Re: [PATCH v3 6/7] KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE enabled

[Catalin Marinas <catalin.marinas@arm.com>]

On Wed, Aug 10, 2022 at 12:30:32PM -0700, Peter Collingbourne wrote:
> Certain VMMs such as crosvm have features (e.g. sandboxing) that depend
> on being able to map guest memory as MAP_SHARED. The current restriction
> on sharing MAP_SHARED pages with the guest is preventing the use of
> those features with MTE. Now that the races between tasks concurrently
> clearing tags on the same page have been fixed, remove this restriction.
> 
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> ---
>  arch/arm64/kvm/mmu.c | 8 --------
>  1 file changed, 8 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index d54be80e31dd..fc65dc20655d 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1075,14 +1075,6 @@ 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;

I think this is fine with the locking in place (BTW, it may be worth
mentioning in the commit message that it's a relaxation of the ABI). I'd
like Steven to have a look as well when he gets the time, in case we
missed anything on the KVM+MTE side.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
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