Re: [RFC 00/16] KVM protected memory extension

Sean Christopherson
2020-06-04T15:48:39Z

+Jun

On Thu, Jun 04, 2020 at 04:15:23PM +0100, Marc Zyngier wrote:
> Hi Kirill,
>
> Thanks for this.
>
> On Fri, 22 May 2020 15:51:58 +0300
> "Kirill A. Shutemov" <kirill@shutemov.name> wrote:
>
> > == Background / Problem ==
> >
> > There are a number of hardware features (MKTME, SEV) which protect guest
> > memory from some unauthorized host access. The patchset proposes a purely
> > software feature that mitigates some of the same host-side read-only
> > attacks.
> >
> >
> > == What does this set mitigate? ==
> >
> > - Host kernel ”accidental” access to guest data (think speculation)
> >
> > - Host kernel induced access to guest data (write(fd, &guest_data_ptr, len))
> >
> > - Host userspace access to guest data (compromised qemu)
> >
> > == What does this set NOT mitigate? ==
> >
> > - Full host kernel compromise. Kernel will just map the pages again.
> >
> > - Hardware attacks
>
> Just as a heads up, we (the Android kernel team) are currently
> involved in something pretty similar for KVM/arm64 in order to bring
> some level of confidentiality to guests.
>
> The main idea is to de-privilege the host kernel by wrapping it in its
> own nested set of page tables which allows us to remove memory
> allocated to guests on a per-page basis. The core hypervisor runs more
> or less independently at its own privilege level. It still is KVM
> though, as we don't intend to reinvent the wheel.
>
> Will has written a much more lingo-heavy description here:
> https://lore.kernel.org/kvmarm/20200327165935.GA8048@willie-the-truck/

Pardon my arm64 ignorance...

IIUC, in this mode, the host kernel runs at EL1? And to switch to a guest
it has to bounce through EL2, which is KVM, or at least a chunk of KVM?
I assume the EL1->EL2->EL1 switch is done by trapping an exception of some
form?

If all of the above are "yes", does KVM already have the necessary logic to
perform the EL1->EL2->EL1 switches, or is that being added as part of the
de-privileging effort?

> This works for one of the virtualization modes that arm64 can use (what
> we call non-VHE, or nVHE for short). The other mode (VHE), is much more
> similar to what happens on other architectures, where the kernel and
> the hypervisor are one single entity. In this case, we cannot use the
> same trick with nested page tables, and have to rely on something that
> would very much look like what you're proposing.
>
> Note that the two modes of the architecture would benefit from this
> work anyway, as I'd like the host to know that we've pulled memory
> from under its feet. Since you have done most of the initial work, I
> intend to give it a go on arm64 shortly and see what sticks.