net: Only allow init netns to set default tcp cong to a restricted algo

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net: Only allow init netns to set default tcp cong to a restricted algo tcp_set_default_congestion_control() is netns-safe in that it writes to &net->ipv4.tcp_congestion_control, but it also sets ca->flags |= TCP_CONG_NON_RESTRICTED which is not namespaced. This has the unintended side-effect of changing the global net.ipv4.tcp_allowed_congestion_control sysctl, despite the fact that it is read-only: 97684f0970f6 ("net: Make tcp_allowed_congestion_control readonly in non-init netns") Resolve this netns "leak" by only allowing the init netns to set the default algorithm to one that is restricted. This restriction could be removed if tcp_allowed_congestion_control were namespace-ified in the future. This bug was uncovered with https://github.com/JonathonReinhart/linux-netns-sysctl-verify

References

• https://git.kernel.org/stable/c/992de06308d9a9584d59b96d294ac676f924e437
• https://git.kernel.org/stable/c/9884f745108f7d25b189bbcd6754e284fb29ab68
• https://git.kernel.org/stable/c/6c1ea8bee75df8fe2184a50fcd0f70bf82986f42
• https://git.kernel.org/stable/c/efe1532a6e1a8e3c343d04fff510f0ed80328f9c
• https://git.kernel.org/stable/c/e7d7bedd507bb732e600403b7a96f9fe48d0ca31
• https://git.kernel.org/stable/c/8d432592f30fcc34ef5a10aac4887b4897884493