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

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Date:   Sun, 18 Nov 2018 08:17:55 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        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 7:53 AM Daniel Colascione <dancol@...gle.com> wrote:
>
> 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.

Linus banned it because of bugs iike the ones in the patch.

>
> > 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

I don't think I ever saw that review.

> 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.

I agree it's weird.  It might be better to have /proc/PID/exit_status
and make *that* pollable.