linux-kernel - Re: [PATCH v4 1/5] KVM: arm64: Block cacheable PFNMAP mapping

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Message-ID: <aCy3OwBe1HLa4O-7@arm.com>
Date: Tue, 20 May 2025 18:09:15 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: ankita@...dia.com
Cc: jgg@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
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Subject: Re: [PATCH v4 1/5] KVM: arm64: Block cacheable PFNMAP mapping

On Sun, May 18, 2025 at 05:47:50AM +0000, ankita@...dia.com wrote:
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 2feb6c6b63af..eaac4db61828 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1466,6 +1466,15 @@ static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
>  	return vma->vm_flags & VM_MTE_ALLOWED;
>  }
>  
> +/*
> + * Determine the memory region cacheability from VMA's pgprot. This
> + * is used to set the stage 2 PTEs.
> + */
> +static unsigned long mapping_type(pgprot_t page_prot)
> +{
> +	return FIELD_GET(PTE_ATTRINDX_MASK, pgprot_val(page_prot));
> +}
> +
>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  			  struct kvm_s2_trans *nested,
>  			  struct kvm_memory_slot *memslot, unsigned long hva,
> @@ -1612,6 +1621,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  
>  	vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
>  
> +	if ((vma->vm_flags & VM_PFNMAP) &&
> +	    mapping_type(vma->vm_page_prot) == MT_NORMAL)
> +		return -EINVAL;
> +
>  	/* Don't use the VMA after the unlock -- it may have vanished */
>  	vma = NULL;
>  
> @@ -2207,6 +2220,12 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  				ret = -EINVAL;
>  				break;
>  			}
> +
> +			/* Cacheable PFNMAP is not allowed */
> +			if (mapping_type(vma->vm_page_prot) == MT_NORMAL) {
> +				ret = -EINVAL;
> +				break;
> +			}

Should we capture MT_NORMAL_TAGGED as well? Presumably no driver sets
VM_MTE_ALLOWED but the same could be set about MT_NORMAL in the
vm_page_prot.

That said, we might as well invert the check, allow if MT_DEVICE_* or
MT_NORMAL_NC (the latter based on VM_ALLOW_ANY_UNCACHED).

-- 
Catalin