# CVE-2026-31586: mm: blk-cgroup: fix use-after-free in cgwb_release_workfn()

> A use-after-free in the Linux kernel block cgroup subsystem allows attackers to potentially execute code or escalate privileges via a race condition.

- **Severity:** High 7.8 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 24 Apr 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-83858, BELL-CVE-2026-31586, DEBIAN-CVE-2026-31586, GHSA-x7q8-xcw2-7mfg, ROOT-OS-DEBIAN-11-CVE-2026-31586, ROOT-OS-DEBIAN-12-CVE-2026-31586, ROOT-OS-DEBIAN-13-CVE-2026-31586, ROOT-OS-UBUNTU-2204-CVE-2026-31586, ROOT-OS-UBUNTU-2404-CVE-2026-31586, UBUNTU-CVE-2026-31586

## What is CVE-2026-31586?

CVE-2026-31586 is a high-severity use after free vulnerability in Linux kernel, published on 24 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

mm: blk-cgroup: fix use-after-free in cgwb_release_workfn()

cgwb_release_workfn() calls css_put(wb->blkcg_css) and then later accesses
wb->blkcg_css again via blkcg_unpin_online().  If css_put() drops the last
reference, the blkcg can be freed asynchronously (css_free_rwork_fn ->
blkcg_css_free -> kfree) before blkcg_unpin_online() dereferences the
pointer to access blkcg->online_pin, resulting in a use-after-free:

  BUG: KASAN: slab-use-after-free in blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367)
  Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531
   Workqueue: cgwb_release cgwb_release_workfn
   Call Trace:
    <TASK>
     blkcg_unpin_online (./include/linux/instrumented.h:112 ./include/linux/atomic/atomic-instrumented.h:400 ./include/linux/refcount.h:389 ./include/linux/refcount.h:432 ./include/linux/refcount.h:450 block/blk-cgroup.c:1367)
     cgwb_release_workfn (mm/backing-dev.c:629)
     process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385)

   Freed by task 1016:
    kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561)
    css_free_rwork_fn (kernel/cgroup/cgroup.c:5542)
    process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385)

** Stack based on commit 66672af7a095 ("Add linux-next specific files
for 20260410")

I am seeing this crash sporadically in Meta fleet across multiple kernel
versions.  A full reproducer is available at:
https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh

(The race window is narrow.  To make it easily reproducible, inject a
msleep(100) between css_put() and blkcg_unpin_online() in
cgwb_release_workfn().  With that delay and a KASAN-enabled kernel, the
reproducer triggers the splat reliably in less than a second.)

Fix this by moving blkcg_unpin_online() before css_put(), so the
cgwb's CSS reference keeps the blkcg alive while blkcg_unpin_online()
accesses it.

Weakness: CWE-416 (Use After Free)

## Which versions are affected by CVE-2026-31586?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31586?

Apply the vendor's patch.

## Is CVE-2026-31586 being exploited?

There is no known exploitation of CVE-2026-31586 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-31586 and fix it if it is affected.

What it is: CVE-2026-31586 is a high-severity use after free vulnerability in Linux kernel, published on 24 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31586 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-31586/ (Markdown: https://insights.sbomify.com/CVE-2026-31586/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31586
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31586
- OSV: https://osv.dev/vulnerability/CVE-2026-31586

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31586/
Summaries are generated and may contain mistakes; the official description above is the record.