VulnerabilityPublished 16 Sep 2026Updated 21 Oct 2026

CVE-2026-89987: Linux kernel · mm/huge_memory: transfer the pmd dirty bit to the folio on zap

Affects Linux kernelSource CVE Database V5

Not known to be exploited · fixed in fe6cf984939d8e12cb33a99673c8d026c5135e68 : update Linux kernel when convenient.

Data as of 26 Oct 2026, 17:16 UTC

Severity

Unrated

Rated by Threat Radar · CVSS not yet scored by NVD

EPSS: 0.2% chance of exploitation in 30 days – higher than 10% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in fe6cf984939d8e12cb33a99673c8d026c5135e68, fd720d3906877b3d2118cd480027e4bc6648f0c5, effe3cc6d4fdd407457eb30f7b0ef094393f0d64 +2

Exploitation evidence

Disclosed: evidence found.
CVE published 16 Sep 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ b5072380eb619786990cd9eab3ade05d09ccd89e, < fe6cf984939d8e12cb33a99673c8d026c5135e68	Fixed in fe6cf984939d8e12cb33a99673c8d026c5135e68
LinuxLinux	Affected ≥ b5072380eb619786990cd9eab3ade05d09ccd89e, < fd720d3906877b3d2118cd480027e4bc6648f0c5	Fixed in fd720d3906877b3d2118cd480027e4bc6648f0c5
LinuxLinux	Affected ≥ b5072380eb619786990cd9eab3ade05d09ccd89e, < effe3cc6d4fdd407457eb30f7b0ef094393f0d64	Fixed in effe3cc6d4fdd407457eb30f7b0ef094393f0d64
LinuxLinux	Affected ≥ b5072380eb619786990cd9eab3ade05d09ccd89e, < 9435cddf1378d149d826bd7bd1448a44bf96868d	Fixed in 9435cddf1378d149d826bd7bd1448a44bf96868d
LinuxLinux	Affected ≥ b5072380eb619786990cd9eab3ade05d09ccd89e, < 6e904f9e8cb0e39f60e1cdbea679f89346a563c2	Fixed in 6e904f9e8cb0e39f60e1cdbea679f89346a563c2
LinuxLinux	Affected =4.8	Fixed in–

Description

In 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.

References

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