block-crypto-fallback: use a bio_set for splitting bios

bio_split with a NULL bs argumen used to fall back to kmalloc the
bio, which does not guarantee forward progress and could to deadlocks.
Now that the overloading of the NULL bs argument to bio_alloc_bioset
has been removed it crashes instead.  Fix all that by using a special
crafted bioset.

Fixes: 3175199a ("block: split bio_kmalloc from bio_alloc_bioset")
Reported-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-crypto-fallback.c

@@ -80,6 +80,7 @@ static struct blk_crypto_keyslot {
 static struct blk_keyslot_manager blk_crypto_ksm;
 static struct workqueue_struct *blk_crypto_wq;
 static mempool_t *blk_crypto_bounce_page_pool;
+static struct bio_set crypto_bio_split;
 
 /*
  * This is the key we set when evicting a keyslot. This *should* be the all 0's
@@ -224,7 +225,8 @@ static bool blk_crypto_split_bio_if_needed(struct bio **bio_ptr)
 	if (num_sectors < bio_sectors(bio)) {
 		struct bio *split_bio;
 
-		split_bio = bio_split(bio, num_sectors, GFP_NOIO, NULL);
+		split_bio = bio_split(bio, num_sectors, GFP_NOIO,
+				      &crypto_bio_split);
 		if (!split_bio) {
 			bio->bi_status = BLK_STS_RESOURCE;
 			return false;
@@ -538,9 +540,13 @@ static int blk_crypto_fallback_init(void)
 
 	prandom_bytes(blank_key, BLK_CRYPTO_MAX_KEY_SIZE);
 
-	err = blk_ksm_init(&blk_crypto_ksm, blk_crypto_num_keyslots);
+	err = bioset_init(&crypto_bio_split, 64, 0, 0);
 	if (err)
 		goto out;
+
+	err = blk_ksm_init(&blk_crypto_ksm, blk_crypto_num_keyslots);
+	if (err)
+		goto fail_free_bioset;
 	err = -ENOMEM;
 
 	blk_crypto_ksm.ksm_ll_ops = blk_crypto_ksm_ll_ops;
@@ -591,6 +597,8 @@ static int blk_crypto_fallback_init(void)
 	destroy_workqueue(blk_crypto_wq);
 fail_free_ksm:
 	blk_ksm_destroy(&blk_crypto_ksm);
+fail_free_bioset:
+	bioset_exit(&crypto_bio_split);
 out:
 	return err;
 }