CVE-2026-31586

In the Linux kernel, the following vulnerability has been resolved:<br /> <br /> mm: blk-cgroup: fix use-after-free in cgwb_release_workfn()<br /> <br /> cgwb_release_workfn() calls css_put(wb-&gt;blkcg_css) and then later accesses<br /> wb-&gt;blkcg_css again via blkcg_unpin_online(). If css_put() drops the last<br /> reference, the blkcg can be freed asynchronously (css_free_rwork_fn -&gt;<br /> blkcg_css_free -&gt; kfree) before blkcg_unpin_online() dereferences the<br /> pointer to access blkcg-&gt;online_pin, resulting in a use-after-free:<br /> <br /> 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)<br /> Write of size 4 at addr ff11000117aa6160 by task kworker/71:1/531<br /> Workqueue: cgwb_release cgwb_release_workfn<br /> Call Trace:<br /> <br /> 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)<br /> cgwb_release_workfn (mm/backing-dev.c:629)<br /> process_scheduled_works (kernel/workqueue.c:3278 kernel/workqueue.c:3385)<br /> <br /> Freed by task 1016:<br /> kfree (./include/linux/kasan.h:235 mm/slub.c:2689 mm/slub.c:6246 mm/slub.c:6561)<br /> css_free_rwork_fn (kernel/cgroup/cgroup.c:5542)<br /> process_scheduled_works (kernel/workqueue.c:3302 kernel/workqueue.c:3385)<br /> <br /> ** Stack based on commit 66672af7a095 ("Add linux-next specific files<br /> for 20260410")<br /> <br /> I am seeing this crash sporadically in Meta fleet across multiple kernel<br /> versions. A full reproducer is available at:<br /> https://github.com/leitao/debug/blob/main/reproducers/repro_blkcg_uaf.sh<br /> <br /> (The race window is narrow. To make it easily reproducible, inject a<br /> msleep(100) between css_put() and blkcg_unpin_online() in<br /> cgwb_release_workfn(). With that delay and a KASAN-enabled kernel, the<br /> reproducer triggers the splat reliably in less than a second.)<br /> <br /> Fix this by moving blkcg_unpin_online() before css_put(), so the<br /> cgwb&amp;#39;s CSS reference keeps the blkcg alive while blkcg_unpin_online()<br /> accesses it.