interfaceV6UsableForTun interpolates the interface name into a /proc path.
A plain filepath.Join + os.Open only cleans the path, so a tunname with
".." (or a symlinked component) could read outside /proc/sys/net/ipv6/conf.
Open under that fixed directory with os.OpenInRoot, which rejects any path
escaping the root (openat-based, so also TOCTOU-resistant), still using
filepath.Join to build the relative name. See https://go.dev/blog/osroot.
Updates #20447
Signed-off-by: Brendan Creane <bcreane@gmail.com>
Brendan CreaneandGitHub
c1edf7f458