[PATCH net] seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation

When an SRv6 packet arrives on an interface enslaved to a VRF,
vrf_ip6_rcv() sets IP6SKB_L3SLAVE in IP6CB, but decap_and_validate()
has never set IPSKB_L3SLAVE in IPCB. The bit stayed clear in the
common case, and with CONFIG_IPV6_MIP6 the leftover frag_max_size of
a reassembled outer packet could even set it, with no VRF involved.
Commit 44930446dde4 ("ipv6: seg6: clear IPv4 control block on IPIP
decapsulation") then made the unreliable bit reliably clear.

The effect of the missing flag is visible with End.DX4 when a
delivery to a local address of the node reaches the socket lookup.
For example, a UDP socket bound to the enslaved ingress interface
does not receive any of the decapsulated packets, while an unbound
socket outside the VRF does.
This contradicts Documentation/networking/vrf.rst: by default the
scope of an unbound UDP or TCP socket is limited to the default VRF.

Set IPSKB_L3SLAVE for IPv4 in decap_and_validate(), which already does
the same for IPv6. The socket lookup then matches the decapsulated
packet like any other packet received on that enslaved interface. Such
a packet matches an unbound UDP or TCP socket only when
udp_l3mdev_accept or tcp_l3mdev_accept is set.

Fixes: 891ef8dd2a8d ("ipv6: sr: implement additional seg6local actions")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
---
 net/ipv6/seg6_local.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/net/ipv6/seg6_local.c b/net/ipv6/seg6_local.c
index 7b5212220185..d1070aec7b72 100644
--- a/net/ipv6/seg6_local.c
+++ b/net/ipv6/seg6_local.c

@@ -257,10 +257,13 @@ static bool decap_and_validate(struct sk_buff *skb, int proto)
 		return false;
 
 	if (proto == IPPROTO_IPIP) {
+		bool l3slave = ipv6_l3mdev_skb(IP6CB(skb)->flags);
 		int iif = IP6CB(skb)->iif;
 
 		memset(IPCB(skb), 0, sizeof(*IPCB(skb)));
 		IPCB(skb)->iif = iif;
+		if (l3slave)
+			IPCB(skb)->flags |= IPSKB_L3SLAVE;
 	} else if (proto == IPPROTO_IPV6) {
 		bool l3slave = ipv6_l3mdev_skb(IP6CB(skb)->flags);
 		int iif = IP6CB(skb)->iif;

-- 
2.43.0