Re: [RFC PATCH] KVM: x86: Disallow KVM_SET_CPUID{2} if the vCPU is in guest mode

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Dec 18, 2019 at 11:38:43AM -0800, Jim Mattson wrote:
> On Wed, Dec 18, 2019 at 9:42 AM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > Reject KVM_SET_CPUID{2} with -EBUSY if the vCPU is in guest mode (L2) to
> > avoid complications and potentially undesirable KVM behavior.  Allowing
> > userspace to change a guest's capabilities while L2 is active would at
> > best result in unexpected behavior in the guest (L1 or L2), and at worst
> > induce bad KVM behavior by breaking fundamental assumptions regarding
> > transitions between L0, L1 and L2.
> 
> This seems a bit contrived. As long as we're breaking the ABI, can we
> disallow changes to CPUID once the vCPU has been powered on?

I can at least concoct scenarios where changing CPUID after KVM_RUN
provides value, e.g. effectively creating a new VM/vCPU without destroying
the kernel's underlying data structures and without putting the file
descriptors, for performance (especially if KVM avoids its hardware on/off
paths) or sandboxing (process has access to a VM fd, but not /dev/kvm).

A truly contrived, but technically architecturally accurate, scenario would
be modeling SGX interaction with the machine check architecutre.  Per the
SDM, #MCs or clearing bits in IA32_MCi_CTL disable SGX, which is reflected
in CPUID:

  Any machine check exception (#MC) that occurs after Intel SGX is first
  enables causes Intel SGX to be disabled, (CPUID.SGX_Leaf.0:EAX[SGX1] == 0)
  It cannot be enabled until after the next reset.

  Any act of clearing bits from '1 to '0 in any of the IA32_MCi_CTL register
  may disable Intel SGX (set CPUID.SGX_Leaf.0:EAX[SGX1] to 0) until the next
  reset.

I doubt a userspace VMM would actively model that behavior, but it's at
least theoretically possible.  Yes, it would technically be possible for
SGX to be disabled while L2 is active, but I don't think it's unreasonable
to require userspace to first force the vCPU out of L2.