[PATCH 5.15.y] dm-integrity: fix infinite loop on discard with large tag size

[Sasha Levin <sashal@kernel.org>]

From: Ben Cressey <ben@cressey.dev>

[ Upstream commit 18d80c77b4c7dd20699e81cedfbbff4e9d198f28 ]

When integrity_metadata handles a discard, it fills a buffer with
DISCARD_FILLER and writes it over the tags, max_blocks blocks at a
time. If the kmalloc fails, the buffer is the on-stack array
checksums_onstack and max_size is set to HASH_MAX_DIGESTSIZE. So if the
tag size is larger than HASH_MAX_DIGESTSIZE, max_blocks is zero, bi_size
is never decremented and the loop never terminates.

Fix this by using sizeof(checksums_onstack) as max_size. The array has
MAX_TAG_SIZE bytes since commit b93b6643e9b5 ("dm integrity: fix a
crash with unusually large tag size"), so max_blocks is at least 1.

Fixes: 84597a44a9d8 ("dm integrity: add optional discard support")
Cc: stable@vger.kernel.org
Reviewed-by: Jose Fernandez (Anthropic) <jose.fernandez@linux.dev>
Signed-off-by: Ben Cressey <ben@cressey.dev>
Assisted-by: Claude:unspecified
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
[ sashal: Adjusted context for filler-only discard handling that calculates max_blocks without subtracting extra_space. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/md/dm-integrity.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/md/dm-integrity.c b/drivers/md/dm-integrity.c
index cebffb0b5a1af..5c81b57a457e4 100644
--- a/drivers/md/dm-integrity.c
+++ b/drivers/md/dm-integrity.c
@@ -1725,7 +1725,7 @@ static void integrity_metadata(struct work_struct *w)
 		if (unlikely(dio->op == REQ_OP_DISCARD)) {
 			sector_t bi_sector = dio->bio_details.bi_iter.bi_sector;
 			unsigned bi_size = dio->bio_details.bi_iter.bi_size;
-			unsigned max_size = likely(checksums != checksums_onstack) ? PAGE_SIZE : HASH_MAX_DIGESTSIZE;
+			unsigned max_size = likely(checksums != checksums_onstack) ? PAGE_SIZE : sizeof(checksums_onstack);
 			unsigned max_blocks = max_size / ic->tag_size;
 			memset(checksums, DISCARD_FILLER, max_size);
 
-- 
2.53.0