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

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Date:   Sun, 18 Nov 2018 07:38:09 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     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 Lutomirski <luto@...nel.org>,
        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 5:59 AM Daniel Colascione <dancol@...gle.com> wrote:
>
> I had been led to believe that the proposal would be a comprehensive
> process API, not an ioctl basically equivalent to my previous patch.
> If you had a more comprehensive proposal, please just share it on LKML
> instead of limiting the discussion to those able to attend these
> various conferences. If there's some determined opposition to a
> general new process API, this opposition needs a fair and full airing,
> as not everyone can attend these conferences.
>
> On Sun, Nov 18, 2018 at 3:17 AM, Christian Brauner <christian@...uner.io> wrote:
> > With this patch an open() call on /proc/<pid> will give userspace a handle
> > to struct pid of the process associated with /proc/<pid>. This allows to
> > maintain a stable handle on a process.
> > I have been discussing various approaches extensively during technical
> > conferences this year culminating in a long argument with Eric at Linux
> > Plumbers. The general consensus was that having a handle on a process
> > will be something that is very simple and easy to maintain
>
> ioctls are the opposite of "easy to maintain". Their
> file-descriptor-specific behavior makes it difficult to use the things
> safely. If you want to take this approach, please make a new system
> call. An ioctl is just a system call with a very strange spelling and
> unfortunate collision semantics.
>
> > with the
> > option of being extensible via a more advanced api if the need arises.
>
> The need *has* arisen; see my exithand patch.
>
> > I
> > believe that this patch is the most simple, dumb, and therefore
> > maintainable solution.
> >
> > The need for this has arisen in order to reliably kill a process without
> > running into issues of the pid being recycled as has been described in the
> > rejected patch [1].
>
> That patch was not "rejected". It was tabled pending the more
> comprehensive process API proposal that was supposed to have emerged.
> This patch is just another variant of the sort of approach we
> discussed on that patch's thread here. As I mentioned on that thread,
> the right approach option is a new system call, not an ioctl.
>
>  To fulfill the need described in that patchset a new
> > ioctl() PROC_FD_SIGNAL is added. It can be used to send signals to a
> > process via a file descriptor:
> >
> > int fd = open("/proc/1234", O_DIRECTORY | O_CLOEXEC);
> > ioctl(fd, PROC_FD_SIGNAL, SIGKILL);
> > close(fd);
> >
> > Note, the stable handle will allow us to carefully extend this feature in
> > the future.
>
> We still need the ability to synchronously wait on a process's death,
> as in my patch set. I will be refreshing that patch set.

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.

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.

This patch that uses ioctl