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

Commit ca522482e3 ("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: ca522482e3 ("dm: pass NULL bdev to bio_alloc_clone")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

include/linux/device-mapper.h

@@ -373,6 +373,12 @@ struct dm_target {
 	 * after returning DM_MAPIO_SUBMITTED from its map function.
 	 */
 	bool accounts_remapped_io:1;
+
+	/*
+	 * Set if the target will submit the DM bio without first calling
+	 * bio_set_dev(). NOTE: ideally a target should _not_ need this.
+	 */
+	bool needs_bio_set_dev:1;
 };
 
 void *dm_per_bio_data(struct bio *bio, size_t data_size);