linux-kernel - Re: [PATCH v1] cgroup,bpf: Add access check for cgroup_get_from

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Date:   Tue, 20 Sep 2016 12:58:17 -0400
From:   Tejun Heo <tj@...nel.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        linux-kernel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Mack <daniel@...que.org>,
        "David S . Miller" <davem@...emloft.net>,
        James Morris <james.l.morris@...cle.com>,
        Kees Cook <keescook@...omium.org>,
        Martin KaFai Lau <kafai@...com>, cgroups@...r.kernel.org
Subject: Re: [PATCH v1] cgroup,bpf: Add access check for cgroup_get_from_fd()

On Tue, Sep 20, 2016 at 06:53:35PM +0200, Mickaël Salaün wrote:
> 
> On 20/09/2016 02:30, Alexei Starovoitov wrote:
> > On Tue, Sep 20, 2016 at 12:49:13AM +0200, Mickaël Salaün wrote:
> >> Add security access check for cgroup backed FD. The "cgroup.procs" file
> >> of the corresponding cgroup should be readable to identify the cgroup,
> >> and writable to prove that the current process can manage this cgroup
> >> (e.g. through delegation). This is similar to the check done by
> >> cgroup_procs_write_permission().
> >>
> >> Fixes: 4ed8ec521ed5 ("cgroup: bpf: Add BPF_MAP_TYPE_CGROUP_ARRAY")
> > 
> > I don't understand what 'fixes' is about.
> > Looks like new feature or tightening?
> > Since cgroup was opened by the process and it got an fd,
> > it had an access, so extra check here looks unnecessary.
> 
> It may not be a "fix", but this patch tighten the access control. The
> current cgroup_get_from_fd() only rely on the access check done on the
> passed FD. However, this FD come from a cgroup directory, not a
> "cgroup.procs" (in this directory). The "cgroup.procs" is used for
> cgroup delegation by cgroup_procs_write_permission(). Checking
> "cgroup.procs" is then more consistent with access checks done by other
> part of the cgroup code. Being able to open a cgroup directory only
> means that the current process is able to list the cgroup hierarchy, not
> necessarily to list the tasks in this cgroups.

Currently, bpf's access control and cgroup's are completely separate
and intentionally so.  I don't see why this matters given the current
model.

Thanks.

-- 
tejun