linux-kernel - [PATCH 5/7] KVM: nVMX: Ensure vmcs01 is the loaded VMCS when freeing nested state

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Date:   Tue, 14 Jul 2020 21:05:55 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dan Cross <dcross@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: [PATCH 5/7] KVM: nVMX: Ensure vmcs01 is the loaded VMCS when freeing nested state

Add a WARN in free_nested() to ensure vmcs01 is loaded prior to freeing
vmcs02 and friends, and explicitly switch to vmcs01 if it's not.  KVM is
supposed to keep is_guest_mode() and loaded_vmcs==vmcs02 synchronized,
but bugs happen and freeing vmcs02 while it's in use will escalate a KVM
error to a use-after-free and potentially crash the kernel.

Do the WARN and switch even in the !vmxon case to help detect latent
bugs.  free_nested() is not a hot path, and the check is cheap.

Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---
 arch/x86/kvm/vmx/nested.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index e9b27c6478da3..5734bff1a5907 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -279,6 +279,9 @@ static void free_nested(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
+	if (WARN_ON_ONCE(vmx->loaded_vmcs != &vmx->vmcs01))
+		vmx_switch_vmcs(vcpu, &vmx->vmcs01);
+
 	if (!vmx->nested.vmxon && !vmx->nested.smm.vmxon)
 		return;
 
-- 
2.26.0