[PATCH 6.18.y 4/4] dm-verity: fix buffer overflow in FEC calculation

[Sasha Levin <sashal@kernel.org>]

From: Mikulas Patocka <mpatocka@redhat.com>

[ Upstream commit 31d6e6c0ba8d5a7bd59660035a089307100c5e8e ]

There's a buffer overflow in dm-verity-fec:

if (neras && *neras <= v->fec->roots)
	fio->erasures[(*neras)++] = i;

This allows *neras to reach roots + 1 (the post-increment pushes it past
roots). This value is then passed as no_eras to decode_rs8(). Inside the
RS decoder (lib/reed_solomon/decode_rs.c:113-121), the erasure locator
polynomial loop writes lambda[j] where j can reach nroots + 1 — one
element past the end of lambda[] (which is sized nroots + 1, valid
indices 0..nroots). The out-of-bounds write lands on syn[0], corrupting
the syndrome buffer.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4-6
Cc: stable@vger.kernel.org
Fixes: a739ff3f543a ("dm verity: add support for forward error correction")
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/md/dm-verity-fec.c | 4 ++--
 drivers/md/dm-verity-fec.h | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-verity-fec.c b/drivers/md/dm-verity-fec.c
index cebcc8fd25d700..d1e4fbc5a21d9b 100644
--- a/drivers/md/dm-verity-fec.c
+++ b/drivers/md/dm-verity-fec.c
@@ -250,7 +250,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 				     (unsigned long long)block, PTR_ERR(bbuf));
 
 			/* assume the block is corrupted */
-			if (neras && *neras <= v->fec->roots)
+			if (neras && *neras < v->fec->roots)
 				fio->erasures[(*neras)++] = i;
 
 			continue;
@@ -268,7 +268,7 @@ static int fec_read_bufs(struct dm_verity *v, struct dm_verity_io *io,
 			 * skip if we have already found the theoretical
 			 * maximum number (i.e. fec->roots) of erasures
 			 */
-			if (neras && *neras <= v->fec->roots &&
+			if (neras && *neras < v->fec->roots &&
 			    fec_is_erasure(v, io, want_digest, bbuf))
 				fio->erasures[(*neras)++] = i;
 		}
diff --git a/drivers/md/dm-verity-fec.h b/drivers/md/dm-verity-fec.h
index 178ecdddbbd310..c19e2d20657284 100644
--- a/drivers/md/dm-verity-fec.h
+++ b/drivers/md/dm-verity-fec.h
@@ -50,7 +50,7 @@ struct dm_verity_fec {
 /* per-bio data */
 struct dm_verity_fec_io {
 	struct rs_control *rs;	/* Reed-Solomon state */
-	int erasures[DM_VERITY_FEC_MAX_ROOTS + 1]; /* erasures for decode_rs8 */
+	int erasures[DM_VERITY_FEC_MAX_ROOTS]; /* erasures for decode_rs8 */
 	u8 *output;		/* buffer for corrected output */
 	unsigned int level;		/* recursion level */
 	unsigned int nbufs;		/* number of buffers allocated */
-- 
2.53.0