linux-kernel - Re: [PATCH v3 09/11] KVM: x86/mmu: serialize vCPUs to zap gfn when guest MTRRs are honored

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Message-ID: <ZJy7pyQYmuYTXC67@google.com>
Date:   Wed, 28 Jun 2023 16:00:55 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, chao.gao@...el.com, kai.huang@...el.com,
        robert.hoo.linux@...il.com
Subject: Re: [PATCH v3 09/11] KVM: x86/mmu: serialize vCPUs to zap gfn when
 guest MTRRs are honored

On Fri, Jun 16, 2023, Yan Zhao wrote:
> diff --git a/arch/x86/kvm/mtrr.c b/arch/x86/kvm/mtrr.c
> index b35dd0bc9cad..688748e3a4d2 100644
> --- a/arch/x86/kvm/mtrr.c
> +++ b/arch/x86/kvm/mtrr.c
> @@ -25,6 +25,8 @@
>  #define IA32_MTRR_DEF_TYPE_FE		(1ULL << 10)
>  #define IA32_MTRR_DEF_TYPE_TYPE_MASK	(0xff)
>  
> +static void kvm_mtrr_zap_gfn_range(struct kvm_vcpu *vcpu,
> +				   gfn_t gfn_start, gfn_t gfn_end);
>  static bool is_mtrr_base_msr(unsigned int msr)
>  {
>  	/* MTRR base MSRs use even numbers, masks use odd numbers. */
> @@ -341,7 +343,7 @@ static void update_mtrr(struct kvm_vcpu *vcpu, u32 msr)
>  		var_mtrr_range(var_mtrr_msr_to_range(vcpu, msr), &start, &end);
>  	}
>  
> -	kvm_zap_gfn_range(vcpu->kvm, gpa_to_gfn(start), gpa_to_gfn(end));
> +	kvm_mtrr_zap_gfn_range(vcpu, gpa_to_gfn(start), gpa_to_gfn(end));
>  }
>  
>  static bool var_mtrr_range_is_valid(struct kvm_mtrr_range *range)
> @@ -437,6 +439,11 @@ int kvm_mtrr_get_msr(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>  void kvm_vcpu_mtrr_init(struct kvm_vcpu *vcpu)
>  {
>  	INIT_LIST_HEAD(&vcpu->arch.mtrr_state.head);
> +
> +	if (vcpu->vcpu_id == 0) {

Eww.  This is actually unsafe, because kvm_arch_vcpu_create() is invoked without
holding kvm->lock.  Oh, and vcpu_id is userspace controlled, so it's *very*
unsafe.  Just initialize these in kvm_arch_init_vm().

> +		spin_lock_init(&vcpu->kvm->arch.mtrr_zap_list_lock);
> +		INIT_LIST_HEAD(&vcpu->kvm->arch.mtrr_zap_list);
> +	}
>  }