linux-kernel - Re: [PATCH] pid: Allow frozen userspace to reboot from non-init pid ns

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Date:   Mon, 9 Oct 2023 13:05:51 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <brauner@...nel.org>,
        Brian Geffon <bgeffon@...gle.com>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pid: Allow frozen userspace to reboot from non-init pid
 ns

On Fri, Sep 29, 2023 at 08:25:42PM -0400, Brian Geffon wrote:
> On Fri, Sep 29, 2023 at 4:09 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Fri, Sep 29, 2023 at 01:44:42PM -0400, Brian Geffon wrote:
> > > When the system has a frozen userspace, for example, during hibernation
> > > the child reaper task will also be frozen. Attmepting to deliver a
> > > signal to it to handle the reboot(2) will ultimately lead to the system
> > > hanging unless userspace is thawed.
> > >
> > > This change checks if the current task is the suspending task and if so
> > > it will allow it to proceed with a reboot from the non-init pid ns.
> >
> > I don't know the code flow too well here, but shouldn't init_pid_ns
> > always be doing the reboot regardless of anything else?
> 
> I think the point of this is, normally the reaper is runnable and so
> an appropriate signal will be delivered allowing them to also clean up
> [2]. In our case, they won't be runnable and doing this wouldn't make
> sense.
> 
> >
> > Also how is this syscall running if current is frozen? This feels weird
> > to me... shouldn't the frozen test be against pid_ns->child_reaper
> > instead of current?
> 
> The task which froze the system won't be frozen to make sure this
> happens it will have the flag PF_SUSPEND_TASK added, so we know if we
> have this flag we're the only running user space task [1].
> 
> >
> > -Kees
> 
> I hope my understanding is correct and it makes sense. Thanks for
> taking the time to review.

Christian, is this something you can take a look at? I'm much less
familiar with this area of the process list logic.

-Kees

-- 
Kees Cook