linux-kernel - Re: [PATCH 2/9] Revert "KVM: SVM: avoid infinite loop on NPF from bad address"

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Date:   Thu, 20 Jan 2022 14:17:17 +0000
From:   Liam Merwick <liam.merwick@...cle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>
Subject: Re: [PATCH 2/9] Revert "KVM: SVM: avoid infinite loop on NPF from bad
 address"

On 20/01/2022 01:07, Sean Christopherson wrote:
> Revert a completely broken check on an "invalid" RIP in SVM's workaround
> for the DecodeAssists SMAP errata.  kvm_vcpu_gfn_to_memslot() obviously
> expects a gfn, i.e. operates in the guest physical address space, whereas
> RIP is a virtual (not even linear) address.  The "fix" worked for the
> problematic KVM selftest because the test identity mapped RIP.
> 
> Fully revert the hack instead of trying to translate RIP to a GPA, as the
> non-SEV case is now handled earlier, and KVM cannot access guest page
> tables to translate RIP.
> 
> This reverts commit e72436bc3a5206f95bb384e741154166ddb3202e.
> 
> Fixes: e72436bc3a52 ("KVM: SVM: avoid infinite loop on NPF from bad address")
> Reported-by: Liam Merwick <liam.merwick@...cle.com>
> Cc: stable@...r.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Liam Merwick <liam.merwick@...cle.com>


> ---
>   arch/x86/kvm/svm/svm.c | 7 -------
>   virt/kvm/kvm_main.c    | 1 -
>   2 files changed, 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index aa1649b8cd8f..85703145eb0a 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4311,13 +4311,6 @@ static bool svm_can_emulate_instruction(struct kvm_vcpu *vcpu, void *insn, int i
>   	if (likely(!insn || insn_len))
>   		return true;
>   
> -	/*
> -	 * If RIP is invalid, go ahead with emulation which will cause an
> -	 * internal error exit.
> -	 */
> -	if (!kvm_vcpu_gfn_to_memslot(vcpu, kvm_rip_read(vcpu) >> PAGE_SHIFT))
> -		return true;
> -
>   	cr4 = kvm_read_cr4(vcpu);
>   	smep = cr4 & X86_CR4_SMEP;
>   	smap = cr4 & X86_CR4_SMAP;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5a1164483e6c..0bacecda79cf 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2248,7 +2248,6 @@ struct kvm_memory_slot *kvm_vcpu_gfn_to_memslot(struct kvm_vcpu *vcpu, gfn_t gfn
>   
>   	return NULL;
>   }
> -EXPORT_SYMBOL_GPL(kvm_vcpu_gfn_to_memslot);
>   
>   bool kvm_is_visible_gfn(struct kvm *kvm, gfn_t gfn)
>   {