mm/huge_memory: use correct flags for device private PMD entry

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Summary

CVECVE-2026-53155
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-25 09:16:32 UTC
Updated2026-06-30 14:44:27 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: mm/huge_memory: use correct flags for device private PMD entry Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") updated set_pmd_migration_entry() to use pmdp_huge_get_and_clear() in the softleaf case, but made no further adjustments to the function itself. Therefore this function continues to incorrectly use pmd_write(), pmd_soft_dirty() and pmd_uffd_wp() to determine whether the installed migration entry should be marked writable, softdirty or uffd-wp respectively. Whilst all are incorrect, the most problematic of these is pmd_write(), as this can lead to corrupted rmap state. On x86-64 _PAGE_SWP_SOFT_DIRTY is aliased to _PAGE_RW. So calling pmd_write() on a softleaf will return the softdirty state encoded in the entry, assuming CONFIG_MEM_SOFT_DIRTY was enabled. This was observed when running the hmm.hmm_device_private.anon_write_child selftest: 1. The test faults in a range then migrates it such that a device-private THP range is established. 2. The parent then migrates it to a device-private writable PMD entry whose folio is entirely AnonExclusive with entire_mapcount=1, softdirty set (accidentally correct write state). 3. The parent forks and the PMD entries are set to device-private read only entries, entire_mapcount=2, softdirty still set. 4. [BUG] The child writes to the range then migrates to RAM - intending to install non-writable migration entries - but replacing parent and child PMD mappings with WRITABLE entries due to misinterpreting the softdirty bit. 5. In remove_migration_pmd(), if !softleaf_is_migration_read(entry) we set the RMAP_EXCLUSIVE flag when calling folio_add_anon_rmap_pmd() for both parent and child, which are therefore AnonExclusive. 6. [SPLAT] Child sets migrated folio entire_mapcount=1, parent sets entire_mapcount=2 and we end up with an AnonExclusive folio with entire_mapcount=2! Assert fires in __folio_add_anon_rmap(): VM_WARN_ON_FOLIO(folio_test_large(folio) && folio_entire_mapcount(folio) > 1 && PageAnonExclusive(cur_page), folio) This patch fixes the issue by correctly referencing the softleaf entry fields for writable, softdirty and uffd-wp in set_pmd_migration_entry(). It also only updates A/D flags if the entry is present as these are otherwise not meaningful for a softleaf entry. This patch also flips the if (!present) { ... } else { ... } logic in set_pmd_migration_entry() so it is easier to understand, and adds some comments to make things clearer. I was able to bisect this to commit 775465fd26a3 ("lib/test_hmm: add zone device private THP test infrastructure") which first exposes this bug as it was the commit that permitted test_hmm to generate the test. However commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") is the commit that actually enabled this behaviour.

Risk And Classification

EPSS: 0.001720000 probability, percentile 0.068870000 (date 2026-07-03)

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 65edfda6f3f2e58f757485a056e4f1775a1404a8 d7251c8d3f7cea76543abac6cf4ed15582c10846 git Not specified
CNA Linux Linux affected 65edfda6f3f2e58f757485a056e4f1775a1404a8 43e7f189769c512c843184a8a5892ac779a6bd90 git Not specified
CNA Linux Linux affected 6.19 Not specified
CNA Linux Linux unaffected 6.19 semver Not specified
CNA Linux Linux unaffected 7.0.13 7.0.* semver Not specified
CNA Linux Linux unaffected 7.1 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/d7251c8d3f7cea76543abac6cf4ed15582c10846 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/43e7f189769c512c843184a8a5892ac779a6bd90 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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