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

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Date:   Wed, 31 Oct 2018 12:17:17 -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 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 :)

> 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.

> > and we can pass the fd around if we want to.
> 
> You can pass the FD around today --- specifically, you just pass the
> /proc/pid directory FD, not the /proc/pid/kill FD. The /proc/pid
> directory FD acts as a process handle. (It's literally a reference to
> a struct pid.) Anyone who receives one of these process handle FDs and
> who wants to use the corresponding kill file can open the kill fd with
> openat(2). What you can't do is pass the /proc/pid/kill FD to another
> security context and use it, but when would you ever want to do that?

Perhaps I don't have a good imagination, because it's not clear to me
when I'd ever use this from a shell instead of the kill binary,
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.

I can understand the desire to have a race free way to do this, but
"it must use write(2)" seems a little unnecessary, given that the
shell use case isn't particularly convincing to me.

Tycho