ipv6: fix out-of-bound access in ip6_parse_tlv()

First problem is that optlen is fetched without checking
there is more than one byte to parse.

Fix this by taking care of IPV6_TLV_PAD1 before
fetching optlen (under appropriate sanity checks against len)

Second problem is that IPV6_TLV_PADN checks of zero
padding are performed before the check of remaining length.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Fixes: c1412fc ("net/ipv6/exthdrs.c: Strict PadN option checking")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

net/ipv6/exthdrs.c

@@ -135,18 +135,23 @@ static bool ip6_parse_tlv(const struct tlvtype_proc *procs,
 	len -= 2;
 
 	while (len > 0) {
-		int optlen = nh[off + 1] + 2;
-		int i;
+		int optlen, i;
 
-		switch (nh[off]) {
-		case IPV6_TLV_PAD1:
-			optlen = 1;
+		if (nh[off] == IPV6_TLV_PAD1) {
 			padlen++;
 			if (padlen > 7)
 				goto bad;
-			break;
+			off++;
+			len--;
+			continue;
+		}
+		if (len < 2)
+			goto bad;
+		optlen = nh[off + 1] + 2;
+		if (optlen > len)
+			goto bad;
 
-		case IPV6_TLV_PADN:
+		if (nh[off] == IPV6_TLV_PADN) {
 			/* RFC 2460 states that the purpose of PadN is
 			 * to align the containing header to multiples
 			 * of 8. 7 is therefore the highest valid value.
@@ -163,12 +168,7 @@ static bool ip6_parse_tlv(const struct tlvtype_proc *procs,
 				if (nh[off + i] != 0)
 					goto bad;
 			}
-			break;
-
-		default: /* Other TLV code so scan list */
-			if (optlen > len)
-				goto bad;
-
+		} else {
 			tlv_count++;
 			if (tlv_count > max_count)
 				goto bad;
@@ -188,7 +188,6 @@ static bool ip6_parse_tlv(const struct tlvtype_proc *procs,
 				return false;
 
 			padlen = 0;
-			break;
 		}
 		off += optlen;
 		len -= optlen;