[PATCH] mm: blk-cgroup: fix use-after-free in cgwb_release_workfn()

[Breno Leitao <leitao@debian.org>]

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.

Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished")
Cc: stable@vger.kernel.org
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 mm/backing-dev.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 7a18fa6c72725..cecbcf9060a65 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);

---
base-commit: 66672af7a095d89f082c5327f3b15bc2f93d558e
change-id: 20260413-blkcg-9b82762430f4

Best regards,
--  
Breno Leitao <leitao@debian.org>