Subject: Re: [PATCH v12 4/8] arm64: kvm: Introduce MTE VM feature

From: Marc Zyngier <[email-masked]>
Date: Mon, 17 May 2021 17:45:46 +0100

On Mon, 17 May 2021 13:32:35 +0100,
Steven Price <[email-masked]> 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.

uber nit in $SUBJECT: "KVM: arm64:" is the preferred prefix (just like
patches 7 and 8).

> 
> Signed-off-by: Steven Price <[email-masked]>
> ---
>  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                 | 37 +++++++++++++++++++++++++++-
>  arch/arm64/kvm/sys_regs.c            |  3 +++
>  include/uapi/linux/kvm.h             |  1 +
>  6 files changed, 48 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..8660f6a03f51 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -822,6 +822,31 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
>  	return PAGE_SIZE;
>  }
>  
> +static int sanitise_mte_tags(struct kvm *kvm, unsigned long size,
> +			     kvm_pfn_t pfn)

Nit: please order the parameters as address, then size.

> +{
> +	if (kvm_has_mte(kvm)) {
> +		/*
> +		 * 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.
> +		 */
> +		unsigned long i, nr_pages = size >> PAGE_SHIFT;
> +		struct page *page = pfn_to_online_page(pfn);
> +
> +		if (!page)
> +			return -EFAULT;

Under which circumstances can this happen? We already have done a GUP
on the page, so I really can't see how the page can vanish from under
our feet.

> +
> +		for (i = 0; i < nr_pages; i++, page++) {
> +			if (!test_and_set_bit(PG_mte_tagged, &page->flags))
> +				mte_clear_page_tags(page_address(page));

You seem to be doing this irrespective of the VMA being created with
PROT_MTE. This is fine form a guest perspective (all its memory should
be MTE capable). However, I can't see any guarantee that the VMM will
actually allocate memslots with PROT_MTE.

Aren't we missing some sanity checks at memslot registration time?

> +		}
> +	}
> +
> +	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 +996,13 @@ 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) {
> +		ret = sanitise_mte_tags(kvm, vma_pagesize, pfn);
> +		if (ret)
> +			goto out_unlock;
> +
>  		clean_dcache_guest_page(pfn, vma_pagesize);
> +	}
>  
>  	if (exec_fault) {
>  		prot |= KVM_PGTABLE_PROT_X;
> @@ -1168,12 +1198,17 @@ bool kvm_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>  bool kvm_set_spte_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
>  {
>  	kvm_pfn_t pfn = pte_pfn(range->pte);
> +	int ret;
>  
>  	if (!kvm->arch.mmu.pgt)
>  		return 0;
>  
>  	WARN_ON(range->end - range->start != 1);
>  
> +	ret = sanitise_mte_tags(kvm, PAGE_SIZE, pfn);
> +	if (ret)
> +		return ret;

Notice the change in return type?

> +
>  	/*
>  	 * We've moved a page around, probably through CoW, so let's treat it
>  	 * just like a translation fault and clean the cache to the PoC.
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 76ea2800c33e..24a844cb79ca 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1047,6 +1047,9 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
>  		break;
>  	case SYS_ID_AA64PFR1_EL1:
>  		val &= ~FEATURE(ID_AA64PFR1_MTE);
> +		if (kvm_has_mte(vcpu->kvm))
> +			val |= FIELD_PREP(FEATURE(ID_AA64PFR1_MTE),
> +					  ID_AA64PFR1_MTE);

Shouldn't this be consistent with what the HW is capable of
(i.e. FEAT_MTE3 if available), and extracted from the sanitised view
of the feature set?

>  		break;
>  	case SYS_ID_AA64ISAR1_EL1:
>  		if (!vcpu_has_ptrauth(vcpu))
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 3fd9a7e9d90c..8c95ba0fadda 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1082,6 +1082,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_SGX_ATTRIBUTE 196
>  #define KVM_CAP_VM_COPY_ENC_CONTEXT_FROM 197
>  #define KVM_CAP_PTP_KVM 198
> +#define KVM_CAP_ARM_MTE 199
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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