25 11:17:21 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: x86/mm: Fix user-space data loss with MADV_FREE and THP Some of users of Polars (a data analytics library) have lost production data from this bug. They seem to have just the right combination of huge pages, MADV_FREE and heavy reclaim pressure. pmd_modify() masks the old value with (_HPAGE_CHG_MASK & ~_PAGE_DIRTY), silently discarding the hardware dirty bit. The subsequent pmd_mksaveddirty() call is supposed to transfer _PAGE_DIRTY into _PAGE_SAVED_DIRTY when write-protecting, but the dirty bit was already stripped from the value, so there is nothing left to transfer. Contrast with pte_modify(), which keeps _PAGE_DIRTY_BITS in its mask, and pud_modify(), which keeps _HPAGE_CHG_MASK untouched: pmd_modify() is the odd one out. Any pmd_modify() on a writable, dirty PMD loses the dirty state. One visible consequence is data loss with MADV_FREE on PMD-mapped THP: memset(buf, 0x5A, size); // PMD-mapped THP, PMD dirty madvise(buf, size, MADV_FREE); // PMD cleaned but left writable, // folio marked lazyfree memset(buf, 0x5A, size); // hardware sets _PAGE_DIRTY again mprotect(buf, size, PROT_READ); // pmd_modify() drops the dirty bit mprotect(buf, size, PROT_READ|PROT_WRITE); // ... memory pressure ... Reclaim (e.g. under memcg pressure) then finds the lazyfree folio with no dirty bit set anywhere and frees it in __discard_anon_folio_pmd_locked(), even though the data was rewritten after MADV_FREE; subsequent reads fault in fresh zero pages. NUMA hinting alone can trigger the same loss, as do_huge_pmd_numa_page() restores the PMD through pmd_modify() as well. PMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping the dirty bit means rewritten data is never written back. Fix it by keeping _PAGE_DIRTY in the preserved mask, exactly like pte_modify() and pud_modify() do. The existing pmd_mksaveddirty()/pmd_clear_saveddirty() pair then performs the hardware-dirty <-> saved-dirty transition based on the write bit, preserving the shadow-stack encoding rules.

Risk And Classification

EPSS: 0.002000000 probability, percentile 0.088480000 (date 2026-09-26)

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected bb3aadf7d446aaf22c725b274e2c194ac5cb2111 e4d569fe2f436a9b58c048fefabf56eab68c1b7e git Not specified
CNA Linux Linux affected bb3aadf7d446aaf22c725b274e2c194ac5cb2111 55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7 git Not specified
CNA Linux Linux affected bb3aadf7d446aaf22c725b274e2c194ac5cb2111 a5ad97cbcfcec31430268441c10eedde73200177 git Not specified
CNA Linux Linux affected bb3aadf7d446aaf22c725b274e2c194ac5cb2111 f7491d7c81db0e7c304a7bd757a76d2fbeaff80e git Not specified
CNA Linux Linux affected 6.6 Not specified
CNA Linux Linux unaffected 6.6 semver Not specified
CNA Linux Linux unaffected 6.12.111 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.53 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.7 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc4 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/e4d569fe2f436a9b58c048fefabf56eab68c1b7e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/a5ad97cbcfcec31430268441c10eedde73200177 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/f7491d7c81db0e7c304a7bd757a76d2fbeaff80e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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