xfrm: interface: fix ipv4 pmtu check to honor ip header df

[ Upstream commit 8fc0e3b ]

Frag needed should only be sent if the header enables DF.

This fix allows packets larger than MTU to pass the xfrm interface
and be fragmented after encapsulation, aligning behavior with
non-interface xfrm.

Fixes: f203b76 ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

net/xfrm/xfrm_interface.c

@@ -305,6 +305,8 @@ xfrmi_xmit2(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
 
 			icmpv6_ndo_send(skb, ICMPV6_PKT_TOOBIG, 0, mtu);
 		} else {
+			if (!(ip_hdr(skb)->frag_off & htons(IP_DF)))
+				goto xmit;
 			icmp_ndo_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 				      htonl(mtu));
 		}
@@ -313,6 +315,7 @@ xfrmi_xmit2(struct sk_buff *skb, struct net_device *dev, struct flowi *fl)
 		return -EMSGSIZE;
 	}
 
+xmit:
 	xfrmi_scrub_packet(skb, !net_eq(xi->net, dev_net(dev)));
 	skb_dst_set(skb, dst);
 	skb->dev = tdev;