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Subject: Re: [PATCH] KVM: nVMX: do not use dangling shadow VMCS after guest
 reset
From:   Liran Alon <liran.alon@oracle.com>
In-Reply-To: <1563554534-46556-3-git-send-email-pbonzini@redhat.com>
Date:   Sat, 20 Jul 2019 00:01:42 +0300
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
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Message-Id: <6D1C57BE-1A1B-4714-B4E5-E0569A60FD1F@oracle.com>
References: <1563554534-46556-3-git-send-email-pbonzini@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On 19 Jul 2019, at 19:42, Paolo Bonzini <pbonzini@redhat.com> wrote:
>=20
> 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.
>=20
> 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.
>=20
> Reported-by: Jan Kiszka <jan.kiszka@siemens.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

First, nested_release_vmcs12() also sets need_vmcs12_to_shadow_sync to =
false explicitly. This can now be removed.

Second, I suggest putting a WARN_ON_ONCE() on copy_vmcs12_to_shadow() in =
case shadow_vmcs=3D=3DNULL.
To assist catching these kind of errors more easily in the future.

Besides that, the fix seems correct to me.
Reviewed-by: Liran Alon <liran.alon@oracle.com>

-Liran

> ---
> arch/x86/kvm/vmx/nested.c | 1 +
> 1 file changed, 1 insertion(+)
>=20
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 6e88f459b323..6119b30347c6 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 =3D false;
> }
>=20
> static inline void nested_release_evmcs(struct kvm_vcpu *vcpu)
> --=20
> 1.8.3.1
>=20