Re: [PATCHv1 6/8] cgroup: restrict cgroup operations within task's cgroupns

["Serge E. Hallyn" <serge-A9i7LUbDfNHQT0dZR+AlfA@public.gmane.org>]

Quoting Aditya Kali (adityakali-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org):
> Restrict following operations within the calling tasks:
> * cgroup_mkdir & cgroup_rmdir
> * cgroup_attach_task
> * writes to cgroup files outside of task's cgroupns-root
> 
> Also, read of /proc/<pid>/cgroup file is now restricted only
> to tasks under same cgroupns-root. If a task tries to look
> at cgroup of another task outside of its cgroupns-root, then
> it won't be able to see anything for the default hierarchy.
> This is same as if the cgroups are not mounted.
> 
> Signed-off-by: Aditya Kali <adityakali-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>

So this is a bit different from some other namespaces - if I
have an open fd to a file, then setns into a mntns where that
file is not addressable, I can still use the file.

I guess not allowing attach to a cgroup outside our ns is a
good failsafe as we'll otherwise risk falling off a cliff in
some code, but I'm not sure the cgroup_file_write/mkdir/rmdir
restrictions are needed.  (And really I can fchdir to a
directory not in my ns, so the cgroup-attach restriction is
any more justified).

Still I'm not strictly opposed ot this, so

Acked-by: Serge Hallyn <serge.hallyn-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org>

just wanted to point this out.

> ---
>  kernel/cgroup.c | 34 +++++++++++++++++++++++++++++++++-
>  1 file changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index f8099b4..2fc0dfa 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -2318,6 +2318,12 @@ static int cgroup_attach_task(struct cgroup *dst_cgrp,
>  	struct task_struct *task;
>  	int ret;
>  
> +	/* Only allow changing cgroups accessible within task's cgroup
> +	 * namespace. i.e. 'dst_cgrp' should be a descendant of task's
> +	 * cgroupns->root_cgrp. */
> +	if (!cgroup_is_descendant(dst_cgrp, task_cgroupns_root(leader)))
> +		return -EPERM;
> +
>  	/* look up all src csets */
>  	down_read(&css_set_rwsem);
>  	rcu_read_lock();
> @@ -2882,6 +2888,10 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
>  	struct cgroup_subsys_state *css;
>  	int ret;
>  
> +	/* Reject writes to cgroup files outside of task's cgroupns-root. */
> +	if (!cgroup_is_descendant(cgrp, task_cgroupns_root(current)))
> +		return -EINVAL;
> +
>  	if (cft->write)
>  		return cft->write(of, buf, nbytes, off);
>  
> @@ -4560,6 +4570,13 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
>  	parent = cgroup_kn_lock_live(parent_kn);
>  	if (!parent)
>  		return -ENODEV;
> +
> +	/* Allow mkdir only within process's cgroup namespace root. */
> +	if (!cgroup_is_descendant(parent, task_cgroupns_root(current))) {
> +		ret = -EPERM;
> +		goto out_unlock;
> +	}
> +
>  	root = parent->root;
>  
>  	/* allocate the cgroup and its ID, 0 is reserved for the root */
> @@ -4822,6 +4839,13 @@ static int cgroup_rmdir(struct kernfs_node *kn)
>  	if (!cgrp)
>  		return 0;
>  
> +	/* Allow rmdir only within process's cgroup namespace root.
> +	 * The process can't delete its own root anyways. */
> +	if (!cgroup_is_descendant(cgrp, task_cgroupns_root(current))) {
> +		cgroup_kn_unlock(kn);
> +		return -EPERM;
> +	}
> +
>  	ret = cgroup_destroy_locked(cgrp);
>  
>  	cgroup_kn_unlock(kn);
> @@ -5051,6 +5075,15 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
>  		if (root == &cgrp_dfl_root && !cgrp_dfl_root_visible)
>  			continue;
>  
> +		cgrp = task_cgroup_from_root(tsk, root);
> +
> +		/* The cgroup path on default hierarchy is shown only if it
> +		 * falls under current task's cgroupns-root.
> +		 */
> +		if (root == &cgrp_dfl_root &&
> +		    !cgroup_is_descendant(cgrp, task_cgroupns_root(current)))
> +			continue;
> +
>  		seq_printf(m, "%d:", root->hierarchy_id);
>  		for_each_subsys(ss, ssid)
>  			if (root->subsys_mask & (1 << ssid))
> @@ -5059,7 +5092,6 @@ int proc_cgroup_show(struct seq_file *m, struct pid_namespace *ns,
>  			seq_printf(m, "%sname=%s", count ? "," : "",
>  				   root->name);
>  		seq_putc(m, ':');
> -		cgrp = task_cgroup_from_root(tsk, root);
>  		path = cgroup_path(cgrp, buf, PATH_MAX);
>  		if (!path) {
>  			retval = -ENAMETOOLONG;
> -- 
> 2.1.0.rc2.206.gedb03e5
> 
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