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

runc is a CLI tool for spawning and running containers according to the OCI specification. In affected versions 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) or 2. 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. Users are advised to upgrade to version 1.1.5. Users unable to upgrade may unshare the cgroup namespace (`(docker|podman|nerdctl) run --cgroupns=private)`. This is the default behavior of Docker/Podman/nerdctl on cgroup v2 hosts. or add `/sys/fs/cgroup` to `maskedPaths`.

CVSS:3.1/AV:L/AC:H/PR:H/UI:N/S:C/C:L/I:L/A:L

权限预留不恰当

runc 安全漏洞

runc是一款用于根据OCI规范生成和运行容器的CLI（命令行界面）工具。 runc 1.1.5之前版本存在安全漏洞，该漏洞源于runc在用户命名空间内执行时没有指定要取消共享的命名空间，在用户命名空间之外执行时可以获得对主机上用户的写访问权限。

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