CVEs-PoC/2021/CVE-2021-47515.md

### [CVE-2021-47515](https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-47515)
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### Description

In the Linux kernel, the following vulnerability has been resolved:seg6: fix the iif in the IPv6 socket control blockWhen an IPv4 packet is received, the ip_rcv_core(...) sets the receivinginterface 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+SRHheader, the seg6_do_srh_encap(...) performs the required encapsulation.In this case, the seg6_do_srh_encap function clears the IPv6 socket controlblock (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: clearIP6CB(skb) on SRH ip4ip6 encapsulation") a long time ago (2019-01-29).Since the IPv6 socket control block and the IPv4 socket control block sharethe 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 ifcommit 0857d6f8c759 ("ipv6: When forwarding count rx stats on the orignetdev") is applied.To fix that issue, we set the IP6CB(skb)->iif with the index of thereceiving interface once again.

### POC

#### Reference
No PoCs from references.

#### Github
- https://github.com/ARPSyndicate/cve-scores