CVE-2021-47515

In the Linux kernel, the following vulnerability has been resolved: 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.