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

["Coly Li" <colyli@fnnas.com>]

On Mon, Mar 30, 2026 at 09:11:53PM +0800, mingzhe.zou@easystack.cn wrote:
> 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>
> 

Hi Mingzhe,

Yeah, this patch is in better shape. Thank for the fix up again.

> ---
> 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));
>  

The patch itself is good. But the previous one (based on closre_sync()) is
in Jens block-tree. Let me ask.

Hi Jens,

Should Mingzhe send a incremental patch based on commit b36478a1fece in
block-7.0 branch of linux-block tree? Or Just use this patch to replace
the in-linux-block-tree one?

Thanks in advance for your hint.

Coly Li