[PATCH] udf: reject descriptors with oversized CRC length

[Michael Bommarito <michael.bommarito@gmail.com>]

udf_read_tagged() skips CRC verification when descCRCLength +
sizeof(struct tag) exceeds the block size.  A crafted UDF image can
set descCRCLength to an oversized value to bypass CRC validation
entirely; the descriptor is then accepted based solely on the 8-bit
tag checksum, which is trivially recomputable.

Reject such descriptors instead of silently accepting them.  A
legitimate single-block descriptor should never have a CRC length that
exceeds the block.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
Found during a filesystem security audit.  The CRC validation
condition in udf_read_tagged() uses OR logic: the first arm
(descCRCLength too large) short-circuits the second arm (CRC
comparison), so an oversized descCRCLength causes the function to
return the buffer head without verifying the CRC.  The descriptor
is accepted based solely on the 8-bit tag checksum.

A crafted UDF image with descCRCLength set to blocksize (e.g. 2048
on a 2048-byte-block filesystem, vs the 2032 limit) in both the
main and reserve Volume Descriptor Sequences mounts successfully
with corrupted descriptor bodies.

Reproduced on UML (ARCH=um, KASAN-enabled v7.0-rc7) with a
mkudffs-generated 20 MiB image, both LVD copies patched to
descCRCLength=2040, CRC left stale, body byte flipped, tag
checksum recomputed.  Mount succeeds (MOUNT=0) with the corrupt
LVD accepted.  With this patch applied, mount fails with EINVAL
and the new "CRC length ... exceeds block size" error is logged.

Reproducer details and UML console logs available on request.

 fs/udf/misc.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/fs/udf/misc.c b/fs/udf/misc.c
index 0788593b6a1d..6928e378fbbd 100644
--- a/fs/udf/misc.c
+++ b/fs/udf/misc.c
@@ -230,8 +230,12 @@ struct buffer_head *udf_read_tagged(struct super_block *sb, uint32_t block,
 	}
 
 	/* Verify the descriptor CRC */
-	if (le16_to_cpu(tag_p->descCRCLength) + sizeof(struct tag) > sb->s_blocksize ||
-	    le16_to_cpu(tag_p->descCRC) == crc_itu_t(0,
+	if (le16_to_cpu(tag_p->descCRCLength) + sizeof(struct tag) > sb->s_blocksize) {
+		udf_err(sb, "block %u: CRC length %u exceeds block size\n",
+			block, le16_to_cpu(tag_p->descCRCLength));
+		goto error_out;
+	}
+	if (le16_to_cpu(tag_p->descCRC) == crc_itu_t(0,
 					bh->b_data + sizeof(struct tag),
 					le16_to_cpu(tag_p->descCRCLength)))
 		return bh;
-- 
2.53.0