linux-kernel - Re: [PATCH MANUALSEL 5.16 4/8] KVM: nVMX: WARN on any attempt to allocate shadow VMCS for vmcs02

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Message-ID: <fd3ffa55-5421-fd18-9dc2-82805b694e14@redhat.com>
Date:   Thu, 10 Feb 2022 17:33:21 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, kvm@...r.kernel.org
Subject: Re: [PATCH MANUALSEL 5.16 4/8] KVM: nVMX: WARN on any attempt to
 allocate shadow VMCS for vmcs02

On 2/9/22 19:56, Sasha Levin wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> [ Upstream commit d6e656cd266cdcc95abd372c7faef05bee271d1a ]
> 
> WARN if KVM attempts to allocate a shadow VMCS for vmcs02.  KVM emulates
> VMCS shadowing but doesn't virtualize it, i.e. KVM should never allocate
> a "real" shadow VMCS for L2.
> 
> The previous code WARNed but continued anyway with the allocation,
> presumably in an attempt to avoid NULL pointer dereference.
> However, alloc_vmcs (and hence alloc_shadow_vmcs) can fail, and
> indeed the sole caller does:
> 
> 	if (enable_shadow_vmcs && !alloc_shadow_vmcs(vcpu))
> 		goto out_shadow_vmcs;
> 
> which makes it not a useful attempt.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Message-Id: <20220125220527.2093146-1-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> Signed-off-by: Sasha Levin <sashal@...nel.org>
> ---
>   arch/x86/kvm/vmx/nested.c | 22 ++++++++++++----------
>   1 file changed, 12 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index c605c2c01394b..9cd68e1fcf602 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -4827,18 +4827,20 @@ static struct vmcs *alloc_shadow_vmcs(struct kvm_vcpu *vcpu)
>   	struct loaded_vmcs *loaded_vmcs = vmx->loaded_vmcs;
>   
>   	/*
> -	 * We should allocate a shadow vmcs for vmcs01 only when L1
> -	 * executes VMXON and free it when L1 executes VMXOFF.
> -	 * As it is invalid to execute VMXON twice, we shouldn't reach
> -	 * here when vmcs01 already have an allocated shadow vmcs.
> +	 * KVM allocates a shadow VMCS only when L1 executes VMXON and frees it
> +	 * when L1 executes VMXOFF or the vCPU is forced out of nested
> +	 * operation.  VMXON faults if the CPU is already post-VMXON, so it
> +	 * should be impossible to already have an allocated shadow VMCS.  KVM
> +	 * doesn't support virtualization of VMCS shadowing, so vmcs01 should
> +	 * always be the loaded VMCS.
>   	 */
> -	WARN_ON(loaded_vmcs == &vmx->vmcs01 && loaded_vmcs->shadow_vmcs);
> +	if (WARN_ON(loaded_vmcs != &vmx->vmcs01 || loaded_vmcs->shadow_vmcs))
> +		return loaded_vmcs->shadow_vmcs;
> +
> +	loaded_vmcs->shadow_vmcs = alloc_vmcs(true);
> +	if (loaded_vmcs->shadow_vmcs)
> +		vmcs_clear(loaded_vmcs->shadow_vmcs);
>   
> -	if (!loaded_vmcs->shadow_vmcs) {
> -		loaded_vmcs->shadow_vmcs = alloc_vmcs(true);
> -		if (loaded_vmcs->shadow_vmcs)
> -			vmcs_clear(loaded_vmcs->shadow_vmcs);
> -	}
>   	return loaded_vmcs->shadow_vmcs;
>   }
>   

NACK, it's just extra care but not particularly useful.

Paolo