[PATCH 4.19 01/23] ipv4: ip_do_fragment: Preserve skb_iif during fragmentation

[Greg Kroah-Hartman]

From: Shmulik Ladkani <shmulik@xxxxxxxxxxxxxxxx>

[ Upstream commit d2f0c961148f65bc73eda72b9fa3a4e80973cb49 ]

Previously, during fragmentation after forwarding, skb->skb_iif isn't
preserved, i.e. 'ip_copy_metadata' does not copy skb_iif from given
'from' skb.

As a result, ip_do_fragment's creates fragments with zero skb_iif,
leading to inconsistent behavior.

Assume for example an eBPF program attached at tc egress (post
forwarding) that examines __sk_buff->ingress_ifindex:
 - the correct iif is observed if forwarding path does not involve
   fragmentation/refragmentation
 - a bogus iif is observed if forwarding path involves
   fragmentation/refragmentatiom

Fix, by preserving skb_iif during 'ip_copy_metadata'.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/ip_output.c |    1 +
 1 file changed, 1 insertion(+)

--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -519,6 +519,7 @@ static void ip_copy_metadata(struct sk_b
 	to->pkt_type = from->pkt_type;
 	to->priority = from->priority;
 	to->protocol = from->protocol;
+	to->skb_iif = from->skb_iif;
 	skb_dst_drop(to);
 	skb_dst_copy(to, from);
 	to->dev = from->dev;