ipv4: fix use-after-free in ip_cmsg_recv_dstaddr()

Caching ip_hdr(skb) before a call to pskb_may_pull() is buggy,
do not do it.

Fixes: 2efd4fca ("ip: in cmsg IP(V6)_ORIGDSTADDR call pskb_may_pull")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Acked-by: NWillem de Bruijn <willemb@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_sockglue.c

@@ -149,7 +149,6 @@ static void ip_cmsg_recv_security(struct msghdr *msg, struct sk_buff *skb)
 static void ip_cmsg_recv_dstaddr(struct msghdr *msg, struct sk_buff *skb)
 {
 	struct sockaddr_in sin;
-	const struct iphdr *iph = ip_hdr(skb);
 	__be16 *ports;
 	int end;
 
@@ -164,7 +163,7 @@ static void ip_cmsg_recv_dstaddr(struct msghdr *msg, struct sk_buff *skb)
 	ports = (__be16 *)skb_transport_header(skb);
 
 	sin.sin_family = AF_INET;
-	sin.sin_addr.s_addr = iph->daddr;
+	sin.sin_addr.s_addr = ip_hdr(skb)->daddr;
 	sin.sin_port = ports[1];
 	memset(sin.sin_zero, 0, sizeof(sin.sin_zero));