Re: [PATCH v12 7/8] KVM: arm64: ioctl to fetch/store tags in a guest

[Catalin Marinas]

On Mon, May 17, 2021 at 01:32:38PM +0100, Steven Price wrote:
> diff --git a/arch/arm64/include/uapi/asm/kvm.h 
> b/arch/arm64/include/uapi/asm/kvm.h
> index 24223adae150..b3edde68bc3e 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -184,6 +184,17 @@ struct kvm_vcpu_events {
>       __u32 reserved[12];
>  };
>  
> +struct kvm_arm_copy_mte_tags {
> +     __u64 guest_ipa;
> +     __u64 length;
> +     void __user *addr;
> +     __u64 flags;
> +     __u64 reserved[2];

I forgot the past discussions, what's the reserved for? Future
expansion?

> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index e89a5e275e25..4b6c83beb75d 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -1309,6 +1309,65 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm 
> *kvm,
>       }
>  }
>  
> +static int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> +                                   struct kvm_arm_copy_mte_tags *copy_tags)
> +{
> +     gpa_t guest_ipa = copy_tags->guest_ipa;
> +     size_t length = copy_tags->length;
> +     void __user *tags = copy_tags->addr;
> +     gpa_t gfn;
> +     bool write = !(copy_tags->flags & KVM_ARM_TAGS_FROM_GUEST);
> +     int ret = 0;
> +
> +     if (copy_tags->reserved[0] || copy_tags->reserved[1])
> +             return -EINVAL;
> +
> +     if (copy_tags->flags & ~KVM_ARM_TAGS_FROM_GUEST)
> +             return -EINVAL;
> +
> +     if (length & ~PAGE_MASK || guest_ipa & ~PAGE_MASK)
> +             return -EINVAL;
> +
> +     gfn = gpa_to_gfn(guest_ipa);
> +
> +     mutex_lock(&kvm->slots_lock);
> +
> +     while (length > 0) {
> +             kvm_pfn_t pfn = gfn_to_pfn_prot(kvm, gfn, write, NULL);
> +             void *maddr;
> +             unsigned long num_tags = PAGE_SIZE / MTE_GRANULE_SIZE;
> +
> +             if (is_error_noslot_pfn(pfn)) {
> +                     ret = -EFAULT;
> +                     goto out;
> +             }
> +
> +             maddr = page_address(pfn_to_page(pfn));
> +
> +             if (!write) {
> +                     num_tags = mte_copy_tags_to_user(tags, maddr, num_tags);
> +                     kvm_release_pfn_clean(pfn);

Do we need to check if PG_mte_tagged is set? If the page was not faulted
into the guest address space but the VMM has the page, does the
gfn_to_pfn_prot() guarantee that a kvm_set_spte_gfn() was called? If
not, this may read stale tags.

> +             } else {
> +                     num_tags = mte_copy_tags_from_user(maddr, tags,
> +                                                        num_tags);
> +                     kvm_release_pfn_dirty(pfn);
> +             }

Same question here, if the we can't guarantee the stage 2 pte being set,
we'd need to set PG_mte_tagged.

> +
> +             if (num_tags != PAGE_SIZE / MTE_GRANULE_SIZE) {
> +                     ret = -EFAULT;
> +                     goto out;
> +             }
> +
> +             gfn++;
> +             tags += num_tags;
> +             length -= PAGE_SIZE;
> +     }
> +
> +out:
> +     mutex_unlock(&kvm->slots_lock);
> +     return ret;
> +}
> +

-- 
Catalin