rootless: `/sys/fs/cgroup` is writable when cgroupns isn't unshared in runc

Impact

It was found that rootless runc makes /sys/fs/cgroup writable in following conditons:

1. when runc is executed inside the user namespace, and the config.json does not specify the cgroup namespace to be unshared (e.g.., (docker|podman|nerdctl) run --cgroupns=host, with Rootless Docker/Podman/nerdctl)
2. or, when runc is executed outside the user namespace, and /sys is mounted with rbind, ro (e.g., runc spec --rootless; this condition is very rare)

A container may gain the write access to user-owned cgroup hierarchy /sys/fs/cgroup/user.slice/... on the host .
Other users's cgroup hierarchies are not affected.

Patches

v1.1.5 (planned)

Workarounds

• Condition 1: Unshare the cgroup namespace ((docker|podman|nerdctl) run --cgroupns=private). This is the default behavior of Docker/Podman/nerdctl on cgroup v2 hosts.
• Condition 2 (very rare): add /sys/fs/cgroup to maskedPaths