KVM: TDX: Handle EPT violation/misconfig exit

For TDX, on EPT violation, call common __vmx_handle_ept_violation() to
trigger x86 MMU code; on EPT misconfiguration, bug the VM since it
shouldn't happen.

EPT violation due to instruction fetch should never be triggered from
shared memory in TDX guest. If such EPT violation occurs, treat it as
broken hardware.

EPT misconfiguration shouldn't happen on neither shared nor secure EPT for
TDX guests.
- TDX module guarantees no EPT misconfiguration on secure EPT. Per TDX
module v1.5 spec section 9.4 "Secure EPT Induced TD Exits":
"By design, since secure EPT is fully controlled by the TDX module, an
EPT misconfiguration on a private GPA indicates a TDX module bug and is
handled as a fatal error."
- For shared EPT, the MMIO caching optimization, which is the only case
where current KVM configures EPT entries to generate EPT
misconfiguration, is implemented in a different way for TDX guests. KVM
configures EPT entries to non-present value without suppressing #VE bit.
It causes #VE in the TDX guest and the guest will call TDG.VP.VMCALL to
request MMIO emulation.

Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Isaku Yamahata <[e-mail]>
Co-developed-by: Adrian Hunter <[e-mail]>
Signed-off-by: Adrian Hunter <[e-mail]>
[binbin: rework changelog]
Co-developed-by: Binbin Wu <[e-mail]>
Signed-off-by: Binbin Wu <[e-mail]>
Message-ID: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/vmx/tdx.c

···
 			return EXIT_REASON_VMCALL;
 
 		return tdcall_to_vmx_exit_reason(vcpu);
+	case EXIT_REASON_EPT_MISCONFIG:
+		/*
+		 * Defer KVM_BUG_ON() until tdx_handle_exit() because this is in
+		 * non-instrumentable code with interrupts disabled.
+		 */
+		return -1u;
 	default:
 		break;
 	}
···
 	tdx->guest_entered = true;
 
 	vcpu->arch.regs_avail &= TDX_REGS_AVAIL_SET;
+
+	if (unlikely(tdx->vp_enter_ret == EXIT_REASON_EPT_MISCONFIG))
+		return EXIT_FASTPATH_NONE;
 
 	if (unlikely((tdx->vp_enter_ret & TDX_SW_ERROR) == TDX_SW_ERROR))
 		return EXIT_FASTPATH_NONE;
···
 	trace_kvm_apicv_accept_irq(vcpu->vcpu_id, delivery_mode, trig_mode, vector);
 }
 
+static int tdx_handle_ept_violation(struct kvm_vcpu *vcpu)
+{
+	unsigned long exit_qual;
+	gpa_t gpa = to_tdx(vcpu)->exit_gpa;
+
+	if (vt_is_tdx_private_gpa(vcpu->kvm, gpa)) {
+		/*
+		 * Always treat SEPT violations as write faults.  Ignore the
+		 * EXIT_QUALIFICATION reported by TDX-SEAM for SEPT violations.
+		 * TD private pages are always RWX in the SEPT tables,
+		 * i.e. they're always mapped writable.  Just as importantly,
+		 * treating SEPT violations as write faults is necessary to
+		 * avoid COW allocations, which will cause TDAUGPAGE failures
+		 * due to aliasing a single HPA to multiple GPAs.
+		 */
+		exit_qual = EPT_VIOLATION_ACC_WRITE;
+	} else {
+		exit_qual = vmx_get_exit_qual(vcpu);
+		/*
+		 * EPT violation due to instruction fetch should never be
+		 * triggered from shared memory in TDX guest.  If such EPT
+		 * violation occurs, treat it as broken hardware.
+		 */
+		if (KVM_BUG_ON(exit_qual & EPT_VIOLATION_ACC_INSTR, vcpu->kvm))
+			return -EIO;
+	}
+
+	trace_kvm_page_fault(vcpu, gpa, exit_qual);
+	return __vmx_handle_ept_violation(vcpu, gpa, exit_qual);
+}
+
 int tdx_handle_exit(struct kvm_vcpu *vcpu, fastpath_t fastpath)
 {
 	struct vcpu_tdx *tdx = to_tdx(vcpu);
···
 
 	if (fastpath != EXIT_FASTPATH_NONE)
 		return 1;
+
+	if (unlikely(vp_enter_ret == EXIT_REASON_EPT_MISCONFIG)) {
+		KVM_BUG_ON(1, vcpu->kvm);
+		return -EIO;
+	}
 
 	/*
 	 * Handle TDX SW errors, including TDX_SEAMCALL_UD, TDX_SEAMCALL_GP and
···
 		return tdx_emulate_io(vcpu);
 	case EXIT_REASON_EPT_MISCONFIG:
 		return tdx_emulate_mmio(vcpu);
+	case EXIT_REASON_EPT_VIOLATION:
+		return tdx_handle_ept_violation(vcpu);
 	case EXIT_REASON_OTHER_SMI:
 		/*
 		 * Unlike VMX, SMI in SEAM non-root mode (i.e. when