KVM: nVMX: Fix setting of CR0 and CR4 in guest mode

The logic for calculating the value with which we call kvm_set_cr0/4 was
broken (will definitely be visible with nested unrestricted guest mode
support). Also, we performed the check regarding CR0_ALWAYSON too early
when in guest mode.

What really needs to be done on both CR0 and CR4 is to mask out L1-owned
bits and merge them in from L1's guest_cr0/4. In contrast, arch.cr0/4
and arch.cr0/4_guest_owned_bits contain the mangled L0+L1 state and,
thus, are not suited as input.

For both CRs, we can then apply the check against VMXON_CRx_ALWAYSON and
refuse the update if it fails. To be fully consistent, we implement this
check now also for CR4. For CR4, we move the check into vmx_set_cr4
while we keep it in handle_set_cr0. This is because the CR0 checks for
vmxon vs. guest mode will diverge soon when adding unrestricted guest
mode support.

Finally, we have to set the shadow to the value L2 wanted to write
originally.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 33 insertions, 15 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index a9d885353108..260da9ac1678 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -3223,7 +3223,9 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
                  */
                 if (!nested_vmx_allowed(vcpu))
                         return 1;
-        } else if (to_vmx(vcpu)->nested.vmxon)
+        }
+        if (to_vmx(vcpu)->nested.vmxon &&
+            ((cr4 & VMXON_CR4_ALWAYSON) != VMXON_CR4_ALWAYSON))
                 return 1;
 
         vcpu->arch.cr4 = cr4;
@@ -4612,34 +4614,50 @@ vmx_patch_hypercall(struct kvm_vcpu *vcpu, unsigned char *hypercall)
 /* called to set cr0 as appropriate for a mov-to-cr0 exit. */
 static int handle_set_cr0(struct kvm_vcpu *vcpu, unsigned long val)
 {
-        if (to_vmx(vcpu)->nested.vmxon &&
-            ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
-                return 1;
-
         if (is_guest_mode(vcpu)) {
+                struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
+                unsigned long orig_val = val;
+
                 /*
                  * We get here when L2 changed cr0 in a way that did not change
                  * any of L1's shadowed bits (see nested_vmx_exit_handled_cr),
-                 * but did change L0 shadowed bits. This can currently happen
-                 * with the TS bit: L0 may want to leave TS on (for lazy fpu
-                 * loading) while pretending to allow the guest to change it.
+                 * but did change L0 shadowed bits. So we first calculate the
+                 * effective cr0 value that L1 would like to write into the
+                 * hardware. It consists of the L2-owned bits from the new
+                 * value combined with the L1-owned bits from L1's guest_cr0.
                  */
-                if (kvm_set_cr0(vcpu, (val & vcpu->arch.cr0_guest_owned_bits) |
-                         (vcpu->arch.cr0 & ~vcpu->arch.cr0_guest_owned_bits)))
+                val = (val & ~vmcs12->cr0_guest_host_mask) |
+                        (vmcs12->guest_cr0 & vmcs12->cr0_guest_host_mask);
+
+                /* TODO: will have to take unrestricted guest mode into
+                 * account */
+                if ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON)
                         return 1;
-                vmcs_writel(CR0_READ_SHADOW, val);
+
+                if (kvm_set_cr0(vcpu, val))
+                        return 1;
+                vmcs_writel(CR0_READ_SHADOW, orig_val);
                 return 0;
-        } else
+        } else {
+                if (to_vmx(vcpu)->nested.vmxon &&
+                    ((val & VMXON_CR0_ALWAYSON) != VMXON_CR0_ALWAYSON))
+                        return 1;
                 return kvm_set_cr0(vcpu, val);
+        }
 }
 
 static int handle_set_cr4(struct kvm_vcpu *vcpu, unsigned long val)
 {
         if (is_guest_mode(vcpu)) {
-                if (kvm_set_cr4(vcpu, (val & vcpu->arch.cr4_guest_owned_bits) |
-                         (vcpu->arch.cr4 & ~vcpu->arch.cr4_guest_owned_bits)))
+                struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
+                unsigned long orig_val = val;
+
+                /* analogously to handle_set_cr0 */
+                val = (val & ~vmcs12->cr4_guest_host_mask) |
+                        (vmcs12->guest_cr4 & vmcs12->cr4_guest_host_mask);
+                if (kvm_set_cr4(vcpu, val))
                         return 1;
-                vmcs_writel(CR4_READ_SHADOW, val);
+                vmcs_writel(CR4_READ_SHADOW, orig_val);
                 return 0;
         } else
                 return kvm_set_cr4(vcpu, val);