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

[Masami Ichikawa <masami256@gmail.com>]

On Wed, Feb 16, 2022 at 9:11 PM Michal Koutný <mkoutny@suse.com> wrote:
>
> Thanks for sharing this Masami, I've been looking into pre-cgroup_ns
> backport too.
>

Thank you for the review.

> 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.
>

I see.  Thank you for the details.
If we treat cgroup_ns as only cgroup_ns that are owned by
init_user_ns,  we don't have to have an extra check.

> [
> 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)
>

Thanks. I got it.

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

Regards,
-- 
/**
* Masami Ichikawa
* personal: masami256@gmail.com
* fedora project: masami@fedoraproject.org
*/