linux-kernel - Re: [PATCH] KVM: VMX: fix SMEP and SMAP without EPT

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Date:	Tue, 3 Nov 2015 10:45:01 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>,
	linux-kernel@...r.kernel.org
Cc:	kvm@...r.kernel.org, Feng Wu <feng.wu@...el.com>,
	Dongxiao Xu <dongxiao.xu@...el.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] KVM: VMX: fix SMEP and SMAP without EPT



On 02/11/2015 22:20, Radim Krčmář wrote:
> The comment in code had it mostly right, but we enable paging for
> emulated real mode regardless of EPT.
> 
> Without EPT (which implies emulated real mode), secondary VCPUs won't
> start unless we disable SM[AE]P when the guest doesn't use paging.
> 
> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b680c2e0e8a3..ab598558a7a4 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -3788,20 +3788,21 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>  		if (!is_paging(vcpu)) {
>  			hw_cr4 &= ~X86_CR4_PAE;
>  			hw_cr4 |= X86_CR4_PSE;
> -			/*
> -			 * SMEP/SMAP is disabled if CPU is in non-paging mode
> -			 * in hardware. However KVM always uses paging mode to
> -			 * emulate guest non-paging mode with TDP.
> -			 * To emulate this behavior, SMEP/SMAP needs to be
> -			 * manually disabled when guest switches to non-paging
> -			 * mode.
> -			 */
> -			hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP);
>  		} else if (!(cr4 & X86_CR4_PAE)) {
>  			hw_cr4 &= ~X86_CR4_PAE;
>  		}
>  	}
>  
> +	if (!enable_unrestricted_guest && !is_paging(vcpu))
> +		/*
> +		 * SMEP/SMAP is disabled if CPU is in non-paging mode in
> +		 * hardware.  However KVM always uses paging mode without
> +		 * unrestricted guest.
> +		 * To emulate this behavior, SMEP/SMAP needs to be manually
> +		 * disabled when guest switches to non-paging mode.
> +		 */
> +		hw_cr4 &= ~(X86_CR4_SMEP | X86_CR4_SMAP);
> +
>  	vmcs_writel(CR4_READ_SHADOW, cr4);
>  	vmcs_writel(GUEST_CR4, hw_cr4);
>  	return 0;
> 

Applied with Cc: stable@...r.kernel.org.

Paolo
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