linux-kernel - Re: [RFC PATCH 00/35] SEV-ES hypervisor support

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Date:   Tue, 15 Sep 2020 17:19:26 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [RFC PATCH 00/35] SEV-ES hypervisor support

On Tue, Sep 15, 2020 at 12:22:05PM -0500, Tom Lendacky wrote:
> On 9/14/20 5:59 PM, Sean Christopherson wrote:
> > Given that we don't yet have publicly available KVM code for TDX, what if I
> > generate and post a list of ioctls() that are denied by either SEV-ES or TDX,
> > organized by the denier(s)?  Then for the ioctls() that are denied by one and
> > not the other, we add a brief explanation of why it's denied?
> > 
> > If that sounds ok, I'll get the list and the TDX side of things posted
> > tomorrow.
> 
> That sounds good.

TDX completely blocks the following ioctl()s:

  kvm_vcpu_ioctl_interrupt
  kvm_vcpu_ioctl_smi
  kvm_vcpu_ioctl_x86_setup_mce
  kvm_vcpu_ioctl_x86_set_mce
  kvm_vcpu_ioctl_x86_get_debugregs
  kvm_vcpu_ioctl_x86_set_debugregs
  kvm_vcpu_ioctl_x86_get_xsave
  kvm_vcpu_ioctl_x86_set_xsave
  kvm_vcpu_ioctl_x86_get_xcrs
  kvm_vcpu_ioctl_x86_set_xcrs
  kvm_arch_vcpu_ioctl_get_regs
  kvm_arch_vcpu_ioctl_set_regs
  kvm_arch_vcpu_ioctl_get_sregs
  kvm_arch_vcpu_ioctl_set_sregs
  kvm_arch_vcpu_ioctl_set_guest_debug
  kvm_arch_vcpu_ioctl_get_fpu
  kvm_arch_vcpu_ioctl_set_fpu

Looking through the code, I think kvm_arch_vcpu_ioctl_get_mpstate() and
kvm_arch_vcpu_ioctl_set_mpstate() should also be disallowed, we just haven't
actually done so.

There are also two helper functions that are "blocked".
dm_request_for_irq_injection() returns false if guest_state_protected, and
post_kvm_run_save() shoves dummy state.

TDX also selectively blocks/skips portions of other ioctl()s so that the
TDX code itself can yell loudly if e.g. .get_cpl() is invoked.  The event
injection restrictions are due to direct injection not being allowed (except
for NMIs); all IRQs have to be routed through APICv (posted interrupts) and
exception injection is completely disallowed.

  kvm_vcpu_ioctl_x86_get_vcpu_events:
	if (!vcpu->kvm->arch.guest_state_protected)
        	events->interrupt.shadow = kvm_x86_ops.get_interrupt_shadow(vcpu);

  kvm_arch_vcpu_put:
        if (vcpu->preempted && !vcpu->kvm->arch.guest_state_protected)
                vcpu->arch.preempted_in_kernel = !kvm_x86_ops.get_cpl(vcpu);

  kvm_vcpu_ioctl_x86_set_vcpu_events:
	u32 allowed_flags = KVM_VCPUEVENT_VALID_NMI_PENDING |
			    KVM_VCPUEVENT_VALID_SIPI_VECTOR |
			    KVM_VCPUEVENT_VALID_SHADOW |
			    KVM_VCPUEVENT_VALID_SMM |
			    KVM_VCPUEVENT_VALID_PAYLOAD;

	if (vcpu->kvm->arch.guest_state_protected)
		allowed_flags = KVM_VCPUEVENT_VALID_NMI_PENDING;


  kvm_arch_vcpu_ioctl_run:
	if (vcpu->kvm->arch.guest_state_protected)
		kvm_sync_valid_fields = KVM_SYNC_X86_EVENTS;
	else
		kvm_sync_valid_fields = KVM_SYNC_X86_VALID_FIELDS;


In addition to the more generic guest_state_protected, we also (obviously
tentatively) have a few other flags to deal with aspects of TDX that I'm
fairly certain don't apply to SEV-ES:

  tsc_immutable - KVM doesn't have write access to the TSC offset of the
                  guest.

  eoi_intercept_unsupported - KVM can't intercept EOIs (doesn't have access
                              to EOI bitmaps) and so can't support level
                              triggered interrupts, at least not without
                              extra pain.

  readonly_mem_unsupported - Secure EPT (analagous to SNP) requires RWX
                             permissions for all private/encrypted memory.
                             S-EPT isn't optional, so we get the joy of
                             adding this right off the bat...