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
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 23acef4156c260e8598397a1a2e8b3a23e919893
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 1bd36e93b542d9dd020190c6607c6a3663405195
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 740ba1ebb223f137ff088ab74d533a13f9167bd8
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 115a5266749dcde7fe4127e8623d19c752088f69
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before dfc8292a1d6782c76b626315605e0585a5a18447
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before ea3af09eb87d8f8708c66747fcf1a2762902e839
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 50879a3c1faf06e661090015d59e2127255cff27
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 67cb119d32f35e32acd0393bbeb318b2bb1fdafe
59b57717fff8b562825d9d25e0180ad7e8048ca9 (git) before 8f5857be99f1ed1fa80991c72449541f634626ee
4.19
Any version before 4.19
5.10.258 (semver)
5.15.209 (semver)
6.1.175 (semver)
6.6.136 (semver)
6.12.83 (semver)
6.18.24 (semver)
6.19.14 (semver)
7.0.1 (semver)
7.1-rc1 (original_commit_for_fix)
References
git.kernel.org/...c/23acef4156c260e8598397a1a2e8b3a23e919893
git.kernel.org/...c/1bd36e93b542d9dd020190c6607c6a3663405195
git.kernel.org/...c/740ba1ebb223f137ff088ab74d533a13f9167bd8
git.kernel.org/...c/115a5266749dcde7fe4127e8623d19c752088f69
git.kernel.org/...c/dfc8292a1d6782c76b626315605e0585a5a18447
git.kernel.org/...c/ea3af09eb87d8f8708c66747fcf1a2762902e839
git.kernel.org/...c/50879a3c1faf06e661090015d59e2127255cff27
git.kernel.org/...c/67cb119d32f35e32acd0393bbeb318b2bb1fdafe
git.kernel.org/...c/8f5857be99f1ed1fa80991c72449541f634626ee