# CVE-2026-31586

> Vulnerability · severity: **HIGH** (CVSS 7.8).

## 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 reliabl

## Key facts

- **CVE ID:** CVE-2026-31586
- **Published:** 2026-04-24
- **CVSS severity:** HIGH
- **CVSS base score:** 7.8
- **CWE codes:** CWE-416

## Affected products

- `linux:linux_kernel`

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31586

## Citation

> AI Analytics. CVE-2026-31586. Retrieved 2026-07-29 from https://api.ai-analytics.org/cve/CVE-2026-31586. Derived from NIST NVD. Licensed CC0.

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