mm/huge_memory: transfer the pmd dirty bit to the folio on zap

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Summary

CVECVE-2026-89987
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-09-16 11:17:09 UTC
Updated2026-09-16 11:17:09 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: mm/huge_memory: transfer the pmd dirty bit to the folio on zap zap_huge_pmd_folio() propagates the pmd young bit to the folio for the file case, but not the dirty bit. The pte path does propagate it, in zap_present_folio_ptes() and so does the pmd split path, in __split_huge_pmd_locked(). For most file mappings the omission is harmless, because writing to a shared file mapping goes through page_mkwrite(), which dirties the folio. tmpfs is different: it has no page_mkwrite(), and vma_wants_writenotify() is false for it, so a *read* fault on a MAP_SHARED tmpfs mapping installs a writable pmd via do_read_fault(). do_read_fault() does not call fault_dirty_shared_page(), so subsequent stores through that mapping set only the hardware dirty bit in the pmd and never call folio_mark_dirty(). A shmem folio allocated by a fault is marked uptodate but not dirty (see the clear: block in shmem_get_folio_gfp()), so PG_dirty is never set at all. Unmapping such a folio - munmap(), or exit_mmap() when the process dies - then loses the only record that it was written, because zap_huge_pmd() drops the pmd without transferring the dirty bit. Reclaim afterwards sees a clean shmem folio: the whole swap-out block in shrink_folio_list() is inside "if (folio_test_dirty(folio))", so pageout() is skipped and the folio falls into __remove_mapping(). There, folio_is_file_lru() is false for a swapbacked folio, so no shadow entry is created and __filemap_remove_folio(folio, NULL) simply empties the i_pages slot. The data is freed without ever being written to swap, and the next fault on that index returns a freshly zeroed folio. This is silent data loss for any process that keeps state in a MAP_SHARED tmpfs segment across an unmap - for example a cache handed from one process generation to the next through /dev/shm. It requires the folio to be PMD-mapped, so it only shows up once shmem THP is enabled (which is what we did in Meta fleet and started noticing crashes); with THP off the pte path transfers the dirty bit correctly. It also only becomes visible when swap is enabled, because with no swap device shmem folios (which are on the anon LRU) are not scanned by reclaim at all, so the clean folio is never dropped. Reproduced on x86_64 with a tmpfs mounted huge=within_size: read-fault a 2MB-backed region, write a known pattern through the resulting mapping, munmap, force reclaim of the cgroup, then re-map and read back. Without this patch the region reads back as zeros and vmstat shows zswpout 0 - the data was discarded rather than swapped. With this patch the region reads back correctly and the pages are swapped out as expected. With huge=never, or when the first touch is a write, the test passes either way.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected b5072380eb619786990cd9eab3ade05d09ccd89e effe3cc6d4fdd407457eb30f7b0ef094393f0d64 git Not specified
CNA Linux Linux affected b5072380eb619786990cd9eab3ade05d09ccd89e 9435cddf1378d149d826bd7bd1448a44bf96868d git Not specified
CNA Linux Linux affected b5072380eb619786990cd9eab3ade05d09ccd89e fe6cf984939d8e12cb33a99673c8d026c5135e68 git Not specified
CNA Linux Linux affected 4.8 Not specified
CNA Linux Linux unaffected 4.8 semver Not specified
CNA Linux Linux unaffected 6.18.52 6.18.* semver Not specified
CNA Linux Linux unaffected 7.2.5 7.2.* semver Not specified
CNA Linux Linux unaffected 7.3-rc2 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/9435cddf1378d149d826bd7bd1448a44bf96868d 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/effe3cc6d4fdd407457eb30f7b0ef094393f0d64 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/fe6cf984939d8e12cb33a99673c8d026c5135e68 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

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