Re: [PATCH v2 2/2] nbd: convert to use blk_mq_get_rq_by_tag()

[Ming Lei]

On Mon, Aug 09, 2021 at 03:08:26PM +0800, yukuai (C) wrote:
> On 2021/08/09 14:28, Ming Lei wrote:
> > On Mon, Aug 09, 2021 at 11:09:27AM +0800, Yu Kuai wrote:
> > > blk_mq_tag_to_rq() might return freed request, use
> > > blk_mq_get_rq_by_tag() instead.
> > > 
> > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > ---
> > >   drivers/block/nbd.c | 11 ++++++-----
> > >   1 file changed, 6 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > > index c38317979f74..9e56975a8eee 100644
> > > --- a/drivers/block/nbd.c
> > > +++ b/drivers/block/nbd.c
> > > @@ -713,11 +713,10 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
> > >   	tag = nbd_handle_to_tag(handle);
> > >   	hwq = blk_mq_unique_tag_to_hwq(tag);
> > >   	if (hwq < nbd->tag_set.nr_hw_queues)
> > > -		req = blk_mq_tag_to_rq(nbd->tag_set.tags[hwq],
> > > -				       blk_mq_unique_tag_to_tag(tag));
> > > -	if (!req || !blk_mq_request_started(req)) {
> > > -		dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n",
> > > -			tag, req);
> > > +		req = blk_mq_get_rq_by_tag(nbd->tag_set.tags[hwq],
> > > +					   blk_mq_unique_tag_to_tag(tag));
> > > +	if (!req) {
> > > +		dev_err(disk_to_dev(nbd->disk), "Unexpected reply %d\n", tag);
> > >   		return ERR_PTR(-ENOENT);
> > >   	}
> > >   	trace_nbd_header_received(req, handle);
> > > @@ -779,6 +778,8 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
> > >   	}
> > >   out:
> > >   	trace_nbd_payload_received(req, handle);
> > > +	if (req)
> > > +		blk_mq_put_rq_ref(req);
> > >   	mutex_unlock(&cmd->lock);
> > >   	return ret ? ERR_PTR(ret) : cmd;
> > 
> > After blk_mq_put_rq_ref() returns, this request may have been freed,
> > so the returned 'cmd' may have been freed too.
> > 
> > As I replied in your another thread, it is driver's responsibility to
> > cover race between normal completion and timeout/error handling, that
> > means the caller of blk_mq_tag_to_rq need to make sure that the request
> > represented by the passed 'tag' can't be freed.
> 
> Hi, Ming
> 
> There are two problems here in nbd, both reported by our syzkaller.
> 
> The first is that blk_mq_tag_to_rq() returned a freed request, which is
> because tags->static_rq[] is freed without clearing tags->rq[].
> Syzkaller log shows that a reply package is sent to client without
> the client's request package. And this patch is trying to solve this
> problem.

It is still driver's problem:

->static_rq is freed in blk_mq_free_tag_set() which is called after
blk_cleanup_disk() returns. Once blk_cleanup_disk() returns, there
shouldn't be any driver activity, including calling blk_mq_tag_to_rq()
by passing one invalid tag.


Thanks, 
Ming