VulnerabilityPublished 3 Nov 2026

CVE-2026-99256: Linux kernel · dm cache: fix issue with background work locking

Affects Linux kernelSource CVE Database V5

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Not known to be exploited · fixed in f5db935d257023c3ef410bd39163b949b0e5a55d : update Linux kernel when convenient.

Data as of 3 Nov 2026, 11:58 UTC

Severity

Medium

Rated Medium by Threat Radar's AI analysis · CVSS not yet scored by NVD

Thin evidence

One scorer or none

Patch: Available – fixed in f5db935d257023c3ef410bd39163b949b0e5a55d, bc9781c0247de107876f32929f1637db93a42b34, 6290507ecc0dd6dd40759729cbbbd49b1d2a63c4 +5

Exploitation evidence

Disclosed: evidence found.
CVE published 3 Nov 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 3 Nov 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade to a fixed release listed under Affected software.

Affected software

Product	Affected	Fixed in
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < f5db935d257023c3ef410bd39163b949b0e5a55d	Fixed in f5db935d257023c3ef410bd39163b949b0e5a55d
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < bc9781c0247de107876f32929f1637db93a42b34	Fixed in bc9781c0247de107876f32929f1637db93a42b34
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < 6290507ecc0dd6dd40759729cbbbd49b1d2a63c4	Fixed in 6290507ecc0dd6dd40759729cbbbd49b1d2a63c4
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < 8433fd005c25fd127021f67c9144a879fb585572	Fixed in 8433fd005c25fd127021f67c9144a879fb585572
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < 104f09e02038b0fc6bbb718b3976af029e15c1d0	Fixed in 104f09e02038b0fc6bbb718b3976af029e15c1d0
LinuxLinux	Affected ≥ b29d4986d0da1a27cd35917cdb433672f5c95d7f, < 73df2ff409f3db95029982fe3656329568e3715d	Fixed in 73df2ff409f3db95029982fe3656329568e3715d

AI-suggested

A fix is available and has been applied in specific Linux kernel commits. Users should update to a Linux kernel version that includes the fix identified by the commits listed in establishedFacts.fixedVersions. No additional mitigation actions are required beyond applying the official patch.

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 <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.

Impact

The vulnerability could lead to kernel warnings and potential instability due to improper locking semantics in the dm cache background work. While no direct exploitation or security breach is described, such kernel warnings indicate synchronization issues that could affect system stability or reliability.

Summary

CVE-2026-99256 addresses a locking issue in the Linux kernel's dm cache component. The dm cache used a rw_semaphore for background_work_lock, but write locks on rw_semaphores require that the locking and unlocking be done by the same process, which was not guaranteed. This caused kernel warnings when suspending and resuming dm cache devices, observable with CONFIG_DEBUG_RWSEMS enabled. The vulnerability was resolved by switching from a rw_semaphore to a spinlock and a wait queue, utilizing existing but previously unused wait queue structures to ensure proper synchronization of background work.

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What to do

Affected software

Description

AI analysisMachine-generated, updated 3 Nov 2026, 11:46 UTC

Technical details

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