linux-kernel - Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM

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Message-ID: <16368a89-99ea-e52c-47b6-bd006933ec1f@redhat.com>
Date:   Fri, 26 Nov 2021 13:20:28 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] KVM: x86: Forbid KVM_SET_CPUID{,2} after KVM_RUN

On 11/22/21 18:58, Vitaly Kuznetsov wrote:
> -	 * KVM does not correctly handle changing guest CPUID after KVM_RUN, as
> -	 * MAXPHYADDR, GBPAGES support, AMD reserved bit behavior, etc.. aren't
> -	 * tracked in kvm_mmu_page_role.  As a result, KVM may miss guest page
> -	 * faults due to reusing SPs/SPTEs.  Alert userspace, but otherwise
> -	 * sweep the problem under the rug.
> -	 *
> -	 * KVM's horrific CPUID ABI makes the problem all but impossible to
> -	 * solve, as correctly handling multiple vCPU models (with respect to
> -	 * paging and physical address properties) in a single VM would require
> -	 * tracking all relevant CPUID information in kvm_mmu_page_role.  That
> -	 * is very undesirable as it would double the memory requirements for
> -	 * gfn_track (see struct kvm_mmu_page_role comments), and in practice
> -	 * no sane VMM mucks with the core vCPU model on the fly.
> +	 * Changing guest CPUID after KVM_RUN is forbidden, see the comment in
> +	 * kvm_arch_vcpu_ioctl().
>   	 */

The second part of the comment still applies to kvm_mmu_after_set_cpuid 
more than to kvm_arch_vcpu_ioctl().

>  		r = -EFAULT;
> [...]
> +		if (vcpu->arch.last_vmentry_cpu != -1)
> +			goto out;
> +
>  		if (copy_from_user(&cpuid, cpuid_arg, sizeof(cpuid)))
>  			goto out;
>  		r = kvm_vcpu_ioctl_set_cpuid(vcpu, &cpuid, cpuid_arg->entries);

This should be an EINVAL.

Tweaked and queued nevertheless, thanks.

Paolo