CVE-2026-31586

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.