CVE-2021-47515: seg6: fix the iif in the IPv6 socket control block

Severity: medium (CVSS 5.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2024-05-24 · updated 2026-08-04
Weakness: CWE-476

Description

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.

Affected products

Linux kernel: from 4.14.98 before 4.14.258; from 4.19.20 before 4.19.221; from 4.20.7 before 5.0; from 5.0.1 before 5.4.165; from 5.5 before 5.10.85; from 5.11 before 5.15.8 — fixed in 4.14.258, 4.19.221, 5.0, 5.10.85, 5.15.8, 5.4.165

How to fix

Upgrade to a fixed version: Linux kernel 4.14.258, 4.19.221, 5.0, 5.10.85. Check the vendor advisory for mitigations.

References

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