[PATCH v2] KVM: nVMX: do not use dangling shadow VMCS after guest reset

[Paolo Bonzini]

If a KVM guest is reset while running a nested guest, free_nested will
disable the shadow VMCS execution control in the vmcs01.  However,
on the next KVM_RUN vmx_vcpu_run would nevertheless try to sync
the VMCS12 to the shadow VMCS which has since been freed.

This causes a vmptrld of a NULL pointer on my machime, but Jan reports
the host to hang altogether.  Let's see how much this trivial patch fixes.

Reported-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
Cc: Liran Alon <liran.alon@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 arch/x86/kvm/vmx/nested.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index 4f23e34f628b..0f1378789bd0 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -194,6 +194,7 @@ static void vmx_disable_shadow_vmcs(struct vcpu_vmx *vmx)
 {
 	secondary_exec_controls_clearbit(vmx, SECONDARY_EXEC_SHADOW_VMCS);
 	vmcs_write64(VMCS_LINK_POINTER, -1ull);
+	vmx->nested.need_vmcs12_to_shadow_sync = false;
 }
 
 static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
@@ -1341,6 +1342,9 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
 	unsigned long val;
 	int i;
 
+	if (WARN_ON(!shadow_vmcs))
+		return;
+
 	preempt_disable();
 
 	vmcs_load(shadow_vmcs);
@@ -1373,6 +1377,9 @@ static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
 	unsigned long val;
 	int i, q;
 
+	if (WARN_ON(!shadow_vmcs))
+		return;
+
 	vmcs_load(shadow_vmcs);
 
 	for (q = 0; q < ARRAY_SIZE(fields); q++) {
@@ -4436,7 +4443,6 @@ static inline void nested_release_vmcs12(struct kvm_vcpu *vcpu)
 		/* copy to memory all shadowed fields in case
 		   they were modified */
 		copy_shadow_to_vmcs12(vmx);
-		vmx->nested.need_vmcs12_to_shadow_sync = false;
 		vmx_disable_shadow_vmcs(vmx);
 	}
 	vmx->nested.posted_intr_nv = -1;
-- 
1.8.3.1