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 7a18fa6c727251f78cf80354d4595af28f865239..cecbcf9060a653267b60b8a9b290e9e4f84faad9 100644 (file)
--- 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);