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

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/23acef4156c260e8598397a1a2e8b3a23e919893
• https://git.kernel.org/stable/c/1bd36e93b542d9dd020190c6607c6a3663405195
• https://git.kernel.org/stable/c/740ba1ebb223f137ff088ab74d533a13f9167bd8
• https://git.kernel.org/stable/c/115a5266749dcde7fe4127e8623d19c752088f69
• https://git.kernel.org/stable/c/dfc8292a1d6782c76b626315605e0585a5a18447
• https://git.kernel.org/stable/c/ea3af09eb87d8f8708c66747fcf1a2762902e839
• https://git.kernel.org/stable/c/50879a3c1faf06e661090015d59e2127255cff27
• https://git.kernel.org/stable/c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe
• https://git.kernel.org/stable/c/8f5857be99f1ed1fa80991c72449541f634626ee
• https://cert-portal.siemens.com/productcert/html/ssa-784472.html