[dm-devel] dm: make sure dm_table is binded before queue request

[Mike Snitzer]

On Wed, Feb 09 2022 at  4:37P -0500,
Zhang Yi <yi.zhang at huawei.com> wrote:

> We found a NULL pointer dereference problem when using dm-mpath target.
> The problem is if we submit IO between loading and binding the table,
> we could neither get a valid dm_target nor a valid dm table when
> submitting request in dm_mq_queue_rq(). BIO based dm target could
> handle this case in dm_submit_bio(). This patch fix this by checking
> the mapping table before submitting request.
> 
> Signed-off-by: Zhang Yi <yi.zhang at huawei.com>
> ---
>  drivers/md/dm-rq.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> index 579ab6183d4d..af2cf71519e9 100644
> --- a/drivers/md/dm-rq.c
> +++ b/drivers/md/dm-rq.c
> @@ -499,8 +499,15 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
>  
>  	if (unlikely(!ti)) {
>  		int srcu_idx;
> -		struct dm_table *map = dm_get_live_table(md, &srcu_idx);
> -
> +		struct dm_table *map;
> +
> +		map = dm_get_live_table(md, &srcu_idx);
> +		if (!map) {
> +			DMERR_LIMIT("%s: mapping table unavailable, erroring io",
> +				    dm_device_name(md));
> +			dm_put_live_table(md, srcu_idx);
> +			return BLK_STS_IOERR;
> +		}
>  		ti = dm_table_find_target(map, 0);
>  		dm_put_live_table(md, srcu_idx);
>  	}
> -- 
> 2.31.1
> 

I think both dm_submit_bio() and now dm_mq_queue_rq() should _not_
error the IO.  This is such a narrow race during device setup that it
best to requeue the IO.

I'll queue this for 5.18:

diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index 6948d5db9092..3dd040a56318 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -491,8 +491,13 @@ static blk_status_t dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
 
 	if (unlikely(!ti)) {
 		int srcu_idx;
-		struct dm_table *map = dm_get_live_table(md, &srcu_idx);
+		struct dm_table *map;
 
+		map = dm_get_live_table(md, &srcu_idx);
+		if (unlikely(!map)) {
+			dm_put_live_table(md, srcu_idx);
+			return BLK_STS_RESOURCE;
+		}
 		ti = dm_table_find_target(map, 0);
 		dm_put_live_table(md, srcu_idx);
 	}
diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 082366d0ad49..c70be6e5ed55 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1533,15 +1533,10 @@ static void dm_submit_bio(struct bio *bio)
 	struct dm_table *map;
 
 	map = dm_get_live_table(md, &srcu_idx);
-	if (unlikely(!map)) {
-		DMERR_LIMIT("%s: mapping table unavailable, erroring io",
-			    dm_device_name(md));
-		bio_io_error(bio);
-		goto out;
-	}
 
-	/* If suspended, queue this IO for later */
-	if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags))) {
+	/* If suspended, or map not yet available, queue this IO for later */
+	if (unlikely(test_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags)) ||
+	    unlikely(!map)) {
 		if (bio->bi_opf & REQ_NOWAIT)
 			bio_wouldblock_error(bio);
 		else if (bio->bi_opf & REQ_RAHEAD)