On 24/06/2021 14:35, Marc Zyngier wrote:
> Hi Steven,
>
> On Mon, 21 Jun 2021 12:17:15 +0100,
> Steven Price <steven.price@xxxxxxx> wrote:
>>
>> The VMM may not wish to have it's own mapping of guest memory mapped
>> with PROT_MTE because this causes problems if the VMM has tag checking
>> enabled (the guest controls the tags in physical RAM and it's unlikely
>> the tags are correct for the VMM).
>>
>> Instead add a new ioctl which allows the VMM to easily read/write the
>> tags from guest memory, allowing the VMM's mapping to be non-PROT_MTE
>> while the VMM can still read/write the tags for the purpose of
>> migration.
>>
>> Reviewed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
>> Signed-off-by: Steven Price <steven.price@xxxxxxx>
>> ---
>> arch/arm64/include/asm/kvm_host.h | 3 ++
>> arch/arm64/include/asm/mte-def.h | 1 +
>> arch/arm64/include/uapi/asm/kvm.h | 11 +++++
>> arch/arm64/kvm/arm.c | 7 +++
>> arch/arm64/kvm/guest.c | 82 +++++++++++++++++++++++++++++++
>> include/uapi/linux/kvm.h | 1 +
>> 6 files changed, 105 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 309e36cc1b42..6a2ac4636d42 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -729,6 +729,9 @@ int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
>> int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>> struct kvm_device_attr *attr);
>>
>> +long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>> + struct kvm_arm_copy_mte_tags *copy_tags);
>> +
>> /* Guest/host FPSIMD coordination helpers */
>> int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
>> void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu);
>> diff --git a/arch/arm64/include/asm/mte-def.h b/arch/arm64/include/asm/mte-def.h
>> index cf241b0f0a42..626d359b396e 100644
>> --- a/arch/arm64/include/asm/mte-def.h
>> +++ b/arch/arm64/include/asm/mte-def.h
>> @@ -7,6 +7,7 @@
>>
>> #define MTE_GRANULE_SIZE UL(16)
>> #define MTE_GRANULE_MASK (~(MTE_GRANULE_SIZE - 1))
>> +#define MTE_GRANULES_PER_PAGE (PAGE_SIZE / MTE_GRANULE_SIZE)
>> #define MTE_TAG_SHIFT 56
>> #define MTE_TAG_SIZE 4
>> #define MTE_TAG_MASK GENMASK((MTE_TAG_SHIFT + (MTE_TAG_SIZE - 1)), MTE_TAG_SHIFT)
>> 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];
>> +};
>> +
>> +#define KVM_ARM_TAGS_TO_GUEST 0
>> +#define KVM_ARM_TAGS_FROM_GUEST 1
>> +
>> /* If you need to interpret the index values, here is the key: */
>> #define KVM_REG_ARM_COPROC_MASK 0x000000000FFF0000
>> #define KVM_REG_ARM_COPROC_SHIFT 16
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 28ce26a68f09..511f3716fe33 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -1359,6 +1359,13 @@ long kvm_arch_vm_ioctl(struct file *filp,
>>
>> return 0;
>> }
>> + case KVM_ARM_MTE_COPY_TAGS: {
>> + struct kvm_arm_copy_mte_tags copy_tags;
>> +
>> + if (copy_from_user(©_tags, argp, sizeof(copy_tags)))
>> + return -EFAULT;
>> + return kvm_vm_ioctl_mte_copy_tags(kvm, ©_tags);
>> + }
>> default:
>> return -EINVAL;
>> }
>> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
>> index 5cb4a1cd5603..4ddb20017b2f 100644
>> --- a/arch/arm64/kvm/guest.c
>> +++ b/arch/arm64/kvm/guest.c
>> @@ -995,3 +995,85 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
>>
>> return ret;
>> }
>> +
>> +long 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 (!kvm_has_mte(kvm))
>> + return -EINVAL;
>> +
>> + 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;
>> + struct page *page;
>> +
>> + if (is_error_noslot_pfn(pfn)) {
>> + ret = -EFAULT;
>> + goto out;
>> + }
>> +
>> + page = pfn_to_online_page(pfn);
>> + if (!page) {
>> + /* Reject ZONE_DEVICE memory */
>> + ret = -EFAULT;
>> + goto out;
>> + }
>> + 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(tags, 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;
>> + }
>> +
>> + /* Set the flag after checking the write completed fully */
>> + if (write)
>> + set_bit(PG_mte_tagged, &page->flags);
>
> This ended up catching my eye as I was merging some other patches.
>
> This set_bit() occurs *after* the page has been released, meaning it
> could have been evicted and reused in the interval. I plan to fix it
> as below. Please let me know if that works for you.
>
> Thanks,
>
> M.
>
> From a78d3206378a7101659fbc2a4bf01cb9376c4793 Mon Sep 17 00:00:00 2001
> From: Marc Zyngier <maz@xxxxxxxxxx>
> Date: Thu, 24 Jun 2021 14:21:05 +0100
> Subject: [PATCH] KVM: arm64: Set the MTE tag bit before releasing the page
>
> Setting a page flag without holding a reference to the page
> is living dangerously. In the tag-writing path, we drop the
> reference to the page by calling kvm_release_pfn_dirty(),
> and only then set the PG_mte_tagged bit.
>
> It would be safer to do it the other way round.
>
> Fixes: f0376edb1ddca ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
> Cc: Steven Price <steven.price@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Well spotted - I'd originally had the set_bit there - but moved it down
when I realised it should only be set if the mte_copy_tags_from_user()
completed successfully. Obviously I hadn't noticed that the page
reference had gone by that point.
Thanks for fixing it. FWIW:
Reviewed-by: Steven Price <steven.price@xxxxxxx>
> ---
> arch/arm64/kvm/guest.c | 12 ++++++++----
> 1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index 4ddb20017b2f..60815ae477cf 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -1053,6 +1053,14 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> } else {
> num_tags = mte_copy_tags_from_user(maddr, tags,
> MTE_GRANULES_PER_PAGE);
> +
> + /*
> + * Set the flag after checking the write
> + * completed fully
> + */
> + if (num_tags == MTE_GRANULES_PER_PAGE)
> + set_bit(PG_mte_tagged, &page->flags);
> +
> kvm_release_pfn_dirty(pfn);
> }
>
> @@ -1061,10 +1069,6 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> goto out;
> }
>
> - /* Set the flag after checking the write completed fully */
> - if (write)
> - set_bit(PG_mte_tagged, &page->flags);
> -
> gfn++;
> tags += num_tags;
> length -= PAGE_SIZE;
>
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