dm: fix dm-raid crash if md_handle_request() splits bio

Commit ca522482e3eaf ("dm: pass NULL bdev to bio_alloc_clone")
introduced the optimization to _not_ perform bio_associate_blkg()'s
relatively costly work when DM core clones its bio. But in doing so it
exposed the possibility for DM's cloned bio to alter DM target
behavior (e.g. crash) if a target were to issue IO without first
calling bio_set_dev().

The DM raid target can trigger an MD crash due to its need to split
the DM bio that is passed to md_handle_request(). The split will
recurse to submit_bio_noacct() using a bio with an uninitialized
->bi_blkg. This NULL bio->bi_blkg causes blk_throtl_bio() to
dereference a NULL blkg_to_tg(bio->bi_blkg).

Fix this in DM core by adding a new 'needs_bio_set_dev' target flag that
will make alloc_tio() call bio_set_dev() on behalf of the target.
dm-raid is the only target that requires this flag. bio_set_dev()
initializes the DM cloned bio's ->bi_blkg, using bio_associate_blkg,
before passing the bio to md_handle_request().

Long-term fix would be to audit and refactor MD code to rely on DM to
split its bio, using dm_accept_partial_bio(), but there are MD raid
personalities (e.g. raid1 and raid10) whose implementation are tightly
coupled to handling the bio splitting inline.

Fixes: ca522482e3eaf ("dm: pass NULL bdev to bio_alloc_clone")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

1 files changed, 6 insertions, 7 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 47bcc5081b2b..f6a6437d0a7c 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -574,9 +574,6 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
 	struct bio *clone;
 
 	clone = bio_alloc_clone(NULL, bio, GFP_NOIO, &md->mempools->io_bs);
-	/* Set default bdev, but target must bio_set_dev() before issuing IO */
-	clone->bi_bdev = md->disk->part0;
-
 	tio = clone_to_tio(clone);
 	tio->flags = 0;
 	dm_tio_set_flag(tio, DM_TIO_INSIDE_DM_IO);
@@ -609,6 +606,7 @@ static void free_io(struct dm_io *io)
 static struct bio *alloc_tio(struct clone_info *ci, struct dm_target *ti,
 			     unsigned target_bio_nr, unsigned *len, gfp_t gfp_mask)
 {
+	struct mapped_device *md = ci->io->md;
 	struct dm_target_io *tio;
 	struct bio *clone;
 
@@ -618,14 +616,10 @@ static struct bio *alloc_tio(struct clone_info *ci, struct dm_target *ti,
 		/* alloc_io() already initialized embedded clone */
 		clone = &tio->clone;
 	} else {
-		struct mapped_device *md = ci->io->md;
-
 		clone = bio_alloc_clone(NULL, ci->bio, gfp_mask,
 					&md->mempools->bs);
 		if (!clone)
 			return NULL;
-		/* Set default bdev, but target must bio_set_dev() before issuing IO */
-		clone->bi_bdev = md->disk->part0;
 
 		/* REQ_DM_POLL_LIST shouldn't be inherited */
 		clone->bi_opf &= ~REQ_DM_POLL_LIST;
@@ -641,6 +635,11 @@ static struct bio *alloc_tio(struct clone_info *ci, struct dm_target *ti,
 	tio->len_ptr = len;
 	tio->old_sector = 0;
 
+	/* Set default bdev, but target must bio_set_dev() before issuing IO */
+	clone->bi_bdev = md->disk->part0;
+	if (unlikely(ti->needs_bio_set_dev))
+		bio_set_dev(clone, md->disk->part0);
+
 	if (len) {
 		clone->bi_iter.bi_size = to_bytes(*len);
 		if (bio_integrity(clone))