Linux Kernel CVE-2025-40173

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

net/ip6_tunnel: Prevent perpetual tunnel growth

Similarly to ipv4 tunnel, ipv6 version updates dev->needed_headroom, too. While ipv4 tunnel headroom adjustment growth was limited in commit 5ae1e9922bbd ("net: ip_tunnel: prevent perpetual headroom growth"), ipv6 tunnel yet increases the headroom without any ceiling.

Reflect ipv4 tunnel headroom adjustment limit on ipv6 version.

Credits to Francesco Ruggeri, who was originally debugging this issue and wrote local Arista-specific patch and a reproducer.

The Linux kernel is affected in versions where the IPv6 tunnel (net/ip6_tunnel) headroom logic lacks the ceiling that IPv4 received in commit 5ae1e9922bbd; the exact affected version ranges are enumerated per stable branch by the eight git.kernel.org stable commits (10fe967e, 11f6066a, 21f4d45e, 402b6985, 48294a67, 566f8d5c, b6eb25d8, eeb43454), each backporting the fix to a different maintained tree. No CPE strings were provided in the input, so precise version boundaries must be read from the individual stable commits and distribution advisories. Canonical/Ubuntu has published fixed kernels in USN-8125-1 (https://ubuntu.com/security/notices/USN-8125-1) and USN-8126-1 (https://ubuntu.com/security/notices/USN-8126-1).