FKIE_CVE-2026-97945

Vulnerability from fkie_nvd - Published: 2026-09-25 11:17 - Updated: 2026-09-25 11:17
Severity
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/a5ad97cbcfcec31430268441c10eedde73200177
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/e4d569fe2f436a9b58c048fefabf56eab68c1b7e
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/f7491d7c81db0e7c304a7bd757a76d2fbeaff80e
Impacted products
Vendor Product Version
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{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "arch/x86/include/asm/pgtable.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "e4d569fe2f436a9b58c048fefabf56eab68c1b7e",
              "status": "affected",
              "version": "bb3aadf7d446aaf22c725b274e2c194ac5cb2111",
              "versionType": "git"
            },
            {
              "lessThan": "55ddbc2ca6d5db64d44c3fb8f8705b4d39eb30b7",
              "status": "affected",
              "version": "bb3aadf7d446aaf22c725b274e2c194ac5cb2111",
              "versionType": "git"
            },
            {
              "lessThan": "a5ad97cbcfcec31430268441c10eedde73200177",
              "status": "affected",
              "version": "bb3aadf7d446aaf22c725b274e2c194ac5cb2111",
              "versionType": "git"
            },
            {
              "lessThan": "f7491d7c81db0e7c304a7bd757a76d2fbeaff80e",
              "status": "affected",
              "version": "bb3aadf7d446aaf22c725b274e2c194ac5cb2111",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "arch/x86/include/asm/pgtable.h"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "6.6"
            },
            {
              "lessThan": "6.6",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.111",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.53",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.2.*",
              "status": "unaffected",
              "version": "7.2.7",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.3-rc4",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "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": "2026-09-25T11:17:21.983",
  "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"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/e4d569fe2f436a9b58c048fefabf56eab68c1b7e"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/f7491d7c81db0e7c304a7bd757a76d2fbeaff80e"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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