linux-kernel - Re: [PATCH v2 05/22] KVM: x86: Retry to-be-emulated insn in "slow" unprotect path iff sp is zapped

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Message-ID: <20240906081755.okkq3hlrm6wibfxy@yy-desk-7060>
Date: Fri, 6 Sep 2024 16:17:55 +0800
From: Yuan Yao <yuan.yao@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Yuan Yao <yuan.yao@...el.com>
Subject: Re: [PATCH v2 05/22] KVM: x86: Retry to-be-emulated insn in "slow"
 unprotect path iff sp is zapped

On Fri, Aug 30, 2024 at 05:15:20PM -0700, Sean Christopherson wrote:
> Resume the guest and thus skip emulation of a non-PTE-writing instruction
> if and only if unprotecting the gfn actually zapped at least one shadow
> page.  If the gfn is write-protected for some reason other than shadow
> paging, attempting to unprotect the gfn will effectively fail, and thus
> retrying the instruction is all but guaranteed to be pointless.  This bug
> has existed for a long time, but was effectively fudged around by the
> retry RIP+address anti-loop detection.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/x86.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 966fb301d44b..c4cb6c6d605b 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8961,14 +8961,14 @@ static bool retry_instruction(struct x86_emulate_ctxt *ctxt,
>  	if (ctxt->eip == last_retry_eip && last_retry_addr == cr2_or_gpa)
>  		return false;
>
> +	if (!vcpu->arch.mmu->root_role.direct)
> +		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
> +
> +	if (!kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa)))
> +		return false;
> +

Reviewed-by: Yuan Yao <yuan.yao@...el.com>

>  	vcpu->arch.last_retry_eip = ctxt->eip;
>  	vcpu->arch.last_retry_addr = cr2_or_gpa;
> -
> -	if (!vcpu->arch.mmu->root_role.direct)
> -		gpa = kvm_mmu_gva_to_gpa_write(vcpu, cr2_or_gpa, NULL);
> -
> -	kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(gpa));
> -
>  	return true;
>  }
>
> --
> 2.46.0.469.g59c65b2a67-goog
>