# CVE-2026-31466: mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

Status: published. Published: 2026-04-22. Updated: 2026-11-05.

In the Linux kernel, the following vulnerability has been resolved:

mm/huge_memory: fix folio isn't locked in softleaf_to_folio()

On arm64 server, we found folio that get from migration entry isn't locked
in softleaf_to_folio().  This issue triggers when mTHP splitting and
zap_nonpresent_ptes() races, and the root cause is lack of memory barrier
in softleaf_to_folio().  The race is as follows:

	CPU0                                             CPU1

deferred_split_scan()                              zap_nonpresent_ptes()
  lock folio
  split_folio()
    unmap_folio()
      change ptes to migration entries
    __split_folio_to_order()                         softleaf_to_folio()
      set flags(including PG_locked) for tail pages    folio = pfn_folio(softleaf_to_pfn(entry))
      smp_wmb()                                        VM_WARN_ON_ONCE(!folio_test_locked(folio))
      prep_compound_page() for tail pages

In __split_folio_to_order(), smp_wmb() guarantees page flags of tail pages
are visible before the tail page becomes non-compound.  smp_wmb() should
be paired with smp_rmb() in softleaf_to_folio(), which is missed.  As a
result, if zap_nonpresent_ptes() accesses migration entry that stores tail
pfn, softleaf_to_folio() may see the updated compound_head of tail page
before page->flags.

This issue will trigger VM_WARN_ON_ONCE() in pfn_swap_entry_folio()
because of the race between folio split and zap_nonpresent_ptes()
leading to a folio incorrectly undergoing modification without a folio
lock being held.

This is a BUG_ON() before commit 93976a20345b ("mm: eliminate further
swapops predicates"), which in merged in v6.19-rc1.

To fix it, add missing smp_rmb() if the softleaf entry is migration entry
in softleaf_to_folio() and softleaf_to_page().

[tujinjiang@huawei.com: update function name and comments]

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 426ee10711586617da869c8bb798214965337617 (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < f1acf5887c2bbaf998dc3fe32c72b7a8b84a3ddd (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 722cfaf6b31d31123439e67b5deac6b1261a3dea (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 7ddcf4a245c1c5a91fdd9698757e3d95179ffe41 (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < b8c49ad888892ad7b77062b9c102b799a3e9b4f8 (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 7ad1997b9bc8032603df8f091761114479285769 (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 8bfb8414e9f2ce6f5f2f0e3d0da52f2d132128e7 (affected, git); ≥ e9b61f19858a5d6c42ce2298cf138279375d0d9b; < 4c5e7f0fcd592801c9cc18f29f80fbee84eb8669 (affected, git). Source: CNA: Linux
- Linux / Linux: 4.5 (affected); ≥ 0; < 4.5 (unaffected, semver); ≥ 5.10.253; ≤ 5.10.* (unaffected, semver); ≥ 5.15.203; ≤ 5.15.* (unaffected, semver); ≥ 6.1.168; ≤ 6.1.* (unaffected, semver); ≥ 6.6.134; ≤ 6.6.* (unaffected, semver); ≥ 6.12.81; ≤ 6.12.* (unaffected, semver); ≥ 6.18.21; ≤ 6.18.* (unaffected, semver); ≥ 6.19.11; ≤ 6.19.* (unaffected, semver); ≥ 7.0; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIMATIC S7-1500 CPU 1518F-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.6; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.5; < * (affected, custom). Source: ADP: siemens-SADP
- Siemens / SIPLUS S7-1500 CPU 1518-4 PN/DP MFP: ≥ V3.1.7; < * (affected, custom). Source: ADP: siemens-SADP

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.09% as of 2026-11-05; percentile 0.57%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/426ee10711586617da869c8bb798214965337617 (CNA: Linux)

- https://git.kernel.org/stable/c/f1acf5887c2bbaf998dc3fe32c72b7a8b84a3ddd (CNA: Linux)

- https://git.kernel.org/stable/c/722cfaf6b31d31123439e67b5deac6b1261a3dea (CNA: Linux)

- https://git.kernel.org/stable/c/7ddcf4a245c1c5a91fdd9698757e3d95179ffe41 (CNA: Linux)

- https://git.kernel.org/stable/c/b8c49ad888892ad7b77062b9c102b799a3e9b4f8 (CNA: Linux)

- https://git.kernel.org/stable/c/7ad1997b9bc8032603df8f091761114479285769 (CNA: Linux)

- https://git.kernel.org/stable/c/8bfb8414e9f2ce6f5f2f0e3d0da52f2d132128e7 (CNA: Linux)

- https://git.kernel.org/stable/c/4c5e7f0fcd592801c9cc18f29f80fbee84eb8669 (CNA: Linux)

- https://cert-portal.siemens.com/productcert/html/ssa-082556.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-019113.html (ADP: siemens-SADP)

- https://cert-portal.siemens.com/productcert/html/ssa-457936.html (ADP: siemens-SADP)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2026-31466; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-31466; retrieved 2026-11-05; source updated Not reported

Revision: 36b01e3aebd74ba9f35514d2db1ba7993b4c1898acc3370408b30e91398bde5c

Canonical: https://y2.dev/cve/CVE-2026-31466

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