netfilter: flowtable: validate vlan header

Ensure there is sufficient room to access the protocol field of the
VLAN header, validate it once before the flowtable lookup.

=====================================================
BUG: KMSAN: uninit-value in nf_flow_offload_inet_hook+0x45a/0x5f0 net/netfilter/nf_flow_table_inet.c:32
 nf_flow_offload_inet_hook+0x45a/0x5f0 net/netfilter/nf_flow_table_inet.c:32
 nf_hook_entry_hookfn include/linux/netfilter.h:154 [inline]
 nf_hook_slow+0xf4/0x400 net/netfilter/core.c:626
 nf_hook_ingress include/linux/netfilter_netdev.h:34 [inline]
 nf_ingress net/core/dev.c:5440 [inline]

Fixes: 4cd91f7 ("netfilter: flowtable: add vlan support")
Reported-by: syzbot+8407d9bb88cd4c6bf61a@syzkaller.appspotmail.com
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_inet.c

@@ -17,6 +17,9 @@ nf_flow_offload_inet_hook(void *priv, struct sk_buff *skb,
 
 	switch (skb->protocol) {
 	case htons(ETH_P_8021Q):
+		if (!pskb_may_pull(skb, skb_mac_offset(skb) + sizeof(*veth)))
+			return NF_ACCEPT;
+
 		veth = (struct vlan_ethhdr *)skb_mac_header(skb);
 		proto = veth->h_vlan_encapsulated_proto;
 		break;

net/netfilter/nf_flow_table_ip.c

@@ -281,6 +281,9 @@ static bool nf_flow_skb_encap_protocol(struct sk_buff *skb, __be16 proto,
 
 	switch (skb->protocol) {
 	case htons(ETH_P_8021Q):
+		if (!pskb_may_pull(skb, skb_mac_offset(skb) + sizeof(*veth)))
+			return false;
+
 		veth = (struct vlan_ethhdr *)skb_mac_header(skb);
 		if (veth->h_vlan_encapsulated_proto == proto) {
 			*offset += VLAN_HLEN;