On 2018-11-19, Christian Brauner <christian@brauner.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
allow process creation this would allow a container to create a process
outside its pidns.

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