[PATCH v3] bcache: fix cached_dev.sb_bio use-after-free and crash

[mingzhe.zou@easystack.cn]

From: Mingzhe Zou <mingzhe.zou@easystack.cn>

In our production environment, we have received multiple crash reports
regarding libceph, which have caught our attention:

```
[6888366.280350] Call Trace:
[6888366.280452]  blk_update_request+0x14e/0x370
[6888366.280561]  blk_mq_end_request+0x1a/0x130
[6888366.280671]  rbd_img_handle_request+0x1a0/0x1b0 [rbd]
[6888366.280792]  rbd_obj_handle_request+0x32/0x40 [rbd]
[6888366.280903]  __complete_request+0x22/0x70 [libceph]
[6888366.281032]  osd_dispatch+0x15e/0xb40 [libceph]
[6888366.281164]  ? inet_recvmsg+0x5b/0xd0
[6888366.281272]  ? ceph_tcp_recvmsg+0x6f/0xa0 [libceph]
[6888366.281405]  ceph_con_process_message+0x79/0x140 [libceph]
[6888366.281534]  ceph_con_v1_try_read+0x5d7/0xf30 [libceph]
[6888366.281661]  ceph_con_workfn+0x329/0x680 [libceph]
```

After analyzing the coredump file, we found that the address of dc->sb_bio
has been freed. We know that cached_dev is only freed when it is stopped.

Since sb_bio is a part of struct cached_dev, rather than an alloc every time.
If the device is stopped while writing to the superblock, the released address
will be accessed at endio.

This patch hopes to wait for sb_write to complete in cached_dev_free.

Signed-off-by: Mingzhe Zou <mingzhe.zou@easystack.cn>

---
v2: fix the crash caused by not calling closure_init in v1
v3: pair the down and up of semaphore sb_write_mutex
---
 drivers/md/bcache/super.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 64bb38c95895..97d9adb0bf96 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1373,6 +1373,14 @@ static CLOSURE_CALLBACK(cached_dev_free)
 
 	mutex_unlock(&bch_register_lock);
 
+	/*
+	 * Wait for any pending sb_write to complete before free.
+	 * The sb_bio is embedded in struct cached_dev, so we must
+	 * ensure no I/O is in progress.
+	 */
+	down(&dc->sb_write_mutex);
+	up(&dc->sb_write_mutex);
+
 	if (dc->sb_disk)
 		folio_put(virt_to_folio(dc->sb_disk));
 
-- 
2.34.1