netfilter: nft_exthdr: fix offset with ipv4_find_option()

There is an incorrect calculation in the offset variable which causes
the nft_skb_copy_to_reg() function to always return -EFAULT. Adding the
start variable is redundant. In the __ip_options_compile() function the
correct offset is specified when finding the function. There is no need
to add the size of the iphdr structure to the offset.

Fixes: dbb5281 ("netfilter: nf_tables: add support for matching IPv4 options")
Signed-off-by: Alexey Kashavkin <akashavkin@gmail.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Author: amkgi

net/netfilter/nft_exthdr.c

@@ -85,15 +85,13 @@ static int ipv4_find_option(struct net *net, struct sk_buff *skb,
 	unsigned char optbuf[sizeof(struct ip_options) + 40];
 	struct ip_options *opt = (struct ip_options *)optbuf;
 	struct iphdr *iph, _iph;
-	unsigned int start;
 	bool found = false;
 	__be32 info;
 	int optlen;
 
 	iph = skb_header_pointer(skb, 0, sizeof(_iph), &_iph);
 	if (!iph)
 		return -EBADMSG;
-	start = sizeof(struct iphdr);
 
 	optlen = iph->ihl * 4 - (int)sizeof(struct iphdr);
 	if (optlen <= 0)
@@ -103,7 +101,7 @@ static int ipv4_find_option(struct net *net, struct sk_buff *skb,
 	/* Copy the options since __ip_options_compile() modifies
 	 * the options.
 	 */
-	if (skb_copy_bits(skb, start, opt->__data, optlen))
+	if (skb_copy_bits(skb, sizeof(struct iphdr), opt->__data, optlen))
 		return -EBADMSG;
 	opt->optlen = optlen;
 
@@ -118,18 +116,18 @@ static int ipv4_find_option(struct net *net, struct sk_buff *skb,
 		found = target == IPOPT_SSRR ? opt->is_strictroute :
 					       !opt->is_strictroute;
 		if (found)
-			*offset = opt->srr + start;
+			*offset = opt->srr;
 		break;
 	case IPOPT_RR:
 		if (!opt->rr)
 			break;
-		*offset = opt->rr + start;
+		*offset = opt->rr;
 		found = true;
 		break;
 	case IPOPT_RA:
 		if (!opt->router_alert)
 			break;
-		*offset = opt->router_alert + start;
+		*offset = opt->router_alert;
 		found = true;
 		break;
 	default: