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

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Date:   Sun, 18 Nov 2018 09:45:18 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        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 9:43 AM Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
> Daniel Colascione <dancol@...gle.com> writes:
>
> > On Sun, Nov 18, 2018 at 9:13 AM, Andy Lutomirski <luto@...nel.org> wrote:
> >> On Sun, Nov 18, 2018 at 8:29 AM Daniel Colascione <dancol@...gle.com> wrote:
> >>>
> >>> On Sun, Nov 18, 2018 at 8:17 AM, Andy Lutomirski <luto@...nel.org> 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.
> >>>
> >>> 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)).
>
> If we did not have the permission check challenges and could perform
> the permission checks in open, write(2) would be on the table.
> Performing write(2) would only be concrend about data.
>
> Unfortunately we have setresuid and exec which make that infeasible
> for the kill operations.

setresuid() should be irrelevant.  If you had permission to kill a
process and the process calls setresuid(), you should still have
permission to kill it.

For execve(), we could make execve() invalidate the fd.  (See other email.)