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

[Andrei Vagin]

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:
> > On Wed, Jun 03, 2020 at 06:23:26PM +0200, Adrian Reber wrote:
> > > This patch introduces CAP_CHECKPOINT_RESTORE, a new capability facilitating
> > > checkpoint/restore for non-root users.
> > > 
> > > Over the last years, The CRIU (Checkpoint/Restore In Userspace) team has been
> > > asked numerous times if it is possible to checkpoint/restore a process as
> > > non-root. The answer usually was: 'almost'.
> > > 
> > > The main blocker to restore a process as non-root was to control the PID of the
> > > restored process. This feature available via the clone3 system call, or via
> > > /proc/sys/kernel/ns_last_pid is unfortunately guarded by CAP_SYS_ADMIN.
> > > 
> > > In the past two years, requests for non-root checkpoint/restore have increased
> > > due to the following use cases:
> > > * 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.
> > > 
> > ...
> > > 
> > > The introduced capability allows to:
> > > * Control PIDs when the current user is CAP_CHECKPOINT_RESTORE capable
> > >   for the corresponding PID namespace via ns_last_pid/clone3.
> > > * Open files in /proc/pid/map_files when the current user is
> > >   CAP_CHECKPOINT_RESTORE capable in the root namespace, useful for recovering
> > >   files that are unreachable via the file system such as deleted files, or memfd
> > >   files.
> > 
> > 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.

Why do you think so bad about me;). 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;

If we want to think about how to convert this capable to ns_capable, we
need to do this in a separate series. And the logic may be that a
process is able to suspend only filters that have been added from the
current user-namespace or its descendants. But we need to think about
this more carefully, maybe there are more pitfalls.