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: 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 cgwbs CSS reference keeps the blkcg alive while blkcg_unpin_online() accesses it.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/115a5266749dcde7fe4127e8623d19c752088f69
• https://git.kernel.org/stable/c/1bd36e93b542d9dd020190c6607c6a3663405195
• https://git.kernel.org/stable/c/23acef4156c260e8598397a1a2e8b3a23e919893
• https://git.kernel.org/stable/c/50879a3c1faf06e661090015d59e2127255cff27
• https://git.kernel.org/stable/c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe
• https://git.kernel.org/stable/c/740ba1ebb223f137ff088ab74d533a13f9167bd8
• https://git.kernel.org/stable/c/8f5857be99f1ed1fa80991c72449541f634626ee
• https://git.kernel.org/stable/c/dfc8292a1d6782c76b626315605e0585a5a18447
• https://git.kernel.org/stable/c/ea3af09eb87d8f8708c66747fcf1a2762902e839