[PATCH] KVM: nVMX: Disable preemption while reading from shadow VMCS

[Jan Kiszka <jan.kiszka@siemens.com>]

In order to access the shadow VMCS, we need to load it. At this point,
vmx->loaded_vmcs->vmcs and the actually loaded one start to differ. If
we now get preempted by Linux, vmx_vcpu_put and, on return, the
vmx_vcpu_load will work against the wrong vmcs. That can cause
copy_shadow_to_vmcs12 to corrupt the vmcs12 state.

Fix the issue by disabling preemption during the copy operation.

copy_vmcs12_to_shadow is safe from this issue as it is executed by
vmx_vcpu_run when preemption is already disabled before vmentry.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
---

This fixes specifically Jailhouse in KVM on CPUs with shadow VMCS
support.

 arch/x86/kvm/vmx.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 04fa1b8..f3de106 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -6417,6 +6417,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
 	const unsigned long *fields = shadow_read_write_fields;
 	const int num_fields = max_shadow_read_write_fields;
 
+	preempt_disable();
+
 	vmcs_load(shadow_vmcs);
 
 	for (i = 0; i < num_fields; i++) {
@@ -6440,6 +6442,8 @@ static void copy_shadow_to_vmcs12(struct vcpu_vmx *vmx)
 
 	vmcs_clear(shadow_vmcs);
 	vmcs_load(vmx->loaded_vmcs->vmcs);
+
+	preempt_enable();
 }
 
 static void copy_vmcs12_to_shadow(struct vcpu_vmx *vmx)
-- 
1.8.4.5

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