linux-kernel - [PATCH 2/2] KVM: VMX: fix memory order between loading vmcs and clearing vmcs

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Date:	Wed, 28 Nov 2012 20:54:14 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
CC:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: [PATCH 2/2] KVM: VMX: fix memory order between loading vmcs and clearing
 vmcs

vmcs->cpu indicates whether it exists on the target cpu, -1 means the vmcs
does not exist on any vcpu

If vcpu load vmcs with vmcs.cpu = -1, it can be directly added to cpu's percpu
list. The list can be corrupted if the cpu prefetch the vmcs's list before
reading vmcs->cpu. Meanwhile, we should remove vmcs from the list before
making vmcs->vcpu == -1 be visible

Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
---
 arch/x86/kvm/vmx.c |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 29e8f42..6056d88 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -1002,6 +1002,15 @@ static void __loaded_vmcs_clear(void *arg)
 	if (per_cpu(current_vmcs, cpu) == loaded_vmcs->vmcs)
 		per_cpu(current_vmcs, cpu) = NULL;
 	list_del(&loaded_vmcs->loaded_vmcss_on_cpu_link);
+
+	/*
+	 * we should ensure updating loaded_vmcs->loaded_vmcss_on_cpu_link
+	 * is before setting loaded_vmcs->vcpu to -1 which is done in
+	 * loaded_vmcs_init. Otherwise, other cpu can see vcpu = -1 fist
+	 * then adds the vmcs into percpu list before it is deleted.
+	 */
+	smp_wmb();
+
 	loaded_vmcs_init(loaded_vmcs);
 }

@@ -1537,6 +1546,14 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

 		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
 		local_irq_disable();
+
+		/*
+		 * Read loaded_vmcs->cpu should be before fetching
+		 * loaded_vmcs->loaded_vmcss_on_cpu_link.
+		 * See the comments in __loaded_vmcs_clear().
+		 */
+		smp_rmb();
+
 		list_add(&vmx->loaded_vmcs->loaded_vmcss_on_cpu_link,
 			 &per_cpu(loaded_vmcss_on_cpu, cpu));
 		local_irq_enable();
-- 
1.7.7.6

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