Re: [PATCH] userfaultfd, capability: introduce CAP_USERFAULTFD

On 2/25/2022 10:17 AM, Axel Rasmussen wrote:

Thanks for the detailed explanation Casey!

On Thu, Feb 24, 2022 at 6:58 PM Peter Xu [off-list ref] wrote:

quoted

On Thu, Feb 24, 2022 at 04:39:44PM -0800, Casey Schaufler wrote:

quoted

What I'd want to see is multiple users where the use of CAP_USERFAULTD
is independent of the use of CAP_SYS_PTRACE. That is, the programs would
never require CAP_SYS_PTRACE. There should be demonstrated real value.
Not just that a compromised program with CAP_SYS_PTRACE can do bad things,
but that the programs with CAP_USERFAULTDD are somehow susceptible to
being exploited to doing those bad things. Hypothetical users are just
that, and often don't materialize.

I kind of have the same question indeed..

The use case we're talking about is VM migration, and the in-question
subject is literally the migration process or thread.  Isn't that a trusted
piece of software already?

Then the question is why the extra capability (in CAP_PTRACE but not in
CAP_UFFD) could bring much risk to the system.  Axel, did I miss something
important?

For me it's just a matter of giving the live migration process as
little power as I can while still letting it do its job.

That's understood. But live migration is a bit of a special case,
and as mentioned above, is trusted to do an oodle of important stuff
correctly.

Live migration is somewhat trusted, and certainly if it can mess with
the memory contents of its own VM, that's no concern. But there are
other processes or threads running alongside it to manage other parts
of the VM, like attached virtual disks. Also it's probably running on
a server which also hosts other VMs, and I think it's a common design
to have them all run as the same user (although, they may be running
in other containers).

That seems unwise. I am often surprised how we're eager to add
new security features to make up for the unwillingness of people
to use the existing ones.

So, it seems unfortunate to me that the live migration process can
just ptrace() any of these other things running alongside it.

I get that. On the other hand, most of the systems you'll run
live migration on are going to have full-up root processes,
possibly even userfaultd (in spite of instructions not to do so).

Casey is right that we can restrict what it can do with e.g. SELinux
or seccomp-ebpf or whatever else. But it seems to me a more fragile
design to give the permissions and then restrict them, vs. just never
giving those permissions in the first place.

If we lived in a universe with a root-less reality I'd agree.

In any case though, it sounds like folks are more amenable to the
device node approach. Honestly, I got that impression from Andrea as
well when we first talked about this some months ago. So, I can pursue
that approach instead.

I think that's more realistic.

quoted

Thanks,
--
Peter Xu