On Tue, Nov 20, 2018 at 08:18:10AM +1100, Aleksa Sarai wrote:

On 2018-11-19, Christian Brauner [off-list ref] wrote:

quoted

On Tue, Nov 20, 2018 at 07:28:57AM +1100, Aleksa Sarai wrote:

quoted

On 2018-11-19, Christian Brauner [off-list ref] wrote:

quoted

+	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

See my next mail.

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/>