netfilter: flowtable: validate pppoe header

Ensure there is sufficient room to access the protocol field of the
PPPoe header. Validate it once before the flowtable lookup, then use a
helper function to access protocol field.

Reported-by: syzbot+b6f07e1c07ef40199081@syzkaller.appspotmail.com
Fixes: 72efd585 ("netfilter: flowtable: add pppoe support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_flow_table.h

@@ -336,7 +336,7 @@ int nf_flow_rule_route_ipv6(struct net *net, struct flow_offload *flow,
 int nf_flow_table_offload_init(void);
 void nf_flow_table_offload_exit(void);
 
-static inline __be16 nf_flow_pppoe_proto(const struct sk_buff *skb)
+static inline __be16 __nf_flow_pppoe_proto(const struct sk_buff *skb)
 {
 	__be16 proto;
 
@@ -352,6 +352,16 @@ static inline __be16 nf_flow_pppoe_proto(const struct sk_buff *skb)
 	return 0;
 }
 
+static inline bool nf_flow_pppoe_proto(struct sk_buff *skb, __be16 *inner_proto)
+{
+	if (!pskb_may_pull(skb, PPPOE_SES_HLEN))
+		return false;
+
+	*inner_proto = __nf_flow_pppoe_proto(skb);
+
+	return true;
+}
+
 #define NF_FLOW_TABLE_STAT_INC(net, count) __this_cpu_inc((net)->ft.stat->count)
 #define NF_FLOW_TABLE_STAT_DEC(net, count) __this_cpu_dec((net)->ft.stat->count)
 #define NF_FLOW_TABLE_STAT_INC_ATOMIC(net, count)	\

net/netfilter/nf_flow_table_inet.c

@@ -21,7 +21,8 @@ nf_flow_offload_inet_hook(void *priv, struct sk_buff *skb,
 		proto = veth->h_vlan_encapsulated_proto;
 		break;
 	case htons(ETH_P_PPP_SES):
-		proto = nf_flow_pppoe_proto(skb);
+		if (!nf_flow_pppoe_proto(skb, &proto))
+			return NF_ACCEPT;
 		break;
 	default:
 		proto = skb->protocol;

net/netfilter/nf_flow_table_ip.c

@@ -273,10 +273,11 @@ static unsigned int nf_flow_xmit_xfrm(struct sk_buff *skb,
 	return NF_STOLEN;
 }
 
-static bool nf_flow_skb_encap_protocol(const struct sk_buff *skb, __be16 proto,
+static bool nf_flow_skb_encap_protocol(struct sk_buff *skb, __be16 proto,
 				       u32 *offset)
 {
 	struct vlan_ethhdr *veth;
+	__be16 inner_proto;
 
 	switch (skb->protocol) {
 	case htons(ETH_P_8021Q):
@@ -287,7 +288,8 @@ static bool nf_flow_skb_encap_protocol(const struct sk_buff *skb, __be16 proto,
 		}
 		break;
 	case htons(ETH_P_PPP_SES):
-		if (nf_flow_pppoe_proto(skb) == proto) {
+		if (nf_flow_pppoe_proto(skb, &inner_proto) &&
+		    inner_proto == proto) {
 			*offset += PPPOE_SES_HLEN;
 			return true;
 		}
@@ -316,7 +318,7 @@ static void nf_flow_encap_pop(struct sk_buff *skb,
 			skb_reset_network_header(skb);
 			break;
 		case htons(ETH_P_PPP_SES):
-			skb->protocol = nf_flow_pppoe_proto(skb);
+			skb->protocol = __nf_flow_pppoe_proto(skb);
 			skb_pull(skb, PPPOE_SES_HLEN);
 			skb_reset_network_header(skb);
 			break;