Re: [PATCH v17 5/6] KVM: arm64: ioctl to fetch/store tags in a guest

[Steven Price]

On 24/06/2021 14:35, Marc Zyngier wrote:
> Hi Steven,
> 
> On Mon, 21 Jun 2021 12:17:15 +0100,
> Steven Price <steven.price@arm.com> 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@arm.com>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>>  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(&copy_tags, argp, sizeof(copy_tags)))
>> +                    return -EFAULT;
>> +            return kvm_vm_ioctl_mte_copy_tags(kvm, &copy_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@kernel.org>
> 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@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>

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@arm.com>

> ---
>  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;
>