Re: [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only

[Andrea Arcangeli]

Hello everyone,

On Fri, May 08, 2020 at 12:54:03PM -0400, Michael S. Tsirkin wrote:
> On Fri, May 08, 2020 at 12:52:34PM -0400, Michael S. Tsirkin wrote:
> > On Wed, Apr 22, 2020 at 05:26:32PM -0700, Daniel Colascione wrote:
> > > This sysctl can be set to either zero or one. When zero (the default)
> > > the system lets all users call userfaultfd with or without
> > > UFFD_USER_MODE_ONLY, modulo other access controls. When
> > > unprivileged_userfaultfd_user_mode_only is set to one, users without
> > > CAP_SYS_PTRACE must pass UFFD_USER_MODE_ONLY to userfaultfd or the API
> > > will fail with EPERM. This facility allows administrators to reduce
> > > the likelihood that an attacker with access to userfaultfd can delay
> > > faulting kernel code to widen timing windows for other exploits.
> > > 
> > > Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
> > 
> > The approach taken looks like a hard-coded security policy.
> > For example, it won't be possible to set the sysctl knob
> > in question on any sytem running kvm. So this is
> > no good for any general purpose system.
> > 
> > What's wrong with using a security policy for this instead?
> 
> In fact I see the original thread already mentions selinux,
> so it's just a question of making this controllable by
> selinux.

I agree it'd be preferable if it was not hardcoded, but then this
patchset is also much simpler than the previous controlling it through
selinux..

I was thinking, an alternative policy that could control it without
hard-coding it, is a seccomp-bpf filter, then you can drop 2/2 as
well, not just 1/6-4/6.

If you keep only 1/2, can't seccomp-bpf enforce userfaultfd to be
always called with flags==0x1 without requiring extra modifications in
the kernel?

Can't you get the feature party with the CAP_SYS_PTRACE capability
too, if you don't wrap those tasks with the ptrace capability under
that seccomp filter?

As far as I can tell, it's unprecedented to create a flag for a
syscall API, with the only purpose of implementing a seccomp-bpf
filter verifying such flag is set, but then if you want to control it
with LSM it's even more complex than doing it with seccomp-bpf, and it
requires more kernel code too. We could always add 2/2 later, such
possibility won't disappear, in fact we could also add 1/6-4/6 later
too if that is not enough.

If we could begin by merging only 1/2 from this new series and be done
with the kernel changes, because we offload the rest of the work to
the kernel eBPF JIT, I think it'd be ideal.

Thanks,
Andrea