[CIFS] Workaround MacOS server problem with SMB2.1 write

 response

Writes fail to Mac servers with SMB2.1 mounts (works with cifs though) due
to them sending an incorrect RFC1001 length for the SMB2.1 Write response.
Workaround this problem. MacOS server sends a write response with 3 bytes
of pad beyond the end of the SMB itself.  The RFC1001 length is 3 bytes
more than the sum of the SMB2.1 header length + the write reponse.

Incorporate feedback from Jeff and JRA to allow servers to send
a tcp frame that is even more than three bytes too long
(ie much longer than the SMB2/SMB3 request that it contains) but
we do log it once now. In the earlier version of the patch I had
limited how far off the length field could be before we fail the request.
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/smb2misc.c

@@ -178,9 +178,24 @@ smb2_check_message(char *buf, unsigned int length)
 		/* Windows 7 server returns 24 bytes more */
 		if (clc_len + 20 == len && command == SMB2_OPLOCK_BREAK_HE)
 			return 0;
-		/* server can return one byte more */
+		/* server can return one byte more due to implied bcc[0] */
 		if (clc_len == 4 + len + 1)
 			return 0;
+
+		/*
+		 * MacOS server pads after SMB2.1 write response with 3 bytes
+		 * of junk. Other servers match RFC1001 len to actual
+		 * SMB2/SMB3 frame length (header + smb2 response specific data)
+		 * Log the server error (once), but allow it and continue
+		 * since the frame is parseable.
+		 */
+		if (clc_len < 4 /* RFC1001 header size */ + len) {
+			printk_once(KERN_WARNING
+				"SMB2 server sent bad RFC1001 len %d not %d\n",
+				len, clc_len - 4);
+			return 0;
+		}
+
 		return 1;
 	}
 	return 0;