linux-kernel - RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported

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Date:   Mon, 24 Jan 2022 07:06:27 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     "Liu, Jing2" <jing2.liu@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for
 vcpu->arch.guest_supported_xcr0

> From: Like Xu <like.xu.linux@...il.com>
> Sent: Sunday, January 23, 2022 1:50 PM
> 
> From: Like Xu <likexu@...cent.com>
> 
> A malicious user space can bypass xstate_get_guest_group_perm() in the
> KVM_GET_SUPPORTED_CPUID mechanism and obtain unpermitted xfeatures,
> since the validity check of xcr0 depends only on guest_supported_xcr0.

Unpermitted xfeatures cannot pass kvm_check_cpuid()...

> 
> Fixes: 445ecdf79be0 ("kvm: x86: Exclude unpermitted xfeatures at
> KVM_GET_SUPPORTED_CPUID")
> Signed-off-by: Like Xu <likexu@...cent.com>
> ---
>  arch/x86/kvm/cpuid.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 3902c28fb6cb..1bd4d560cbdd 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -266,7 +266,8 @@ static void kvm_vcpu_after_set_cpuid(struct
> kvm_vcpu *vcpu)
>  		vcpu->arch.guest_supported_xcr0 = 0;
>  	else
>  		vcpu->arch.guest_supported_xcr0 =
> -			(best->eax | ((u64)best->edx << 32)) &
> supported_xcr0;
> +			(best->eax | ((u64)best->edx << 32)) &
> +			(supported_xcr0 & xstate_get_guest_group_perm());
> 
>  	/*
>  	 * Bits 127:0 of the allowed SECS.ATTRIBUTES (CPUID.0x12.0x1)
> enumerate
> --
> 2.33.1