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Date:   Mon, 19 Nov 2018 22:37:24 +0100
From:   Christian Brauner <christian@brauner.io>
To:     Daniel Colascione <dancol@google.com>
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        linux-man <linux-man@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall
Message-ID: <20181119213722.z54huio5g4kuldxk@brauner.io>
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On Mon, Nov 19, 2018 at 01:26:22PM -0800, Daniel Colascione wrote:
> On Mon, Nov 19, 2018 at 1:21 PM, Christian Brauner <christian@brauner.io> wrote:
> > That can be done without a loop by comparing the level counter for the
> > two pid namespaces.
> >
> >>
> >> 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.)
> 
> Why is any of this even necessary? Why does the child namespace we're
> considering even have a file descriptor to its ancestor's procfs? If

Because you can send file descriptors between processes and container
runtimes tend to do that.

> it has one of these FDs, it can already *read* all sorts of
> information it really shouldn't be able to acquire, so the additional
> ability to send a signal (subject to the usual permission checks)
> feels like sticking a finger in a dike that's already well-perforated.
> IMHO, we shouldn't bother with this check. The patch would be simpler
> without it.

We will definitely not allow signaling processes in an ancestor pid
namespace! That is a security issue! I can imagine container runtimes
killing their monitoring process etc. pp. Not happening, unless someone
with deep expertise in signals can convince me otherwise.