cifs: sanitize length checking in coalesce_t2 (try #3)

There are a couple of places in this code where these values can wrap or
go negative, and that could potentially end up overflowing the buffer.
Ensure that that doesn't happen. Do all of the length calculation and
checks first, and only perform the memcpy after they pass.

Also, increase some stack variables to 32 bits to ensure that they don't
wrap without being detected.

Finally, change the error codes to be a bit more descriptive of any
problems detected. -EINVAL isn't very accurate.

Cc: [e-mail]
Reported-and-Acked-by: David Howells <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/connect.c

···
 	char *data_area_of_target;
 	char *data_area_of_buf2;
 	int remaining;
-	__u16 byte_count, total_data_size, total_in_buf, total_in_buf2;
+	unsigned int byte_count, total_in_buf;
+	__u16 total_data_size, total_in_buf2;
 
 	total_data_size = get_unaligned_le16(&pSMBt->t2_rsp.TotalDataCount);
 
···
 	remaining = total_data_size - total_in_buf;
 
 	if (remaining < 0)
-		return -EINVAL;
+		return -EPROTO;
 
 	if (remaining == 0) /* nothing to do, ignore */
 		return 0;
···
 	data_area_of_target += total_in_buf;
 
 	/* copy second buffer into end of first buffer */
-	memcpy(data_area_of_target, data_area_of_buf2, total_in_buf2);
 	total_in_buf += total_in_buf2;
+	/* is the result too big for the field? */
+	if (total_in_buf > USHRT_MAX)
+		return -EPROTO;
 	put_unaligned_le16(total_in_buf, &pSMBt->t2_rsp.DataCount);
+
+	/* fix up the BCC */
 	byte_count = get_bcc_le(pTargetSMB);
 	byte_count += total_in_buf2;
+	/* is the result too big for the field? */
+	if (byte_count > USHRT_MAX)
+		return -EPROTO;
 	put_bcc_le(byte_count, pTargetSMB);
 
 	byte_count = pTargetSMB->smb_buf_length;
 	byte_count += total_in_buf2;
-
-	/* BB also add check that we are not beyond maximum buffer size */
-
+	/* don't allow buffer to overflow */
+	if (byte_count > CIFSMaxBufSize)
+		return -ENOBUFS;
 	pTargetSMB->smb_buf_length = byte_count;
+
+	memcpy(data_area_of_target, data_area_of_buf2, total_in_buf2);
 
 	if (remaining == total_in_buf2) {
 		cFYI(1, "found the last secondary response");