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

[Christian Brauner]

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.

        if (unlikely(data & PTRACE_O_SUSPEND_SECCOMP)) {
                if (!IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) ||
                    !IS_ENABLED(CONFIG_SECCOMP))
                        return -EINVAL;

                if (!capable(CAP_SYS_ADMIN))
                        return -EPERM;

                if (seccomp_mode(&current->seccomp) != SECCOMP_MODE_DISABLED ||
                    current->ptrace & PT_SUSPEND_SECCOMP)
                        return -EPERM;
        }

Christian