CVE-2026-97945 (GCVE-0-2026-97945)
Vulnerability from cvelistv5 – Published: 2026-09-25 10:22 – Updated: 2026-10-19 09:11
VLAI
EPSS
VEX
Title
x86/mm: Fix user-space data loss with MADV_FREE and THP
Summary
In 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.
Severity
No CVSS data available.
Assigner
References
Impacted products
2 products
| Vendor | Product | Version | CPE status | |
|---|---|---|---|---|
| Linux | Linux |
Affected:
bb3aadf7d446aaf22c725b274e2c194ac5cb2111 , < b5dec6dfb593d81fec15ba43c4fe5d58f10a49a2
(git)
Affected: bb3aadf7d446aaf22c725b274e2c194ac5cb2111 , < e4d569fe2f436a9b58c048fefabf56eab68c1b7e (git) Affected: bb3aadf7d446aaf22c725b274e2c194ac5cb2111 , < 55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7 (git) Affected: bb3aadf7d446aaf22c725b274e2c194ac5cb2111 , < a5ad97cbcfcec31430268441c10eedde73200177 (git) Affected: bb3aadf7d446aaf22c725b274e2c194ac5cb2111 , < f7491d7c81db0e7c304a7bd757a76d2fbeaff80e (git) |
guessed | |
| Linux | Linux |
Affected:
6.6
Unaffected: 0 , < 6.6 (semver) Unaffected: 6.6.158 , ≤ 6.6.* (semver) Unaffected: 6.12.111 , ≤ 6.12.* (semver) Unaffected: 6.18.53 , ≤ 6.18.* (semver) Unaffected: 7.2.7 , ≤ 7.2.* (semver) Unaffected: 7.3 , ≤ * (original_commit_for_fix) |
guessed |
{
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}
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"nodes": [
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"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Fix user-space data loss with MADV_FREE and THP\n\nSome of users of Polars (a data analytics library) have lost production\ndata from this bug. They seem to have just the right combination of\nhuge pages, MADV_FREE and heavy reclaim pressure.\n\npmd_modify() masks the old value with (_HPAGE_CHG_MASK \u0026 ~_PAGE_DIRTY),\nsilently discarding the hardware dirty bit. The subsequent\npmd_mksaveddirty() call is supposed to transfer _PAGE_DIRTY into\n_PAGE_SAVED_DIRTY when write-protecting, but the dirty bit was already\nstripped from the value, so there is nothing left to transfer.\n\nContrast with pte_modify(), which keeps _PAGE_DIRTY_BITS in its mask,\nand pud_modify(), which keeps _HPAGE_CHG_MASK untouched: pmd_modify()\nis the odd one out. Any pmd_modify() on a writable, dirty PMD loses\nthe dirty state.\n\nOne visible consequence is data loss with MADV_FREE on PMD-mapped THP:\n\n memset(buf, 0x5A, size); // PMD-mapped THP, PMD dirty\n madvise(buf, size, MADV_FREE); // PMD cleaned but left writable,\n // folio marked lazyfree\n memset(buf, 0x5A, size); // hardware sets _PAGE_DIRTY again\n mprotect(buf, size, PROT_READ); // pmd_modify() drops the dirty bit\n mprotect(buf, size, PROT_READ|PROT_WRITE);\n // ... memory pressure ...\n\nReclaim (e.g. under memcg pressure) then finds the lazyfree folio with\nno dirty bit set anywhere and frees it in\n__discard_anon_folio_pmd_locked(), even though the data was rewritten\nafter MADV_FREE; subsequent reads fault in fresh zero pages. NUMA\nhinting alone can trigger the same loss, as do_huge_pmd_numa_page()\nrestores the PMD through pmd_modify() as well.\n\nPMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping\nthe dirty bit means rewritten data is never written back.\n\nFix it by keeping _PAGE_DIRTY in the preserved mask, exactly like\npte_modify() and pud_modify() do. The existing\npmd_mksaveddirty()/pmd_clear_saveddirty() pair then performs the\nhardware-dirty \u003c-\u003e saved-dirty transition based on the write bit,\npreserving the shadow-stack encoding rules."
