tftp: don't implicity trust the format of recevied packets

The TFTP OACK code trusts that the incoming packet is formated as
ASCII text and can be processed by string functions. It also has a
loop limit overflow bug where if the packet length is less than 8, it
ends up looping over *all* of memory to find the 'blksize' string.

This patch solves the problem by forcing the packet to be null
terminated and using strstr() to search for the sub string.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

net/tftp.c

@@ -238,9 +238,9 @@ TftpSend (void)
 static void
 TftpHandler (uchar * pkt, unsigned dest, unsigned src, unsigned len)
 {
+	char * blksize;
 	ushort proto;
 	ushort *s;
-	int i;
 
 	if (dest != TftpOurPort) {
 #ifdef CONFIG_MCAST_TFTP
@@ -272,22 +272,22 @@ TftpHandler (uchar * pkt, unsigned dest, unsigned src, unsigned len)
 
 	case TFTP_OACK:
 #ifdef ET_DEBUG
-		printf("Got OACK: %s %s\n", pkt, pkt+strlen(pkt)+1);
+		printf("Got OACK:\n");
+		print_buffer (0, pkt, 1, len, 16);
 #endif
 		TftpState = STATE_OACK;
 		TftpServerPort = src;
+
 		/* Check for 'blksize' option */
-		for (i=0;i<len-8;i++) {
-			if (strcmp ((char*)pkt+i,"blksize") == 0) {
-				TftpBlkSize = (unsigned short)
-					simple_strtoul((char*)pkt+i+8,NULL,10);
+		pkt[len] = 0; /* NULL terminate so string ops work */
+		blksize = strstr((char*)pkt, "blksize");
+		if ((blksize) && (blksize + 8 < (char*)pkt + len)) {
+			TftpBlkSize = simple_strtoul(blksize + 8, NULL, 10);
 #ifdef ET_DEBUG
-				printf ("Blocksize ack: %s, %d\n",
-					(char*)pkt+i+8,TftpBlkSize);
+			printf("Blocksize ack: %d\n", TftpBlkSize);
 #endif
-				break;
-			}
 		}
+
 #ifdef CONFIG_MCAST_TFTP
 		parse_multicast_oack((char *)pkt,len-1);
 		if ((Multicast) && (!MasterClient))