Re: [PATCH] capabilities: Introduce CAP_RESTORE

From: Adrian Reber
Date: Wed May 27 2020 - 10:14:27 EST

On Tue, May 26, 2020 at 08:59:29AM -0500, Eric W. Biederman wrote:
> Adrian Reber <areber@xxxxxxxxxx> writes:
> > On Fri, May 22, 2020 at 09:40:37AM -0700, Casey Schaufler wrote:
> >> What are the other blockers? Are you going to suggest additional new
> >> capabilities to clear them?
> >
> > As mentioned somewhere else access to /proc/<pid>/map_files/ would be
> > helpful. Right now I am testing with a JVM and it works without root
> > just with the attached patch. Without access to /proc/<pid>/map_files/
> > not everything CRIU can do will actually work, but we are a lot closer
> > to what our users have been asking for.
> The current permission checks on /proc/<pid>/map_files/ are simply
> someone being over-cautious.
> Someone needs to think through the threat landscape and figure out what
> permission checks are actually needed.
> Making the permission check ns_capable instead of capable is a
> no-brainer. Figuring out which user_ns to test against might be a
> we bit harder.
> We could probably even allow the owner of the process to open the files
> but that requires someone doing the work of thinking through how
> being able to opening files that you have mmaped might be a problem.

As mentioned in the other thread, CRIU can work with read access to

> >> > There are probably a few more things guarded by CAP_SYS_ADMIN required
> >> > to run checkpoint/restore as non-root,
> >>
> >> If you need CAP_SYS_ADMIN anyway you're not gaining anything by
> >> separating out CAP_RESTORE.
> >
> > No, as described we can checkpoint and restore a JVM with this patch and
> > it also solves the problem the set_ns_last_pid fork() loop daemon tries
> > to solve. It is not enough to support the full functionality of CRIU as
> > map_files is also important, but we do not need CAP_SYS_ADMIN and
> > CAP_RESTORE. Only CAP_RESTORE would be necessary.
> >
> > With a new capability users can enable checkpoint/restore as non-root
> > without giving CRIU access to any of the other possibilities offered by
> > CAP_SYS_ADMIN. Setting a PID and map_files have been introduced for CRIU
> > and used to live behind CONFIG_CHECKPOINT_RESTORE. Having a capability
> > for checkpoint/restore would make it easier for CRIU users to run it as
> > non-root and make it very clear what is possible when giving CRIU the
> > new capability. No other things would be allowed than necessary for
> > checkpoint/restore. Setting a PID is most important for the restore part
> > and reading map_files would be helpful during checkpoint. So it actually
> > should be called CAP_CHECKPOINT_RESTORE as Christian mentioned in
> > another email.
> Please if one is for checkpoint and one is for restore asking for a pair
> of capabilities is probably more appropriate.

I will send out a v2 with a renamed capability soon and also include
map_files to be readable with that capability.

> >> > but by applying this patch I can
> >> > already checkpoint and restore processes as non-root. As there are
> >> > already multiple workarounds I would prefer to do it correctly in the
> >> > kernel to avoid that CRIU users are starting to invent more workarounds.
> >>
> >> You've presented a couple of really inappropriate implementations
> >> that would qualify as workarounds. But the other two are completely
> >> appropriate within the system security policy. They don't "get around"
> >> the problem, they use existing mechanisms as they are intended.
> >
> > I agree with the user namespace approach to be appropriate, but not the
> > CAP_SYS_ADMIN approach as CRIU only needs a tiny subset (2 things) of
> > what CAP_SYS_ADMIN allows.
> If we are only talking 2 things can you please include in your patchset
> a patch enabling those 2 things?

The two things are setting a PID via ns_last_pid/clone3() and reading

> But even more than this we need a request that asks not for the least
> you can possibly ask for but asks for what you need to do a good job.

Also in this thread Kamil mentioned that they also need calling prctl
with PR_SET_MM during restore in their production setup.

> I am having visions of a recurring discussion that says can we add one
> more permission check to CAP_RESTORE or CAP_CHECKPOINT when they are
> things we could know today.

I will prepare a new version of this patch using CAP_CHECKPOINT_RESTORE
for ns_last_pid/clone3(), map_files, and prctl with PR_SET_MM.