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

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Date:   Sun, 18 Nov 2018 08:33:52 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>,
        Daniel Colascione <dancol@...gle.com>
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>,
        Jan Engelhardt <jengelh@...i.de>
Subject: Re: [PATCH] proc: allow killing processes via file descriptors

On 11/18/18 8:17 AM, Andy Lutomirski wrote:
> 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.
> 

If there is a new exit_status file, it could even be more than
8 bits of exit status:

See https://lore.kernel.org/lkml/alpine.LSU.2.20.1507091257010.9602@nerf40.vanv.qr/T/#u
and http://austingroupbugs.net/view.php?id=594#c1317

-- 
~Randy