ipv6: xfrm: Handle errors reported by xfrm6_find_1stfragopt()

xfrm6_find_1stfragopt() may now return an error code and we must
not treat it as a length.

Fixes: 2423496 ("ipv6: Prevent overrun when parsing v6 header options")
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Craig Gallek <kraig@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/xfrm6_mode_ro.c

@@ -47,6 +47,8 @@ static int xfrm6_ro_output(struct xfrm_state *x, struct sk_buff *skb)
 	iph = ipv6_hdr(skb);
 
 	hdr_len = x->type->hdr_offset(x, skb, &prevhdr);
+	if (hdr_len < 0)
+		return hdr_len;
 	skb_set_mac_header(skb, (prevhdr - x->props.header_len) - skb->data);
 	skb_set_network_header(skb, -x->props.header_len);
 	skb->transport_header = skb->network_header + hdr_len;

net/ipv6/xfrm6_mode_transport.c

@@ -30,6 +30,8 @@ static int xfrm6_transport_output(struct xfrm_state *x, struct sk_buff *skb)
 	skb_set_inner_transport_header(skb, skb_transport_offset(skb));
 
 	hdr_len = x->type->hdr_offset(x, skb, &prevhdr);
+	if (hdr_len < 0)
+		return hdr_len;
 	skb_set_mac_header(skb, (prevhdr - x->props.header_len) - skb->data);
 	skb_set_network_header(skb, -x->props.header_len);
 	skb->transport_header = skb->network_header + hdr_len;