linux-kernel - Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

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Date:   Thu, 1 Nov 2018 14:33:28 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...capital.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Christian Brauner <christian@...uner.io>,
        Tyler Hicks <tyhicks@...onical.com>,
        Akihiro Suda <suda.akihiro@....ntt.co.jp>,
        Aleksa Sarai <asarai@...e.de>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v8 1/2] seccomp: add a return code to trap to userspace

On Thu, Nov 01, 2018 at 03:48:05PM +0100, Oleg Nesterov wrote:
> On 10/30, Tycho Andersen wrote:
> >
> > > I am not sure I understand the value of signaled/SECCOMP_NOTIF_FLAG_SIGNALED...
> > > I mean, why it is actually useful?
> > >
> > > Sorry if this was already discussed.
> >
> > :) no problem, many people have complained about this. This is an
> > implementation of Andy's suggestion here:
> > https://lkml.org/lkml/2018/3/15/1122
> >
> > You can see some more detailed discussion here:
> > https://lkml.org/lkml/2018/9/21/138
> 
> Cough, sorry, I simply can't understand what are you talking about ;)
> It seems that I need to read all the previous emails... So let me ask
> a stupid question below.
> 
> > > But my main concern is that either way wait_for_completion_killable() allows
> > > to trivially create a process which doesn't react to SIGSTOP, not good...
> > >
> > > Note also that this can happen if, say, both the tracer and tracee run in the
> > > same process group and SIGSTOP is sent to their pgid, if the tracer gets the
> > > signal first the tracee won't stop.
> > >
> > > Of freezer. try_to_freeze_tasks() can fail if it freezes the tracer before
> > > it does SECCOMP_IOCTL_NOTIF_SEND.
> >
> > I think in general the way this is intended to be used these things
> > wouldn't happen.
> 
> Why?

The intent is to run the tracer on the host and have it trace
containers, which would live in a different freezer cgroup, process
group, etc. Of course you could use it in a situation where they would
be, so the concern is still valid, but I'm not sure why you'd do that.

> > was malicious and had the ability to create a user namespace to
> > exhaust pids this way,
> 
> Not sure I understand how this connects to my question... nevermind.
> 
> > so perhaps we should drop this part of the
> > patch. I have no real need for it, but perhaps Andy can elaborate?
> 
> Yes I think it would be nice to avoid wait_for_completion_killable().
> 
> So please help me to understand the problem. Once again, why can not
> seccomp_do_user_notification() use wait_for_completion_interruptible() only?
>
> This is called before the task actually starts the syscall, so
> -ERESTARTNOINTR if signal_pending() can't hurt.

The idea was that when the tracee gets a signal, it notifies the
tracer exactly once, and then waits for the tracer to decide what to
do. So if we use another wait_for_completion_interruptible(), doesn't
it just get re-woken immediately because the signal is still pending?

...actually I just tested it, and it doesn't. So it seems we could use
_interruptible() here and achieve the same thing.

> Now lets suppose seccomp_do_user_notification() simply does
> 
> 	err = wait_for_completion_interruptible(&n.ready);
> 
> 	if (err < 0 && state != SECCOMP_NOTIFY_REPLIED) {
> 		syscall_set_return_value(ERESTARTNOINTR);
> 		list_del(&n.list);
> 		return -1;
> 	}
> 
> (I am ignoring the locking/etc). Now the obvious problem is that the listener
> doing SECCOMP_IOCTL_NOTIF_SEND can't distinguish -ENOENT from the case when the
> tracee was killed, yes?
> 
> Is it that important?

The answer to this question depends on how we want the listener to be
able to react. For example, if the listener is in the middle of doing
a mount() on behalf of the task and it gets a signal and we return
immediately, the listener will complete the mount(), try to respond
with success and get -ENOENT. If the task handles the signal and
restarts the mount(), it'll happen twice unless the listener undoes
it when it sees the -ENOENT. If we send another notification with the
SIGNALED flag, the listener has a better picture of what's going on,
which might be nice.

Tycho