Re: [PATCH v4 1/3] capabilities: Introduce CAP_CHECKPOINT_RESTORE

[Adrian Reber <areber@redhat.com>]

On Wed, Jul 01, 2020 at 10:27:08AM +0200, Christian Brauner wrote:
> On Wed, Jul 01, 2020 at 08:49:04AM +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.
> > 
> > We have seen the following workarounds:
> > * Use a setuid wrapper around CRIU:
> >   See https://github.com/FredHutch/slurm-examples/blob/master/checkpointer/lib/checkpointer/checkpointer-suid.c
> > * Use a setuid helper that writes to ns_last_pid.
> >   Unfortunately, this helper delegation technique is impossible to use with
> >   clone3, and is thus prone to races.
> >   See https://github.com/twosigma/set_ns_last_pid
> > * Cycle through PIDs with fork() until the desired PID is reached:
> >   This has been demonstrated to work with cycling rates of 100,000 PIDs/s
> >   See https://github.com/twosigma/set_ns_last_pid
> > * Patch out the CAP_SYS_ADMIN check from the kernel
> > * Run the desired application in a new user and PID namespace to provide
> >   a local CAP_SYS_ADMIN for controlling PIDs. This technique has limited use in
> >   typical container environments (e.g., Kubernetes) as /proc is
> >   typically protected with read-only layers (e.g., /proc/sys) for hardening
> >   purposes. Read-only layers prevent additional /proc mounts (due to proc's
> >   SB_I_USERNS_VISIBLE property), making the use of new PID namespaces limited as
> >   certain applications need access to /proc matching their PID namespace.
> > 
> > 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.
> > 
> > See corresponding selftest for an example with clone3().
> > 
> > Signed-off-by: Adrian Reber <areber@redhat.com>
> > Signed-off-by: Nicolas Viennot <Nicolas.Viennot@twosigma.com>
> > ---
> 
> I think that now looks reasonable. A few comments.
> 
> Before we proceed, please split the addition of
> checkpoint_restore_ns_capable() out into a separate patch.
> In fact, I think the cleanest way of doing this would be:
> - 0/n capability: add CAP_CHECKPOINT_RESTORE
> - 1/n pid: use checkpoint_restore_ns_capable() for set_tid
> - 2/n pid_namespace: use checkpoint_restore_ns_capable() for ns_last_pid
> - 3/n: proc: require checkpoint_restore_ns_capable() in init userns for map_files
> 
> (commit subjects up to you of course) and a nice commit message for each
> time we relax a permissions on something so we have a clear separate
> track record for each change in case we need to revert something. Then
> the rest of the patches in this series. Testing patches probably last.

Yes, makes sense. I was thinking about this already, but I was not sure
if it I should do it or not. But I had the same idea already.

		Adrian