[PATCH] KVM: nVMX: Hide shadow VMCS right after VMCLEAR

[Paolo Bonzini]

From: Hyunwoo Kim <imv4bel@xxxxxxxxx>

free_nested() frees the shadow VMCS while vmcs01 still points to it. But
because it is asynchronous with respect to loaded_vmcs_clear(), the vCPU
might migrate before the pointer is cleared and __loaded_vmcs_clear()
may then execute VMCLEAR.

The VMCS needs to stay attached until its explicit VMCLEAR completes, but
then it can be hidden and the page safely freed.

Fixes: 355f4fb1405e ("kvm: nVMX: VMCLEAR an active shadow VMCS after last use")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 220d42ebc82e..ddf6df7bee93 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -336,6 +336,7 @@ static void nested_put_vmcs12_pages(struct kvm_vcpu *vcpu)
 static void free_nested(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	struct vmcs *shadow_vmcs;
 
 	if (WARN_ON_ONCE(vmx->loaded_vmcs != &vmx->vmcs01))
 		vmx_switch_vmcs(vcpu, &vmx->vmcs01);
@@ -353,9 +354,15 @@ static void free_nested(struct kvm_vcpu *vcpu)
 	vmx->nested.current_vmptr = INVALID_GPA;
 	if (enable_shadow_vmcs) {
 		vmx_disable_shadow_vmcs(vmx);
-		vmcs_clear(vmx->vmcs01.shadow_vmcs);
-		free_vmcs(vmx->vmcs01.shadow_vmcs);
+
+		/*
+		 * Keep the pointer visible until after VMCLEAR, so migration
+		 * can clear an active shadow VMCS on the old CPU.
+		 */
+		shadow_vmcs = vmx->vmcs01.shadow_vmcs;
+		vmcs_clear(shadow_vmcs);
 		vmx->vmcs01.shadow_vmcs = NULL;
+		free_vmcs(shadow_vmcs);
 	}
 	kfree(vmx->nested.cached_vmcs12);
 	vmx->nested.cached_vmcs12 = NULL;
-- 
2.55.0