Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT_RESTORE

From: Andrei Vagin <avagin@gmail.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: "Adrian Reber" <areber@redhat.com>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Pavel Emelyanov" <ovzxemul@gmail.com>,
	"Oleg Nesterov" <oleg@redhat.com>,
	"Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Nicolas Viennot" <Nicolas.Viennot@twosigma.com>,
	"Michał Cłapiński" <mclapinski@google.com>,
	"Kamil Yurtsever" <kyurtsever@google.com>,
	"Dirk Petersen" <dipeit@gmail.com>,
	"Christine Flood" <chf@redhat.com>,
	"Casey Schaufler" <casey@schaufler-ca.com>,
	"Mike Rapoport" <rppt@linux.ibm.com>,
	"Radostin Stoyanov" <rstoyanov1@gmail.com>,
	"Cyrill Gorcunov" <gorcunov@openvz.org>,
	"Serge Hallyn" <serge@hallyn.com>,
	"Stephen Smalley" <stephen.smalley.work@gmail.com>,
	"Sargun Dhillon" <sargun@sargun.me>,
	"Arnd Bergmann" <arnd@arndb.de>,
	linux-security-module@vger.kernel.org,
	linux-kernel@vger.kernel.org, selinux@vger.kernel.org,
	"Eric Paris" <eparis@parisplace.org>,
	"Jann Horn" <jannh@google.com>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] capabilities: Introduce CAP_CHECKPOINT_RESTORE
Date: Wed, 10 Jun 2020 00:59:28 -0700	[thread overview]
Message-ID: <20200610075928.GA172301@gmail.com> (raw)
In-Reply-To: <20200609161427.4eoozs3kkgablmaa@wittgenstein>

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