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From:   Daniel Colascione <dancol@google.com>
Date:   Sun, 18 Nov 2018 09:17:31 -0800
Message-ID: <CAKOZues3_uh1mnQr=VPCxoWiY0wfiUq0oFXcVuiHF7rPmUXsuw@mail.gmail.com>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Christian Brauner <christian@brauner.io>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Serge E. Hallyn" <serge@hallyn.com>, Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.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>,
        Kees Cook <keescook@chromium.org>
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On Sun, Nov 18, 2018 at 9:13 AM, Andy Lutomirski <luto@kernel.org> wrote:
> On Sun, Nov 18, 2018 at 8:29 AM Daniel Colascione <dancol@google.com> wrote:
>>
>> On Sun, Nov 18, 2018 at 8:17 AM, Andy Lutomirski <luto@kernel.org> wrote:
>> > On Sun, Nov 18, 2018 at 7:53 AM Daniel Colascione <dancol@google.com> wrote:
>> >>
>> >> On Sun, Nov 18, 2018 at 7:38 AM, Andy Lutomirski <luto@kernel.org> wrote:
>> >> > I fully agree that a more comprehensive, less expensive API for
>> >> > managing processes would be nice.  But I also think that this patch
>> >> > (using the directory fd and ioctl) is better from a security
>> >> > perspective than using a new file in /proc.
>> >>
>> >> That's an assertion, not an argument. And I'm not opposed to an
>> >> operation on the directory FD, now that it's clear Linus has banned
>> >> "write(2)-as-a-command" APIs. I just insist that we implement the API
>> >> with a system call instead of a less-reliable ioctl due to the
>> >> inherent namespace collision issues in ioctl command names.
>> >
>> > Linus banned it because of bugs iike the ones in the patch.
>>
>> Maybe: he didn't provide a reason. What's your point?
>
> My point is that an API that involves a file like /proc/PID/kill is
> very tricky to get right.  Here are some considerations:

Moot. write(2) for this interface is off the table anyway. The right
approach here is a system call that accepts a /proc/pid directory file
descriptor, a signal number, and a signal information field (as in
sigqueue(2)).

> Now if we had an ioctlat() API, maybe it would make sense.  But we
> don't, and I think it would be a bit crazy to add one.

A process is not a driver. Why won't this idea of using an ioctl for
the kill-process-by-dfd thing just won't die? An ioctl has *zero*
advantage in this context.