Re: [PATCH for 4.4.y-cip] cgroup-v1: Require capabilities to set release_agent

["Michal Koutný" <mkoutny@suse.com>]

Thanks for sharing this Masami, I've been looking into pre-cgroup_ns
backport too.

On Wed, Feb 16, 2022 at 08:40:37AM +0900, Masami Ichikawa <masami256@gmail.com> wrote:
> [masami: Backport patch from 4.9. Adjust to use current_user_ns() to get current user_ns.
> Fix conflict in cgroup_release_agent_write().]

The condition to allow modifying release_agent is two-fold:
a) caller is capabable(CAP_SYS_ADMIN),
b) cgroup_ns is owned by init_user_ns.

In pre-cgroup_ns kernels, it is IMO safer to consider all (=the only)
cgroup_ns owned by init_user_ns.

So the (positive) condition translates into capable(CAP_SYS_ADMIN) only.

[
Additionally, there's invariant/implication
  capable(CAP_XXX) -> (current_user_ns() == &init_user_ns) ,
so the expression
  (current_user_ns() != &init_user_ns) || !capable(CAP_SYS_ADMIN)
simplifies to
  !capable(CAP_SYS_ADMIN) .
]


> @@ -2839,6 +2856,14 @@ static ssize_t cgroup_release_agent_write(struct kernfs_open_file *of,
>  
>  	BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
>  
> +	/*
> +	 * Release agent gets called with all capabilities,
> +	 * require capabilities to set release agent.
> +	 */
> +	if ((of->file->f_cred->user_ns != &init_user_ns) ||
> +		!capable(CAP_SYS_ADMIN))
> +			return -EPERM;
> +

Following the reasoning above, the check simplifies too but it should be
be against the opener, not the writer:
  file_ns_capable(of->file, &init_user_ns, CAP_SYS_ADMIN)

(It seems to be to be incorrect even in the original commit.
So I'll send a patch there to rectify that.)


Michal