KVM: nVMX: Allow VMREAD when Enlightened VMCS is in use

Hyper-V TLFS explicitly forbids VMREAD and VMWRITE instructions when
Enlightened VMCS interface is in use:

"Any VMREAD or VMWRITE instructions while an enlightened VMCS is
active is unsupported and can result in unexpected behavior.""

Windows 11 + WSL2 seems to ignore this, attempts to VMREAD VMCS field
0x4404 ("VM-exit interruption information") are observed. Failing
these attempts with nested_vmx_failInvalid() makes such guests
unbootable.

Microsoft confirms this is a Hyper-V bug and claims that it'll get fixed
eventually but for the time being we need a workaround. (Temporary) allow
VMREAD to get data from the currently loaded Enlightened VMCS.

Note: VMWRITE instructions remain forbidden, it is not clear how to
handle them properly and hopefully won't ever be needed.

Reviewed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20220112170134.1904308-6-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/evmcs.h

@@ -98,6 +98,18 @@ static __always_inline int evmcs_field_offset(unsigned long field,
 	return evmcs_field->offset;
 }
 
+static inline u64 evmcs_read_any(struct hv_enlightened_vmcs *evmcs,
+				 unsigned long field, u16 offset)
+{
+	/*
+	 * vmcs12_read_any() doesn't care whether the supplied structure
+	 * is 'struct vmcs12' or 'struct hv_enlightened_vmcs' as it takes
+	 * the exact offset of the required field, use it for convenience
+	 * here.
+	 */
+	return vmcs12_read_any((void *)evmcs, field, offset);
+}
+
 #if IS_ENABLED(CONFIG_HYPERV)
 
 static __always_inline int get_evmcs_offset(unsigned long field,

arch/x86/kvm/vmx/nested.c

@@ -7,6 +7,7 @@
 #include <asm/mmu_context.h>
 
 #include "cpuid.h"
+#include "evmcs.h"
 #include "hyperv.h"
 #include "mmu.h"
 #include "nested.h"
@@ -5101,6 +5102,10 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
 	if (!nested_vmx_check_permission(vcpu))
 		return 1;
 
+	/* Decode instruction info and find the field to read */
+	field = kvm_register_read(vcpu, (((instr_info) >> 28) & 0xf));
+
+	if (!evmptr_is_valid(vmx->nested.hv_evmcs_vmptr)) {
 		/*
 		 * In VMX non-root operation, when the VMCS-link pointer is INVALID_GPA,
 		 * any VMREAD sets the ALU flags for VMfailInvalid.
@@ -5110,9 +5115,6 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
 		     get_vmcs12(vcpu)->vmcs_link_pointer == INVALID_GPA))
 			return nested_vmx_failInvalid(vcpu);
 
-	/* Decode instruction info and find the field to read */
-	field = kvm_register_read(vcpu, (((instr_info) >> 28) & 0xf));
-
 		offset = get_vmcs12_field_offset(field);
 		if (offset < 0)
 			return nested_vmx_fail(vcpu, VMXERR_UNSUPPORTED_VMCS_COMPONENT);
@@ -5122,6 +5124,27 @@ static int handle_vmread(struct kvm_vcpu *vcpu)
 
 		/* Read the field, zero-extended to a u64 value */
 		value = vmcs12_read_any(vmcs12, field, offset);
+	} else {
+		/*
+		 * Hyper-V TLFS (as of 6.0b) explicitly states, that while an
+		 * enlightened VMCS is active VMREAD/VMWRITE instructions are
+		 * unsupported. Unfortunately, certain versions of Windows 11
+		 * don't comply with this requirement which is not enforced in
+		 * genuine Hyper-V. Allow VMREAD from an enlightened VMCS as a
+		 * workaround, as misbehaving guests will panic on VM-Fail.
+		 * Note, enlightened VMCS is incompatible with shadow VMCS so
+		 * all VMREADs from L2 should go to L1.
+		 */
+		if (WARN_ON_ONCE(is_guest_mode(vcpu)))
+			return nested_vmx_failInvalid(vcpu);
+
+		offset = evmcs_field_offset(field, NULL);
+		if (offset < 0)
+			return nested_vmx_fail(vcpu, VMXERR_UNSUPPORTED_VMCS_COMPONENT);
+
+		/* Read the field, zero-extended to a u64 value */
+		value = evmcs_read_any(vmx->nested.hv_evmcs, field, offset);
+	}
 
 	/*
 	 * Now copy part of this value to register or memory, as requested.