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.
Product status
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before dfc8292a1d6782c76b626315605e0585a5a18447
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before ea3af09eb87d8f8708c66747fcf1a2762902e839
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before 50879a3c1faf06e661090015d59e2127255cff27
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before 67cb119d32f35e32acd0393bbeb318b2bb1fdafe
6.12.83 (semver)
6.18.24 (semver)
6.19.14 (semver)
7.0.1 (semver)
References
git.kernel.org/...c/dfc8292a1d6782c76b626315605e0585a5a18447
git.kernel.org/...c/ea3af09eb87d8f8708c66747fcf1a2762902e839
git.kernel.org/...c/50879a3c1faf06e661090015d59e2127255cff27
git.kernel.org/...c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe