Wind River Support Network

Description

In 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.

CVEs

• CVE-2026-45942