Vulnerability Instance

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`