netfilter: synproxy: fix BUG_ON triggered by corrupt TCP packets

TCP packets hitting the SYN proxy through the SYNPROXY target are not
validated by TCP conntrack. When th->doff is below 5, an underflow happens
when calculating the options length, causing skb_header_pointer() to
return NULL and triggering the BUG_ON().

Handle this case gracefully by checking for NULL instead of using BUG_ON().

Reported-by: Martin Topholm <[e-mail]>
Tested-by: Martin Topholm <[e-mail]>
Signed-off-by: Patrick McHardy <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_synproxy.h

···
 
 struct tcphdr;
 struct xt_synproxy_info;
-extern void synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
+extern bool synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
 				   const struct tcphdr *th,
 				   struct synproxy_options *opts);
 extern unsigned int synproxy_options_size(const struct synproxy_options *opts);

net/ipv4/netfilter/ipt_SYNPROXY.c

···
 	if (th == NULL)
 		return NF_DROP;
 
-	synproxy_parse_options(skb, par->thoff, th, &opts);
+	if (!synproxy_parse_options(skb, par->thoff, th, &opts))
+		return NF_DROP;
 
 	if (th->syn && !(th->ack || th->fin || th->rst)) {
 		/* Initial SYN from client */
···
 
 		/* fall through */
 	case TCP_CONNTRACK_SYN_SENT:
-		synproxy_parse_options(skb, thoff, th, &opts);
+		if (!synproxy_parse_options(skb, thoff, th, &opts))
+			return NF_DROP;
 
 		if (!th->syn && th->ack &&
 		    CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
···
 		if (!th->syn || !th->ack)
 			break;
 
-		synproxy_parse_options(skb, thoff, th, &opts);
+		if (!synproxy_parse_options(skb, thoff, th, &opts))
+			return NF_DROP;
+
 		if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 			synproxy->tsoff = opts.tsval - synproxy->its;
 

net/ipv6/netfilter/ip6t_SYNPROXY.c

···
 	if (th == NULL)
 		return NF_DROP;
 
-	synproxy_parse_options(skb, par->thoff, th, &opts);
+	if (!synproxy_parse_options(skb, par->thoff, th, &opts))
+		return NF_DROP;
 
 	if (th->syn && !(th->ack || th->fin || th->rst)) {
 		/* Initial SYN from client */
···
 
 		/* fall through */
 	case TCP_CONNTRACK_SYN_SENT:
-		synproxy_parse_options(skb, thoff, th, &opts);
+		if (!synproxy_parse_options(skb, thoff, th, &opts))
+			return NF_DROP;
 
 		if (!th->syn && th->ack &&
 		    CTINFO2DIR(ctinfo) == IP_CT_DIR_ORIGINAL) {
···
 		if (!th->syn || !th->ack)
 			break;
 
-		synproxy_parse_options(skb, thoff, th, &opts);
+		if (!synproxy_parse_options(skb, thoff, th, &opts))
+			return NF_DROP;
+
 		if (opts.options & XT_SYNPROXY_OPT_TIMESTAMP)
 			synproxy->tsoff = opts.tsval - synproxy->its;
 

net/netfilter/nf_synproxy_core.c

···
 int synproxy_net_id;
 EXPORT_SYMBOL_GPL(synproxy_net_id);
 
-void
+bool
 synproxy_parse_options(const struct sk_buff *skb, unsigned int doff,
 		       const struct tcphdr *th, struct synproxy_options *opts)
 {
···
 	u8 buf[40], *ptr;
 
 	ptr = skb_header_pointer(skb, doff + sizeof(*th), length, buf);
-	BUG_ON(ptr == NULL);
+	if (ptr == NULL)
+		return false;
 
 	opts->options = 0;
 	while (length > 0) {
···
 
 		switch (opcode) {
 		case TCPOPT_EOL:
-			return;
+			return true;
 		case TCPOPT_NOP:
 			length--;
 			continue;
 		default:
 			opsize = *ptr++;
 			if (opsize < 2)
-				return;
+				return true;
 			if (opsize > length)
-				return;
+				return true;
 
 			switch (opcode) {
 			case TCPOPT_MSS:
···
 			length -= opsize;
 		}
 	}
+	return true;
 }
 EXPORT_SYMBOL_GPL(synproxy_parse_options);