[PATCH v2 3/3] ecryptfs: reject too-small tag 70 packets

ecryptfs_parse_tag_70_packet() subtracts fixed metadata fields from the
parsed packet body size to derive the encrypted filename size.  A
malformed packet with a body smaller than those fixed fields can underflow
that size calculation.

Reject tag 70 packets before the subtraction unless the body contains the
signature, cipher code, and at least one byte of encrypted filename data.

Fixes: 9c79f34f7ee7 ("eCryptfs: Filename Encryption: Tag 70 packets")
Signed-off-by: Yichong Chen <[email protected]>
---
 fs/ecryptfs/keystore.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/ecryptfs/keystore.c b/fs/ecryptfs/keystore.c
index 02b5d77..a525a8c 100644
--- a/fs/ecryptfs/keystore.c
+++ b/fs/ecryptfs/keystore.c
@@ -894,6 +894,12 @@ ecryptfs_parse_tag_70_packet(char **filename, size_t *filename_size,
 		       "rc = [%d]\n", __func__, rc);
 		goto out;
 	}
+	if (s->parsed_tag_70_packet_size < (ECRYPTFS_SIG_SIZE + 2)) {
+		ecryptfs_printk(KERN_WARNING, "Invalid packet size [%zd]\n",
+				s->parsed_tag_70_packet_size);
+		rc = -EINVAL;
+		goto out;
+	}
 	s->block_aligned_filename_size = (s->parsed_tag_70_packet_size
 					  - ECRYPTFS_SIG_SIZE - 1);
 	if ((1 + s->packet_size_len + s->parsed_tag_70_packet_size)
-- 
2.51.0