CVE-2026-31586
HighCVSS 7.8Exploitation Probability (EPSS)
Low risk7th percentile - higher than 7% of all known CVEs
Summary
W jądrze systemu Linux zidentyfikowano podatność, która dotyczy funkcji cgwb_release_workfn(). Problem polega na tym, że po wywołaniu css_put() następuje ponowny dostęp do wskaźnika blkcg_css, co może prowadzić do sytuacji use-after-free. Naprawa polega na zmianie kolejności wywołań, aby zapewnić, że blkcg pozostaje aktywny podczas dostępu do niego.
Risk Assessment
Podatność ta może prowadzić do niestabilności systemu, w tym do awarii jądra, co może wpłynąć na dostępność usług w organizacji. W przypadku wykorzystania tej luki przez złośliwego użytkownika, może to prowadzić do nieautoryzowanego dostępu do pamięci.
Recommendation
Zaleca się aktualizację jądra systemu Linux do wersji, która zawiera poprawkę dla tej podatności. Dodatkowo, monitorowanie systemu pod kątem nieprawidłowego działania oraz wdrożenie odpowiednich procedur bezpieczeństwa jest wskazane.
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Original NVD description (English source)
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.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

