netfilter: nf_reject_ipv4: Fix use-after-free in send_reset

niph is not updated after pskb_expand_head changes the skb head. It
still points to the freed data, which is then used to update tot_len and
checksum. This could cause use-after-free poison crash.

Update niph, if ip_route_me_harder does not fail.

This only affects the interaction with REJECT targets and br_netfilter.

Signed-off-by: Tejaswi Tanikella <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/ipv4/netfilter/nf_reject_ipv4.c

···
 	if (ip_route_me_harder(net, nskb, RTN_UNSPEC))
 		goto free_nskb;
 
+	niph = ip_hdr(nskb);
+
 	/* "Never happens" */
 	if (nskb->len > dst_mtu(skb_dst(nskb)))
 		goto free_nskb;