}
],
"providerMetadata": {
"dateUpdated": "2026-10-19T09:11:11.655Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
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"url": "https://git.kernel.org/stable/c/b5dec6dfb593d81fec15ba43c4fe5d58f10a49a2"
},
{
"url": "https://git.kernel.org/stable/c/e4d569fe2f436a9b58c048fefabf56eab68c1b7e"
},
{
"url": "https://git.kernel.org/stable/c/55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7"
},
{
"url": "https://git.kernel.org/stable/c/a5ad97cbcfcec31430268441c10eedde73200177"
},
{
"url": "https://git.kernel.org/stable/c/f7491d7c81db0e7c304a7bd757a76d2fbeaff80e"
}
],
"title": "x86/mm: Fix user-space data loss with MADV_FREE and THP",
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"cveId": "CVE-2026-97945",
"datePublished": "2026-09-25T10:22:55.981Z",
"dateReserved": "2026-09-25T10:18:58.204Z",
"dateUpdated": "2026-10-19T09:11:11.655Z",
"state": "PUBLISHED"
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"date": "2026-11-14",
"epss": "0.00206",
"percentile": "0.10664"
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"cve": "CVE-2026-97945",
"id": "msrc_CVE-2026-97945",
"initial_release_date": "2026-09-27T23:41:55.000Z",
"product_status:fixed": "1",
"product_status:known_affected": "1",
"source": "Microsoft CSAF VEX",
"status": "final",
"title": "x86/mm: Fix user-space data loss with MADV_FREE and THP",
"url": "https://msrc.microsoft.com/csaf/vex/2026/msrc_cve-2026-97945.json",
"version": "3"
},
"nvd": {
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"cveTags": [],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nx86/mm: Fix user-space data loss with MADV_FREE and THP\n\nSome of users of Polars (a data analytics library) have lost production\ndata from this bug. They seem to have just the right combination of\nhuge pages, MADV_FREE and heavy reclaim pressure.\n\npmd_modify() masks the old value with (_HPAGE_CHG_MASK \u0026 ~_PAGE_DIRTY),\nsilently discarding the hardware dirty bit. The subsequent\npmd_mksaveddirty() call is supposed to transfer _PAGE_DIRTY into\n_PAGE_SAVED_DIRTY when write-protecting, but the dirty bit was already\nstripped from the value, so there is nothing left to transfer.\n\nContrast with pte_modify(), which keeps _PAGE_DIRTY_BITS in its mask,\nand pud_modify(), which keeps _HPAGE_CHG_MASK untouched: pmd_modify()\nis the odd one out. Any pmd_modify() on a writable, dirty PMD loses\nthe dirty state.\n\nOne visible consequence is data loss with MADV_FREE on PMD-mapped THP:\n\n memset(buf, 0x5A, size); // PMD-mapped THP, PMD dirty\n madvise(buf, size, MADV_FREE); // PMD cleaned but left writable,\n // folio marked lazyfree\n memset(buf, 0x5A, size); // hardware sets _PAGE_DIRTY again\n mprotect(buf, size, PROT_READ); // pmd_modify() drops the dirty bit\n mprotect(buf, size, PROT_READ|PROT_WRITE);\n // ... memory pressure ...\n\nReclaim (e.g. under memcg pressure) then finds the lazyfree folio with\nno dirty bit set anywhere and frees it in\n__discard_anon_folio_pmd_locked(), even though the data was rewritten\nafter MADV_FREE; subsequent reads fault in fresh zero pages. NUMA\nhinting alone can trigger the same loss, as do_huge_pmd_numa_page()\nrestores the PMD through pmd_modify() as well.\n\nPMD-mapped file THPs are affected too: mprotect()/NUMA hinting dropping\nthe dirty bit means rewritten data is never written back.\n\nFix it by keeping _PAGE_DIRTY in the preserved mask, exactly like\npte_modify() and pud_modify() do. The existing\npmd_mksaveddirty()/pmd_clear_saveddirty() pair then performs the\nhardware-dirty \u003c-\u003e saved-dirty transition based on the write bit,\npreserving the shadow-stack encoding rules."
}
],
"id": "CVE-2026-97945",
"lastModified": "2026-10-19T10:17:45.740",
"metrics": {},
"published": "2026-09-25T11:17:21.983",
"references": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7"
},
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"url": "https://git.kernel.org/stable/c/a5ad97cbcfcec31430268441c10eedde73200177"
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"url": "https://git.kernel.org/stable/c/b5dec6dfb593d81fec15ba43c4fe5d58f10a49a2"
},
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"url": "https://git.kernel.org/stable/c/f7491d7c81db0e7c304a7bd757a76d2fbeaff80e"
}
],
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"vulnStatus": "Received"
}
},
"redhat_vex": {
"aggregate_severity": "Moderate",
"current_release_date": "2026-11-06T02:33:20+00:00",
"cve": "CVE-2026-97945",
"id": "CVE-2026-97945",
"initial_release_date": "2026-09-25T00:00:00+00:00",
"product_status:known_affected": "124",
"product_status:known_not_affected": "79",
"source": "Red Hat CSAF VEX",
"status": "final",
"title": "kernel: x86/mm: Fix user-space data loss with MADV_FREE and THP",
"url": "https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-97945.json",
"version": "3"
}
}
}
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Sightings
| Author | Source | Type | Date | Other |
|---|
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Browse all ATT&CK techniques and the vulnerabilities related to each.
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