ext4: fix e4b bitmap inconsistency reports

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Summary

CVECVE-2026-45942
StatePUBLISHED
AssignerLinux
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-05-27 14:17:10 UTC
Updated2026-05-27 14:48:03 UTC
DescriptionIn the Linux kernel, the following vulnerability has been resolved: ext4: fix e4b bitmap inconsistency reports A bitmap inconsistency issue was observed during stress tests under mixed huge-page workloads. Ext4 reported multiple e4b bitmap check failures like: ext4_mb_complex_scan_group:2508: group 350, 8179 free clusters as per group info. But got 8192 blocks Analysis and experimentation confirmed that the issue is caused by a race condition between page migration and bitmap modification. Although this timing window is extremely narrow, it is still hit in practice: folio_lock ext4_mb_load_buddy __migrate_folio check ref count folio_mc_copy __filemap_get_folio folio_try_get(folio) ...... mb_mark_used ext4_mb_unload_buddy __folio_migrate_mapping folio_ref_freeze folio_unlock The root cause of this issue is that the fast path of load_buddy only increments the folio's reference count, which is insufficient to prevent concurrent folio migration. We observed that the folio migration process acquires the folio lock. Therefore, we can determine whether to take the fast path in load_buddy by checking the lock status. If the folio is locked, we opt for the slow path (which acquires the lock) to close this concurrency window. Additionally, this change addresses the following issues: When the DOUBLE_CHECK macro is enabled to inspect bitmap-related issues, the following error may be triggered: corruption in group 324 at byte 784(6272): f in copy != ff on disk/prealloc Analysis reveals that this is a false positive. There is a specific race window where the bitmap and the group descriptor become momentarily inconsistent, leading to this error report: ext4_mb_load_buddy ext4_mb_load_buddy __filemap_get_folio(create|lock) folio_lock ext4_mb_init_cache folio_mark_uptodate __filemap_get_folio(no lock) ...... mb_mark_used mb_mark_used_double mb_cmp_bitmaps mb_set_bits(e4b->bd_bitmap) folio_unlock The original logic assumed that since mb_cmp_bitmaps is called when the bitmap is newly loaded from disk, the folio lock would be sufficient to prevent concurrent access. However, this overlooks a specific race condition: if another process attempts to load buddy and finds the folio is already in an uptodate state, it will immediately begin using it without holding folio lock.

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 57e83bfbe1e412ac42daced2086f3c6f9a17bba0 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 c05033cfc5c7699cd4df8d48cef94d01da755f24 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 29a07d691d282faf38c33d4b61839b89399110f9 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 f29709a7a3fc38f5015d850504762cdef0e151f9 git Not specified
CNA Linux Linux affected 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 bdc56a9c46b2a99c12313122b9352b619a2e719e git Not specified
CNA Linux Linux affected 6.6.130 semver Not specified
CNA Linux Linux affected 6.12.75 semver Not specified
CNA Linux Linux affected 6.18.14 semver Not specified
CNA Linux Linux affected 6.19.4 semver Not specified
CNA Linux Linux unaffected 6.6.130 6.6.* semver Not specified
CNA Linux Linux unaffected 6.12.75 6.12.* semver Not specified
CNA Linux Linux unaffected 6.18.14 6.18.* semver Not specified
CNA Linux Linux unaffected 6.19.4 6.19.* semver Not specified
CNA Linux Linux unaffected 7.0 * original_commit_for_fix Not specified

References

ReferenceSourceLinkTags
git.kernel.org/stable/c/57e83bfbe1e412ac42daced2086f3c6f9a17bba0 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/bdc56a9c46b2a99c12313122b9352b619a2e719e 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/c05033cfc5c7699cd4df8d48cef94d01da755f24 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/29a07d691d282faf38c33d4b61839b89399110f9 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
git.kernel.org/stable/c/f29709a7a3fc38f5015d850504762cdef0e151f9 416baaa9-dc9f-4396-8d5f-8c081fb06d67 git.kernel.org
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