Re: [PATCH v3 5/7] KVM: arm64: unify the tests for VMAs in memslots when MTE is enabled

[Steven Price <steven.price@arm.com>]

On 10/08/2022 20:30, Peter Collingbourne wrote:
> Previously we allowed creating a memslot containing a private mapping that
> was not VM_MTE_ALLOWED, but would later reject KVM_RUN with -EFAULT. Now
> we reject the memory region at memslot creation time.
> 
> Since this is a minor tweak to the ABI (a VMM that created one of
> these memslots would fail later anyway), no VMM to my knowledge has
> MTE support yet, and the hardware with the necessary features is not
> generally available, we can probably make this ABI change at this point.
> 
> Signed-off-by: Peter Collingbourne <pcc@google.com>

Reviewed-by: Steven Price <steven.price@arm.com>

> ---
>  arch/arm64/kvm/mmu.c | 25 ++++++++++++++++---------
>  1 file changed, 16 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 750a69a97994..d54be80e31dd 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1073,6 +1073,19 @@ static void sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>  	}
>  }
>  
> +static bool kvm_vma_mte_allowed(struct vm_area_struct *vma)
> +{
> +	/*
> +	 * VM_SHARED mappings are not allowed with MTE to avoid races
> +	 * when updating the PG_mte_tagged page flag, see
> +	 * sanitise_mte_tags for more details.
> +	 */
> +	if (vma->vm_flags & VM_SHARED)
> +		return false;
> +
> +	return vma->vm_flags & VM_MTE_ALLOWED;
> +}
> +
>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  			  struct kvm_memory_slot *memslot, unsigned long hva,
>  			  unsigned long fault_status)
> @@ -1249,9 +1262,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	}
>  
>  	if (fault_status != FSC_PERM && !device && kvm_has_mte(kvm)) {
> -		/* Check the VMM hasn't introduced a new VM_SHARED VMA */
> -		if ((vma->vm_flags & VM_MTE_ALLOWED) &&
> -		    !(vma->vm_flags & VM_SHARED)) {
> +		/* Check the VMM hasn't introduced a new disallowed VMA */
> +		if (kvm_vma_mte_allowed(vma)) {
>  			sanitise_mte_tags(kvm, pfn, vma_pagesize);
>  		} else {
>  			ret = -EFAULT;
> @@ -1695,12 +1707,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  		if (!vma)
>  			break;
>  
> -		/*
> -		 * VM_SHARED mappings are not allowed with MTE to avoid races
> -		 * when updating the PG_mte_tagged page flag, see
> -		 * sanitise_mte_tags for more details.
> -		 */
> -		if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED) {
> +		if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) {
>  			ret = -EINVAL;
>  			break;
>  		}

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