seg6: fix the iif in the IPv6 socket control block

When an IPv4 packet is received, the ip_rcv_core(...) sets the receiving
interface index into the IPv4 socket control block (v5.16-rc4,
net/ipv4/ip_input.c line 510):

    IPCB(skb)->iif = skb->skb_iif;

If that IPv4 packet is meant to be encapsulated in an outer IPv6+SRH
header, the seg6_do_srh_encap(...) performs the required encapsulation.
In this case, the seg6_do_srh_encap function clears the IPv6 socket control
block (v5.16-rc4 net/ipv6/seg6_iptunnel.c line 163):

    memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));

The memset(...) was introduced in commit ef489749aae5 ("ipv6: sr: clear
IP6CB(skb) on SRH ip4ip6 encapsulation") a long time ago (2019-01-29).

Since the IPv6 socket control block and the IPv4 socket control block share
the same memory area (skb->cb), the receiving interface index info is lost
(IP6CB(skb)->iif is set to zero).

As a side effect, that condition triggers a NULL pointer dereference if
commit 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orig
netdev") is applied.

To fix that issue, we set the IP6CB(skb)->iif with the index of the
receiving interface once again.

Fixes: ef489749aae5 ("ipv6: sr: clear IP6CB(skb) on SRH ip4ip6 encapsulation")
Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20211208195409.12169-1-andrea.mayer@uniroma2.it
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
(cherry picked from commit ae68d93354e5bf5191ee673982251864ea24dd5c)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

diff --git a/net/ipv6/seg6_iptunnel.c b/net/ipv6/seg6_iptunnel.c
index 3adc5d9211ad695bb1cdf66405be3dfe1a539c76..d64855010948db23eb5ebe5ce0bc4dfff2634afb 100644 (file)
--- a/net/ipv6/seg6_iptunnel.c
+++ b/net/ipv6/seg6_iptunnel.c
@@ -161,6 +161,14 @@ int seg6_do_srh_encap(struct sk_buff *skb, struct ipv6_sr_hdr *osrh, int proto)
                hdr->hop_limit = ip6_dst_hoplimit(skb_dst(skb));
 
                memset(IP6CB(skb), 0, sizeof(*IP6CB(skb)));
+
+               /* the control block has been erased, so we have to set the
+                * iif once again.
+                * We read the receiving interface index directly from the
+                * skb->skb_iif as it is done in the IPv4 receiving path (i.e.:
+                * ip_rcv_core(...)).
+                */
+               IP6CB(skb)->iif = skb->skb_iif;
        }
 
        hdr->nexthdr = NEXTHDR_ROUTING;