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

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

On Tue, Mar 31, 2026 at 09:56:18AM +0800, Coly Li wrote:
> 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?
> 

Hi Mingzhe,

Considering no responds from Jens at this moment, I assume that an incremental
patch againt the already-in-linux-block patch might be more convenient for him.
Then he can simply apply the incremental patch without extra rebase stuffs.

Can you re-compose a patch against block-7.0 branch of linux-block tree?

Thanks.

Coly Li