Commit 8f5857be99f1 for kernel

commit 8f5857be99f1ed1fa80991c72449541f634626ee
Author: Breno Leitao <leitao@debian.org>
Date:   Mon Apr 13 03:09:19 2026 -0700

    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.

    Link: https://lore.kernel.org/20260413-blkcg-v1-1-35b72622d16c@debian.org
    Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished")
    Signed-off-by: Breno Leitao <leitao@debian.org>
    Reviewed-by: Dennis Zhou <dennis@kernel.org>
    Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
    Cc: David Hildenbrand <david@kernel.org>
    Cc: Jens Axboe <axboe@kernel.dk>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Josef Bacik <josef@toxicpanda.com>
    Cc: JP Kobryn <inwardvessel@gmail.com>
    Cc: Liam Howlett <liam.howlett@oracle.com>
    Cc: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
    Cc: Martin KaFai Lau <martin.lau@linux.dev>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Mike Rapoport <rppt@kernel.org>
    Cc: Suren Baghdasaryan <surenb@google.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7a18fa6c7272..cecbcf9060a6 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -618,12 +618,13 @@ static void cgwb_release_workfn(struct work_struct *work)
 	wb_shutdown(wb);

 	css_put(wb->memcg_css);
-	css_put(wb->blkcg_css);
-	mutex_unlock(&wb->bdi->cgwb_release_mutex);

 	/* triggers blkg destruction if no online users left */
 	blkcg_unpin_online(wb->blkcg_css);

+	css_put(wb->blkcg_css);
+	mutex_unlock(&wb->bdi->cgwb_release_mutex);
+
 	fprop_local_destroy_percpu(&wb->memcg_completions);

 	spin_lock_irq(&cgwb_lock);