Re: [PATCH v17 2/6] KVM: arm64: Introduce MTE VM feature

[Marc Zyngier <maz@kernel.org>]

On Mon, 21 Jun 2021 18:00:20 +0100,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi,
> 
> On Mon, Jun 21, 2021 at 12:18 PM 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.
> >
> > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> > 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                 | 64 +++++++++++++++++++++++++++-
> >  arch/arm64/kvm/sys_regs.c            |  7 +++
> >  include/uapi/linux/kvm.h             |  1 +
> >  6 files changed, 79 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
> > index 01b9857757f2..fd418955e31e 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;
> >  };
> 
> nit: newline before the comment/new member
> 
> >
> >  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 11541b94b328..0418399e0a20 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 c10207fed2f3..52326b739357 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 +1010,18 @@ 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) {
> > +                       ret = -EFAULT;
> > +                       goto out_unlock;
> > +               }
> > +               ret = sanitise_mte_tags(kvm, pfn, vma_pagesize);
> > +               if (ret)
> > +                       goto out_unlock;
> > +
> 
> nit: Would it make sense to bring in sanitise_mte_tags under the
> kvm_has_mte. I know that a check is done in kvm_has_mte as well, but
> since you're already checking, it might make the code a bit clearer.

I think it makes more sense once merged with -next, as the CMO has
been moved into the PT code. I came up with the following resolution:

	if (vma_pagesize == PAGE_SIZE && !(force_pte || device))
		vma_pagesize = transparent_hugepage_adjust(memslot, hva,
							   &pfn, &fault_ipa);

	if (fault_status != FSC_PERM && !device && kvm_has_mte(kvm)) {
		/* Check the VMM hasn't introduced a new VM_SHARED VMA */
		if (!(vma->vm_flags & VM_SHARED))
			ret = sanitise_mte_tags(kvm, pfn, vma_pagesize);
		else
			ret = -EFAULT;
		if (ret)
			goto out_unlock;
	}

	if (writable)
		prot |= KVM_PGTABLE_PROT_W;


However, there is a more annoying issue here, which is that the vma is
accessed outside of the mm lock. I *think* we're safe because if an
unmap happens in parallel, the MMU notifier will kick and we will be
in one of two cases:

- the unmap occurs before we take the kvm->mmu_lock, and the mmu
  notifier seq_lock is want saves us (we will drop everything and take
  the fault again),

- it occurs once we hold the lock, and this blocks the unmap.

Either way, I'd be more confident if the shared state was sampled
inside the locked section.

Thoughts?

	M.

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

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