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

From: Marc Zyngier
Date: Tue Jun 22 2021 - 06:25:43 EST

Hi Fuad,

On Tue, 22 Jun 2021 09:56:22 +0100,
Fuad Tabba <tabba@xxxxxxxxxx> wrote:
>
> Hi,
>
>
> On Mon, Jun 21, 2021 at 12:18 PM Steven Price <steven.price@xxxxxxx> 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@xxxxxxx>
> > Signed-off-by: Steven Price <steven.price@xxxxxxx>
> > ---
> > 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.