ksmbd: use buf_data_size instead of recalculation in smb3_decrypt_req()

Tom suggested to use buf_data_size that is already calculated, to verify
these offsets.

Cc: Tom Talpey <tom@talpey.com>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Ralph Böhme <slow@samba.org>
Suggested-by: Tom Talpey <tom@talpey.com>
Acked-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/ksmbd/smb2pdu.c

@@ -8395,20 +8395,18 @@ int smb3_decrypt_req(struct ksmbd_work *work)
 	struct smb2_hdr *hdr;
 	unsigned int pdu_length = get_rfc1002_len(buf);
 	struct kvec iov[2];
-	unsigned int buf_data_size = pdu_length + 4 -
+	int buf_data_size = pdu_length + 4 -
 		sizeof(struct smb2_transform_hdr);
 	struct smb2_transform_hdr *tr_hdr = (struct smb2_transform_hdr *)buf;
 	int rc = 0;
 
-	if (pdu_length + 4 <
-	    sizeof(struct smb2_transform_hdr) + sizeof(struct smb2_hdr)) {
+	if (buf_data_size < sizeof(struct smb2_hdr)) {
 		pr_err("Transform message is too small (%u)\n",
 		       pdu_length);
 		return -ECONNABORTED;
 	}
 
-	if (pdu_length + 4 <
-	    le32_to_cpu(tr_hdr->OriginalMessageSize) + sizeof(struct smb2_transform_hdr)) {
+	if (buf_data_size < le32_to_cpu(tr_hdr->OriginalMessageSize)) {
 		pr_err("Transform message is broken\n");
 		return -ECONNABORTED;
 	}