linux-kernel - Re: [PATCH] KVM: x86/mmu: Reject direct bits in gpa passed to KVM_PRE_FAULT

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Message-ID: <cfd99e56-551b-49c5-b486-05c9f6d8cf11@intel.com>
Date: Thu, 12 Jun 2025 13:29:40 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: seanjc@...gle.com, yan.y.zhao@...el.com
Subject: Re: [PATCH] KVM: x86/mmu: Reject direct bits in gpa passed to
 KVM_PRE_FAULT_MEMORY

On 6/12/2025 12:49 PM, Paolo Bonzini wrote:
> Only let userspace pass the same addresses that were used in KVM_SET_USER_MEMORY_REGION
> (or KVM_SET_USER_MEMORY_REGION2); gpas in the the upper half of the address space
> are an implementation detail of TDX and KVM.
> 
> Extracted from a patch by Sean Christopherson <seanjc@...gle.com>.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>   arch/x86/kvm/mmu/mmu.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index a4040578b537..4e06e2e89a8f 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4903,6 +4903,9 @@ long kvm_arch_vcpu_pre_fault_memory(struct kvm_vcpu *vcpu,
>   	if (!vcpu->kvm->arch.pre_fault_allowed)
>   		return -EOPNOTSUPP;
>   
> +	if (kvm_is_gfn_alias(vcpu->kvm, gpa_to_gfn(range->gpa)))
> +		return -EINVAL;

Do we need to worry about the case (range->gpa + range->size) becomes alias?

>   	/*
>   	 * reload is efficient when called repeatedly, so we can do it on
>   	 * every iteration.