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

stable inclusion
from stable-5.10.30
commit 3b74ce529ece253fb70efa69548ef8e160fb6b2d
bugzilla: 51791

--------------------------------

[ Upstream commit 8fc0e3b6 ]

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: f203b76d ("xfrm: Add virtual xfrm interfaces")
Signed-off-by: NEyal Birger <eyal.birger@gmail.com>
Reviewed-by: NSabrina Dubroca <sd@queasysnail.net>
Signed-off-by: NSteffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NChen Jun <chenjun102@huawei.com>
Acked-by: N  Weilong Chen <chenweilong@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

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;