linux-kernel - Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT

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Date:   Wed, 10 Jun 2020 00:59:28 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Adrian Reber <areber@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Nicolas Viennot <Nicolas.Viennot@...sigma.com>,
        Michał Cłapiński <mclapinski@...gle.com>,
        Kamil Yurtsever <kyurtsever@...gle.com>,
        Dirk Petersen <dipeit@...il.com>,
        Christine Flood <chf@...hat.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Radostin Stoyanov <rstoyanov1@...il.com>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Sargun Dhillon <sargun@...gun.me>,
        Arnd Bergmann <arnd@...db.de>,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, selinux@...r.kernel.org,
        Eric Paris <eparis@...isplace.org>,
        Jann Horn <jannh@...gle.com>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT_RESTORE

On Tue, Jun 09, 2020 at 06:14:27PM +0200, Christian Brauner wrote:
> On Tue, Jun 09, 2020 at 09:06:27AM -0700, Andrei Vagin wrote:
> > On Tue, Jun 09, 2020 at 09:44:22AM +0200, Christian Brauner wrote:
> > > On Mon, Jun 08, 2020 at 08:42:21PM -0700, Andrei Vagin wrote:
...
> > > > PTRACE_O_SUSPEND_SECCOMP is needed for C/R and it is protected by
> > > > CAP_SYS_ADMIN too.
> > > 
> > > This is currently capable(CAP_SYS_ADMIN) (init_ns capable) why is it
> > > safe to allow unprivileged users to suspend security policies? That
> > > sounds like a bad idea.
> > 
...
> > I don't suggest to remove or
> > downgrade this capability check. The patch allows all c/r related
> > operations if the current has CAP_CHECKPOINT_RESTORE.
> > 
> > So in this case the check:
> >      if (!capable(CAP_SYS_ADMIN))
> >              return -EPERM;
> > 
> > will be converted in:
> >      if (!capable(CAP_SYS_ADMIN) && !capable(CAP_CHECKPOINT_RESTORE))
> >              return -EPERM;
> 
> Yeah, I got that but what's the goal here? Isn't it that you want to
> make it safe to install the criu binary with the CAP_CHECKPOINT_RESTORE
> fscap set so that unprivileged users can restore their own processes
> without creating a new user namespace or am I missing something? The
> use-cases in the cover-letter make it sound like that's what this is
> leading up to:
> > > > > * Checkpoint/Restore in an HPC environment in combination with a resource
> > > > >   manager distributing jobs where users are always running as non-root.
> > > > >   There is a desire to provide a way to checkpoint and restore long running
> > > > >   jobs.
> > > > > * Container migration as non-root
> > > > > * We have been in contact with JVM developers who are integrating
> > > > >   CRIU into a Java VM to decrease the startup time. These checkpoint/restore
> > > > >   applications are not meant to be running with CAP_SYS_ADMIN.
> 
> But maybe I'm just misunderstanding crucial bits (likely (TM)).

I think you understand this right. The goal is to make it possible to
use C/R functionality for unprivileged processes. And for me, here are
two separate tasks. The first one is how to allow unprivileged users to
use C/R from the root user namespace. This is what we discuss here.

And another one is how to allow to use C/R functionality from a non-root
user namespaces. The second task is about downgrading capable to
ns_capable for map_files and PTRACE_O_SUSPEND_SECCOMP.

Thanks,
Andrei