Code:  int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 {
	u16 offset = sizeof(struct ipv6hdr);
 	unsigned int packet_len = skb_tail_pointer(skb) -
 		skb_network_header(skb);
 	int found_rhdr = 0;
 	*nexthdr = &ipv6_hdr(skb)->nexthdr;
 
 	while (offset <= packet_len) {
 		struct ipv6_opt_hdr *exthdr;
 
 		switch (**nexthdr) {
 
		case NEXTHDR_HOP:
			break;
		case NEXTHDR_ROUTING:
			found_rhdr = 1;
			break;
		case NEXTHDR_DEST:
#if IS_ENABLED(CONFIG_IPV6_MIP6)
			if (ipv6_find_tlv(skb, offset, IPV6_TLV_HAO) >= 0)
				break;
#endif
			if (found_rhdr)
				return offset;
			break;
		default:
			return offset;
		}

		if (offset + sizeof(struct ipv6_opt_hdr) > packet_len)
			return -EINVAL;
 
 		exthdr = (struct ipv6_opt_hdr *)(skb_network_header(skb) +
 						 offset);
		offset += ipv6_optlen(exthdr);
 		*nexthdr = &exthdr->nexthdr;
 	}
 
	return -EINVAL;
}

Commit Message: ipv6: avoid overflow of offset in ip6_find_1stfragopt

In some cases, offset can overflow and can cause an infinite loop in
ip6_find_1stfragopt(). Make it unsigned int to prevent the overflow, and
cap it at IPV6_MAXPLEN, since packets larger than that should be invalid.

This problem has been here since before the beginning of git history.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
CWE ID: CWE-190
 int ip6_find_1stfragopt(struct sk_buff *skb, u8 **nexthdr)
 {
	unsigned int offset = sizeof(struct ipv6hdr);
 	unsigned int packet_len = skb_tail_pointer(skb) -
 		skb_network_header(skb);
 	int found_rhdr = 0;
 	*nexthdr = &ipv6_hdr(skb)->nexthdr;
 
 	while (offset <= packet_len) {
 		struct ipv6_opt_hdr *exthdr;
		unsigned int len;
 
 		switch (**nexthdr) {
 
		case NEXTHDR_HOP:
			break;
		case NEXTHDR_ROUTING:
			found_rhdr = 1;
			break;
		case NEXTHDR_DEST:
#if IS_ENABLED(CONFIG_IPV6_MIP6)
			if (ipv6_find_tlv(skb, offset, IPV6_TLV_HAO) >= 0)
				break;
#endif
			if (found_rhdr)
				return offset;
			break;
		default:
			return offset;
		}

		if (offset + sizeof(struct ipv6_opt_hdr) > packet_len)
			return -EINVAL;
 
 		exthdr = (struct ipv6_opt_hdr *)(skb_network_header(skb) +
 						 offset);
		len = ipv6_optlen(exthdr);
		if (len + offset >= IPV6_MAXPLEN)
			return -EINVAL;
		offset += len;
 		*nexthdr = &exthdr->nexthdr;
 	}
 
	return -EINVAL;
}
