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Date:   Tue, 20 Nov 2018 07:28:57 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Christian Brauner <christian@brauner.io>
Cc:     ebiederm@xmission.com, linux-kernel@vger.kernel.org,
        serge@hallyn.com, jannh@google.com, luto@kernel.org,
        akpm@linux-foundation.org, oleg@redhat.com,
        viro@zeniv.linux.org.uk, linux-fsdevel@vger.kernel.org,
        linux-api@vger.kernel.org, dancol@google.com, timmurray@google.com,
        linux-man@vger.kernel.org, Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall
Message-ID: <20181119202857.k5zw742xjfrw677j@yavin>
References: <20181119103241.5229-1-christian@brauner.io>
 <20181119103241.5229-3-christian@brauner.io>
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On 2018-11-19, Christian Brauner <christian@brauner.io> wrote:
> +	if (info) {
> +		ret =3D __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 =3D -EPERM;
> +		if ((kinfo.si_code >=3D 0 || kinfo.si_code =3D=3D SI_TKILL) &&
> +		    (task_pid(current) !=3D 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.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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