linux-kernel - Re: [PATCH 1/2] KVM: X86: Fix CR3 reserve bits

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Date:   Sun, 13 May 2018 00:53:10 -0700 (PDT)
From:   Liran Alon <liran.alon@...cle.com>
To:     <kernellwp@...il.com>
Cc:     <rkrcmar@...hat.com>, <pbonzini@...hat.com>,
        <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>,
        <junaids@...gle.com>
Subject: Re: [PATCH 1/2] KVM: X86: Fix CR3 reserve bits


----- kernellwp@...il.com wrote:

> From: Wanpeng Li <wanpengli@...cent.com>
> 
> MSB of CR3 is a reserved bit if the PCIDE bit is not set in CR4. 
> It should be checked when PCIDE bit is not set, however commit 
> 'd1cd3ce900441 ("KVM: MMU: check guest CR3 reserved bits based on 
> its physical address width")' removes the bit 63 checking 
> unconditionally. This patch fixes it by checking bit 63 of CR3 
> when PCIDE bit is not set in CR4.
> 
> Fixes: d1cd3ce900441 (KVM: MMU: check guest CR3 reserved bits based on
> its physical address width)
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Junaid Shahid <junaids@...gle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/emulate.c | 4 +++-
>  arch/x86/kvm/x86.c     | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index b3705ae..b21f427 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4189,7 +4189,9 @@ static int check_cr_write(struct
> x86_emulate_ctxt *ctxt)
>  				maxphyaddr = eax & 0xff;
>  			else
>  				maxphyaddr = 36;
> -			rsvd = rsvd_bits(maxphyaddr, 62);
> +			if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_PCIDE)
> +				new_val &= ~CR3_PCID_INVD;
> +			rsvd = rsvd_bits(maxphyaddr, 63);

I would prefer instead to do this:
if (ctxt->ops->get_cr(ctxt, 4) & X86_CR4_PCIDE)
    rsvd &= ~CR3_PCID_INVD;
It makes more sense as opposed to temporary removing the CR3_PCID_INVD bit from new_val.

>  		}
>  
>  		if (new_val & rsvd)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 87e4805..9a90668 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -863,7 +863,7 @@ int kvm_set_cr3(struct kvm_vcpu *vcpu, unsigned
> long cr3)
>  	}
>  
>  	if (is_long_mode(vcpu) &&
> -	    (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 62)))
> +	    (cr3 & rsvd_bits(cpuid_maxphyaddr(vcpu), 63)))
>  		return 1;
>  	else if (is_pae(vcpu) && is_paging(vcpu) &&
>  		   !load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3))
> -- 
> 2.7.4