linux-kernel - Re: [PATCH] proc: allow killing processes via file descriptors

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Date:   Sun, 18 Nov 2018 07:53:27 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors

On Sun, Nov 18, 2018 at 7:38 AM, Andy Lutomirski <luto@...nel.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.

> I have an old patch to make proc directory fds pollable:
>
> https://lore.kernel.org/patchwork/patch/345098/
>
> That patch plus the one in this thread might make a nice addition to
> the kernel even if we expect something much better to come along
> later.

I've always commented on that patch. You never addressed my technical
objections. Why are you bringing up this patch again as if that
discussion had never happened? To review, that patch has various race
conditions, and even if it were technically correct, it'd be an abuse
of directory objects (in what other circumstance do we poll
directories?) and not logically generalizable to a model in which we
expose process exit status via the exit-monitoring API.