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

[Steven Price]

On 20/05/2021 13:05, Catalin Marinas wrote:
> 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?

Yes - for future expansion. Marc asked for them[1]:

> I'd be keen on a couple of reserved __64s. Just in case...

[1] https://lore.kernel.org/r/87ft14xl9e.wl-maz%40kernel.org

>> 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.

Ah, I hadn't thought about that... No I don't believe gfn_to_pfn_prot()
will fault it into the guest.

>> +            } 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.

This is arguably worse as we'll be writing tags into the guest but
without setting PG_mte_tagged - so they'll be lost when the guest then
faults the pages in. Which sounds like it should break migration.

I think the below should be safe, and avoids the overhead of setting the
flag just for reads.

Thanks,

Steve

----8<----
                page = pfn_to_page(pfn);
                maddr = page_address(page);

                if (!write) {
                        if (test_bit(PG_mte_tagged, &page->flags))
                                num_tags = mte_copy_tags_to_user(tags, maddr,
                                                        MTE_GRANULES_PER_PAGE);
                        else
                                /* No tags in memory, so write zeros */
                                num_tags = MTE_GRANULES_PER_PAGE -
                                        clear_user(tag, MTE_GRANULES_PER_PAGE);
                        kvm_release_pfn_clean(pfn);
                } else {
                        num_tags = mte_copy_tags_from_user(maddr, tags,
                                                        MTE_GRANULES_PER_PAGE);
                        kvm_release_pfn_dirty(pfn);
                }

                if (num_tags != MTE_GRANULES_PER_PAGE) {
                        ret = -EFAULT;
                        goto out;
                }

                if (write)
                        test_and_set_bit(PG_mte_tagged, &page->flags);