Use-after-free in Linux kernel blk-cgroup cgwb_release_workfn()
CVE-2026-31586 is a Linux kernel memory-safety flaw in the mm/block cgroup writeback path, specifically in cgwb_release_workfn(). The bug occurs because cgwb_release_workfn() calls css_put(wb->blkcg_css) and subsequently accesses the same wb->blkcg_css object again through blkcg_unpin_online(). If css_put() drops the final reference, the blkcg CSS object may be freed asynchronously via css_free_rwork_fn() -> blkcg_css_free() -> kfree() before blkcg_unpin_online() dereferences it. This creates a race leading to a slab use-after-free in blkcg_unpin_online(). The issue was observed across multiple kernel versions, and a public reproducer was reported. The upstream fix reorders operations so blkcg_unpin_online() executes before css_put(), ensuring the CSS reference remains valid while the object is still being accessed.
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