linux-kernel - Re: [PATCH] KVM: x86: cleanup CR3 reserved bits checks

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Message-ID: <YBmbM8PToDWr9ti/@google.com>
Date:   Tue, 2 Feb 2021 10:34:27 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: cleanup CR3 reserved bits checks

On Tue, Feb 02, 2021, Paolo Bonzini wrote:
> If not in long mode, the low bits of CR3 are reserved but not enforced to
> be zero, so remove those checks.  If in long mode, however, the MBZ bits
> extend down to the highest physical address bit of the guest, excluding
> the encryption bit.
> 
> Make the checks consistent with the above, and match them between
> nested_vmcb_checks and KVM_SET_SREGS.
> 

Fixes + Cc:stable@?

> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

Reviewed-by: Sean Christopherson <seanjc@...gle.com> 

> ---
>  arch/x86/kvm/svm/nested.c | 12 ++----------
>  arch/x86/kvm/svm/svm.h    |  3 ---
>  arch/x86/kvm/x86.c        |  2 ++
>  3 files changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index eecb548bdda6..9ee542ea3f56 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -244,18 +244,10 @@ static bool nested_vmcb_checks(struct vcpu_svm *svm, struct vmcb *vmcb12)
>  
>  	vmcb12_lma = (vmcb12->save.efer & EFER_LME) && (vmcb12->save.cr0 & X86_CR0_PG);
>  
> -	if (!vmcb12_lma) {
> -		if (vmcb12->save.cr4 & X86_CR4_PAE) {
> -			if (vmcb12->save.cr3 & MSR_CR3_LEGACY_PAE_RESERVED_MASK)
> -				return false;
> -		} else {
> -			if (vmcb12->save.cr3 & MSR_CR3_LEGACY_RESERVED_MASK)
> -				return false;
> -		}
> -	} else {
> +	if (vmcb12_lma) {
>  		if (!(vmcb12->save.cr4 & X86_CR4_PAE) ||
>  		    !(vmcb12->save.cr0 & X86_CR0_PE) ||
> -		    (vmcb12->save.cr3 & MSR_CR3_LONG_MBZ_MASK))
> +		    (vmcb12->save.cr3 & svm->vcpu.arch.cr3_lm_rsvd_bits))

Gah, I was too slow as usual.  I have a series to clean up GPA validity checks,
this one included.  I'll base that series on this patch, if I get it sent before
this hits kvm/queue...

>  			return false;
>  	}
>  	if (!kvm_is_valid_cr4(&svm->vcpu, vmcb12->save.cr4))
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 0fe874ae5498..6e7d070f8b86 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -403,9 +403,6 @@ static inline bool gif_set(struct vcpu_svm *svm)
>  }
>  
>  /* svm.c */
> -#define MSR_CR3_LEGACY_RESERVED_MASK		0xfe7U
> -#define MSR_CR3_LEGACY_PAE_RESERVED_MASK	0x7U
> -#define MSR_CR3_LONG_MBZ_MASK			0xfff0000000000000U
>  #define MSR_INVALID				0xffffffffU
>  
>  extern int sev;
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b748bf0d6d33..97674204bf44 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9660,6 +9660,8 @@ static bool kvm_is_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
>  		 */
>  		if (!(sregs->cr4 & X86_CR4_PAE) || !(sregs->efer & EFER_LMA))
>  			return false;
> +		if (sregs->cr3 & vcpu->arch.cr3_lm_rsvd_bits)
> +			return false;
>  	} else {
>  		/*
>  		 * Not in 64-bit mode: EFER.LMA is clear and the code
> -- 
> 2.26.2
>