linux-kernel - Re: [PATCH v3] Implement /proc/pid/kill

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Date:   Wed, 31 Oct 2018 14:06:37 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Christian Brauner <christian.brauner@...onical.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v3] Implement /proc/pid/kill

On Wed, Oct 31, 2018 at 07:33:06PM +0000, Daniel Colascione wrote:
> On Wed, Oct 31, 2018 at 6:17 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> > On Wed, Oct 31, 2018 at 06:00:49PM +0000, Daniel Colascione wrote:
> >> On Wed, Oct 31, 2018 at 5:54 PM, Tycho Andersen <tycho@...ho.ws> wrote:
> >> > Why not just use an ioctl() like Jann suggested instead of this big
> >> > security check? Then we avoid the whole setuid writer thing entirely,
> >>
> >> Don't you think a system call would be better than a new ioctl?
> >
> > We already have a kill() system call :)
> 
> kill(2) is useless this purpose: it accepts a numeric PID, but we'd
> need it to accept a process file descriptor instead. It's true that
> the existing kill(1) binary might be the vehicle for using a
> hypothetical new system call, but that's a separate matter.
> 
> >> With either an ioctl or a new system call, though, the shell would
> >> need a helper program to use the facility, whereas with the existing
> >> approach, the shell can use the new facility without any additional
> >> binaries.
> >
> > ...and a binary to use it!
> >
> > The nice thing about an ioctl is that it avoids this class of attacks
> > entirely.
> 
> Let's stop talking about adding an ioctl. Ioctls have problems with
> namespacing of the request argument; it's not safe, in general, to
> issue an ioctl against a file descriptor of an unknown type.

So don't lose track of the fd type. I'm not sure I see this as a big
problem.

> You don't know how that FD will interpret your request code. The two
> good options before us are a write(2) interface and a new system
> call. I think both are defensible. But I don't see a good reason to
> consider adding an ioctl instead of a system call.

https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1729911.html
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1729921.html

maybe? :)

> All of this is moot if the new comprehensive process interface that
> comes out of LPC ends up being better anyway.

+1, I think a way to do all of this sort of thing would be nice.

> > either. Using this from the shell is still racy, because if I do
> > something like:
> >
> > echo 9 > /proc/$pid/kill
> >
> > There's exactly the same race that there is with kill, that $pid might
> > be something else.
> 
> > Of course I could do some magic with bind mounts or
> > my pwd or something to keep it alive, but I can already do that today
> > with kill.
> 
> You can't do it today with kill. The idea that keeping a open file
> descriptor to a /proc/pid or a file within it prevents PID reuse is
> widespread, but incorrect.

Good to know :)

Tycho