cifs: read overflow in is_valid_oplock_break()

We need to verify that the "data_offset" is within bounds.
Reported-by: NDr Silvio Cesare of InfoSect <silvio.cesare@gmail.com>
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NSteve French <stfrench@microsoft.com>
Reviewed-by: NAurelien Aptel <aaptel@suse.com>

fs/cifs/misc.c

@@ -402,9 +402,17 @@ is_valid_oplock_break(char *buffer, struct TCP_Server_Info *srv)
 			(struct smb_com_transaction_change_notify_rsp *)buf;
 		struct file_notify_information *pnotify;
 		__u32 data_offset = 0;
+		size_t len = srv->total_read - sizeof(pSMBr->hdr.smb_buf_length);
+
 		if (get_bcc(buf) > sizeof(struct file_notify_information)) {
 			data_offset = le32_to_cpu(pSMBr->DataOffset);
 
+			if (data_offset >
+			    len - sizeof(struct file_notify_information)) {
+				cifs_dbg(FYI, "invalid data_offset %u\n",
+					 data_offset);
+				return true;
+			}
 			pnotify = (struct file_notify_information *)
 				((char *)&pSMBr->hdr.Protocol + data_offset);
 			cifs_dbg(FYI, "dnotify on %s Action: 0x%x\n",