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

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Date:   Mon, 19 Nov 2018 21:55:20 +0100
From:   Christian Brauner <christian@...uner.io>
To:     Aleksa Sarai <cyphar@...har.com>
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 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

> 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/>