---
id: "CVE-2026-99256"
kind: "vulnerability"
aliases: ["CVE-2026-99256"]
title: "Linux kernel: dm cache lock misuse triggers kernel warning on suspend and resume"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["device-mapper", "dm-cache", "kernel", "linux", "runtime"]
not_affected_if: ["default-config", "no-dm-cache"]
published_at: "2026-11-03T12:17:32+00:00"
first_seen_at: "2026-11-03T12:20:39+00:00"
updated_at: "2026-11-03T12:22:51+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-99256", "first_seen_at": "2026-11-03T12:20:39+00:00"}]
---

# Linux kernel: dm cache lock misuse triggers kernel warning on suspend and resume

The device-mapper cache target in the Linux kernel released a read-write semaphore from a different process than the one that locked it, which trips a debug warning when a cache device is suspended and resumed. The sources describe a kernel warning and a lock fix; they do not describe code execution or data exposure.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check whether the kernel is built with `CONFIG_DEBUG_RWSEMS` and whether the host uses dm-cache (`dmsetup status` lists cache targets)
- Check the running kernel version with `uname -r`

## Timeline

- 2026-11-03T12:20:39+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-99256

## From the sources

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 <cache>, run: 'dmsetup
suspend <cache> && dmsetup resume <cache>'. 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.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_