KVM: nVMX: Allow setting the VMXE bit in CR4

This patch allows the guest to enable the VMXE bit in CR4, which is a
prerequisite to running VMXON.

Whether to allow setting the VMXE bit now depends on the architecture (svm
or vmx), so its checking has moved to kvm_x86_ops->set_cr4(). This function
now returns an int: If kvm_x86_ops->set_cr4() returns 1, __kvm_set_cr4()
will also return 1, and this will cause kvm_set_cr4() will throw a #GP.

Turning on the VMXE bit is allowed only when the nested VMX feature is
enabled, and turning it off is forbidden after a vmxon.

Signed-off-by: Nadav Har'El <nyh@il.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

4 files changed, 22 insertions, 7 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index ff17deb6e98b..d167039ecdf4 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -555,7 +555,7 @@ struct kvm_x86_ops {
         void (*decache_cr4_guest_bits)(struct kvm_vcpu *vcpu);
         void (*set_cr0)(struct kvm_vcpu *vcpu, unsigned long cr0);
         void (*set_cr3)(struct kvm_vcpu *vcpu, unsigned long cr3);
-        void (*set_cr4)(struct kvm_vcpu *vcpu, unsigned long cr4);
+        int (*set_cr4)(struct kvm_vcpu *vcpu, unsigned long cr4);
         void (*set_efer)(struct kvm_vcpu *vcpu, u64 efer);
         void (*get_idt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
         void (*set_idt)(struct kvm_vcpu *vcpu, struct desc_ptr *dt);
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 506e4fe23adc..475d1c948501 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1496,11 +1496,14 @@ static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
         update_cr0_intercept(svm);
 }
 
-static void svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+static int svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
         unsigned long host_cr4_mce = read_cr4() & X86_CR4_MCE;
         unsigned long old_cr4 = to_svm(vcpu)->vmcb->save.cr4;
 
+        if (cr4 & X86_CR4_VMXE)
+                return 1;
+
         if (npt_enabled && ((old_cr4 ^ cr4) & X86_CR4_PGE))
                 svm_flush_tlb(vcpu);
 
@@ -1510,6 +1513,7 @@ static void svm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
         cr4 |= host_cr4_mce;
         to_svm(vcpu)->vmcb->save.cr4 = cr4;
         mark_dirty(to_svm(vcpu)->vmcb, VMCB_CR);
+        return 0;
 }
 
 static void svm_set_segment(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 3a727ca02f24..eda2cb619c25 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2121,7 +2121,7 @@ static void ept_save_pdptrs(struct kvm_vcpu *vcpu)
                   (unsigned long *)&vcpu->arch.regs_dirty);
 }
 
-static void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4);
+static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4);
 
 static void ept_update_paging_mode_cr0(unsigned long *hw_cr0,
                                         unsigned long cr0,
@@ -2219,11 +2219,23 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
         vmcs_writel(GUEST_CR3, guest_cr3);
 }
 
-static void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
+static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 {
         unsigned long hw_cr4 = cr4 | (to_vmx(vcpu)->rmode.vm86_active ?
                     KVM_RMODE_VM_CR4_ALWAYS_ON : KVM_PMODE_VM_CR4_ALWAYS_ON);
 
+        if (cr4 & X86_CR4_VMXE) {
+                /*
+                 * To use VMXON (and later other VMX instructions), a guest
+                 * must first be able to turn on cr4.VMXE (see handle_vmon()).
+                 * So basically the check on whether to allow nested VMX
+                 * is here.
+                 */
+                if (!nested_vmx_allowed(vcpu))
+                        return 1;
+        } else if (to_vmx(vcpu)->nested.vmxon)
+                return 1;
+
         vcpu->arch.cr4 = cr4;
         if (enable_ept) {
                 if (!is_paging(vcpu)) {
@@ -2236,6 +2248,7 @@ static void vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
 
         vmcs_writel(CR4_READ_SHADOW, cr4);
         vmcs_writel(GUEST_CR4, hw_cr4);
+        return 0;
 }
 
 static void vmx_get_segment(struct kvm_vcpu *vcpu,
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d88de565d0c0..460932b62c5b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -615,11 +615,9 @@ int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
                                    kvm_read_cr3(vcpu)))
                 return 1;
 
-        if (cr4 & X86_CR4_VMXE)
+        if (kvm_x86_ops->set_cr4(vcpu, cr4))
                 return 1;
 
-        kvm_x86_ops->set_cr4(vcpu, cr4);
-
         if ((cr4 ^ old_cr4) & pdptr_bits)
                 kvm_mmu_reset_context(vcpu);