linux-kernel - Re: [PATCH v1 2/2] signal: add procfd

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Message-ID: <20181119211810.73ptfhnwdmkngfi4@yavin>
Date:   Tue, 20 Nov 2018 08:18:10 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Christian Brauner <christian@...uner.io>
Cc:     ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        serge@...lyn.com, jannh@...gle.com, luto@...nel.org,
        akpm@...ux-foundation.org, oleg@...hat.com,
        viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-api@...r.kernel.org, dancol@...gle.com, timmurray@...gle.com,
        linux-man@...r.kernel.org, Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

On 2018-11-19, Christian Brauner <christian@...uner.io> wrote:
> On Tue, Nov 20, 2018 at 07:28:57AM +1100, Aleksa Sarai wrote:
> > On 2018-11-19, Christian Brauner <christian@...uner.io> wrote:
> > > +	if (info) {
> > > +		ret = __copy_siginfo_from_user(sig, &kinfo, info);
> > > +		if (unlikely(ret))
> > > +			goto err;
> > > +		/*
> > > +		 * Not even root can pretend to send signals from the kernel.
> > > +		 * Nor can they impersonate a kill()/tgkill(), which adds
> > > +		 * source info.
> > > +		 */
> > > +		ret = -EPERM;
> > > +		if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
> > > +		    (task_pid(current) != pid))
> > > +			goto err;
> > > +	} else {
> > > +		prepare_kill_siginfo(sig, &kinfo);
> > > +	}
> > 
> > I wonder whether we should also have a pidns restriction here, since
> > currently it isn't possible for a container process using a pidns to
> > signal processes outside its pidns. AFAICS, this isn't done through an
> > explicit check -- it's a side-effect of processes in a pidns not being
> > able to address non-descendant-pidns processes.
> > 
> > But maybe it's reasonable to allow sending a procfd to a different pidns
> > and the same operations working on it? If we extend the procfd API to
> 
> No, I don't think so. I really don't want any fancy semantics in here.
> Fancy doesn't get merged and fancy is hard to maintain. So we should do
> something like:
> 
> if (proc_pid_ns() != current_pid_ns)
> 	return EINVAL

This isn't quite sufficient. The key thing is that you have to be in an
*ancestor* (or same) pidns, not the *same* pidns. Ideally you can re-use
the check already in pidns_get_parent, and expose it. It would be
something as trivial as:

bool pidns_is_descendant(struct pid_namespace *ns,
                         struct pid_namespace *ancestor)
{
    for (;;) {
        if (!ns)
            return false;
        if (ns == ancestor)
            break;
        ns = ns->parent;
    }
    return true;
}

And you can rewrite pidns_get_parent to use it. So you would instead be
doing:

    if (pidns_is_descendant(proc_pid_ns, task_active_pid_ns(current)))
        return -EPERM;

(Or you can just copy the 5-line loop into procfd_signal -- though I
imagine we'll need this for all of the procfd_* APIs.)

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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