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

[Marc Zyngier <maz@kernel.org>]

Hi Fuad,

On Tue, 22 Jun 2021 09:56:22 +0100,
Fuad Tabba <tabba@google.com> wrote:
> 
> Hi,
> 
> 
> On Mon, Jun 21, 2021 at 12:18 PM 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);
> > +
> > +               gfn++;
> > +               tags += num_tags;
> > +               length -= PAGE_SIZE;
> > +       }
> > +
> > +out:
> > +       mutex_unlock(&kvm->slots_lock);
> > +       /* If some data has been copied report the number of bytes copied */
> > +       if (length != copy_tags->length)
> > +               return copy_tags->length - length;
> 
> I'm not sure if this is actually an issue, but a couple of comments on
> the return value if there is an error after a partial copy has been
> done. If mte_copy_tags_to_user or mte_copy_tags_from_user don't return
> MTE_GRANULES_PER_PAGE, then the check for num_tags would fail, but
> some of the tags would have been copied, which wouldn't be reflected
> in length. That said, on a write the tagged bit wouldn't be set, and
> on read then the return value would be conservative, but not
> incorrect.
>
> That said, even though it is described that way in the documentation
> (rather deep in the description though), it might be confusing to
> return a non-negative value on an error. The other kvm ioctl I could
> find that does something similar, KVM_S390_GET_IRQ_STATE, seems to
> always return a -ERROR on error, rather than the number of bytes
> copied.

My mental analogy for this ioctl is the read()/write() syscalls, which
return the number of bytes that have been transferred in either
direction.

I agree that there are some corner cases (a tag copy that fails
because of a faulty page adjacent to a valid page will still report
some degree of success), but it is also important to report what has
actually been done in either direction.

Thanks,

	M.

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