Re: [PATCH v16 3/7] KVM: arm64: Introduce MTE VM feature

[Steven Price]

On 21/06/2021 10:01, Marc Zyngier wrote:
> On Fri, 18 Jun 2021 14:28:22 +0100,
> Steven Price <steven.price@arm.com> wrote:
>>
>> Add a new VM feature 'KVM_ARM_CAP_MTE' which enables memory tagging
>> for a VM. This will expose the feature to the guest and automatically
>> tag memory pages touched by the VM as PG_mte_tagged (and clear the tag
>> storage) to ensure that the guest cannot see stale tags, and so that
>> the tags are correctly saved/restored across swap.
>>
>> Actually exposing the new capability to user space happens in a later
>> patch.
>>
>> Signed-off-by: Steven Price <steven.price@arm.com>
>> ---
>>  arch/arm64/include/asm/kvm_emulate.h |  3 ++
>>  arch/arm64/include/asm/kvm_host.h    |  3 ++
>>  arch/arm64/kvm/hyp/exception.c       |  3 +-
>>  arch/arm64/kvm/mmu.c                 | 62 +++++++++++++++++++++++++++-
>>  arch/arm64/kvm/sys_regs.c            |  7 ++++
>>  include/uapi/linux/kvm.h             |  1 +
>>  6 files changed, 77 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_emulate.h 
>> b/arch/arm64/include/asm/kvm_emulate.h
>> index f612c090f2e4..6bf776c2399c 100644
>> --- a/arch/arm64/include/asm/kvm_emulate.h
>> +++ b/arch/arm64/include/asm/kvm_emulate.h
>> @@ -84,6 +84,9 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
>>      if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE) ||
>>          vcpu_el1_is_32bit(vcpu))
>>              vcpu->arch.hcr_el2 |= HCR_TID2;
>> +
>> +    if (kvm_has_mte(vcpu->kvm))
>> +            vcpu->arch.hcr_el2 |= HCR_ATA;
>>  }
>>  
>>  static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)
>> diff --git a/arch/arm64/include/asm/kvm_host.h 
>> b/arch/arm64/include/asm/kvm_host.h
>> index 7cd7d5c8c4bc..afaa5333f0e4 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -132,6 +132,8 @@ struct kvm_arch {
>>  
>>      u8 pfr0_csv2;
>>      u8 pfr0_csv3;
>> +    /* Memory Tagging Extension enabled for the guest */
>> +    bool mte_enabled;
>>  };
>>  
>>  struct kvm_vcpu_fault_info {
>> @@ -769,6 +771,7 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
>>  #define kvm_arm_vcpu_sve_finalized(vcpu) \
>>      ((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
>>  
>> +#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled)
>>  #define kvm_vcpu_has_pmu(vcpu)                                      \
>>      (test_bit(KVM_ARM_VCPU_PMU_V3, (vcpu)->arch.features))
>>  
>> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
>> index 73629094f903..56426565600c 100644
>> --- a/arch/arm64/kvm/hyp/exception.c
>> +++ b/arch/arm64/kvm/hyp/exception.c
>> @@ -112,7 +112,8 @@ static void enter_exception64(struct kvm_vcpu *vcpu, 
>> unsigned long target_mode,
>>      new |= (old & PSR_C_BIT);
>>      new |= (old & PSR_V_BIT);
>>  
>> -    // TODO: TCO (if/when ARMv8.5-MemTag is exposed to guests)
>> +    if (kvm_has_mte(vcpu->kvm))
>> +            new |= PSR_TCO_BIT;
>>  
>>      new |= (old & PSR_DIT_BIT);
>>  
>> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
>> index c5d1f3c87dbd..f5305b7561ad 100644
>> --- a/arch/arm64/kvm/mmu.c
>> +++ b/arch/arm64/kvm/mmu.c
>> @@ -822,6 +822,45 @@ transparent_hugepage_adjust(struct kvm_memory_slot 
>> *memslot,
>>      return PAGE_SIZE;
>>  }
>>  
>> +/*
>> + * The page will be mapped in stage 2 as Normal Cacheable, so the VM will be
>> + * able to see the page's tags and therefore they must be initialised 
>> first. If
>> + * PG_mte_tagged is set, tags have already been initialised.
>> + *
>> + * The race in the test/set of the PG_mte_tagged flag is handled by:
>> + * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
>> + *   racing to santise the same page
>> + * - mmap_lock protects between a VM faulting a page in and the VMM 
>> performing
>> + *   an mprotect() to add VM_MTE
>> + */
>> +static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
>> +                         unsigned long size)
>> +{
>> +    unsigned long i, nr_pages = size >> PAGE_SHIFT;
>> +    struct page *page;
>> +
>> +    if (!kvm_has_mte(kvm))
>> +            return 0;
>> +
>> +    /*
>> +     * pfn_to_online_page() is used to reject ZONE_DEVICE pages
>> +     * that may not support tags.
>> +     */
>> +    page = pfn_to_online_page(pfn);
>> +
>> +    if (!page)
>> +            return -EFAULT;
>> +
>> +    for (i = 0; i < nr_pages; i++, page++) {
>> +            if (!test_bit(PG_mte_tagged, &page->flags)) {
>> +                    mte_clear_page_tags(page_address(page));
>> +                    set_bit(PG_mte_tagged, &page->flags);
>> +            }
>> +    }
>> +
>> +    return 0;
>> +}
>> +
>>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>>                        struct kvm_memory_slot *memslot, unsigned long hva,
>>                        unsigned long fault_status)
>> @@ -971,8 +1010,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, 
>> phys_addr_t fault_ipa,
>>      if (writable)
>>              prot |= KVM_PGTABLE_PROT_W;
>>  
>> -    if (fault_status != FSC_PERM && !device)
>> +    if (fault_status != FSC_PERM && !device) {
>> +            /* Check the VMM hasn't introduced a new VM_SHARED VMA */
>> +            if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED)
>> +                    return -EINVAL;
> 
> nit: I'd rather we return -EFAULT here. That's consistent with other
> cases where we can't satisfy the mapping.

Sure, and it would be even better if I dropped the lock (goto
out_unlock) - I'll fix that too!

Thanks,

Steve