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    SubjectRe: [PATCH v17 5/6] KVM: arm64: ioctl to fetch/store tags in a guest
    Hi Steven,

    On Mon, 21 Jun 2021 12:17:15 +0100,
    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);

    This ended up catching my eye as I was merging some other patches.

    This set_bit() occurs *after* the page has been released, meaning it
    could have been evicted and reused in the interval. I plan to fix it
    as below. Please let me know if that works for you.

    Thanks,

    M.

    From a78d3206378a7101659fbc2a4bf01cb9376c4793 Mon Sep 17 00:00:00 2001
    From: Marc Zyngier <maz@kernel.org>
    Date: Thu, 24 Jun 2021 14:21:05 +0100
    Subject: [PATCH] KVM: arm64: Set the MTE tag bit before releasing the page

    Setting a page flag without holding a reference to the page
    is living dangerously. In the tag-writing path, we drop the
    reference to the page by calling kvm_release_pfn_dirty(),
    and only then set the PG_mte_tagged bit.

    It would be safer to do it the other way round.

    Fixes: f0376edb1ddca ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
    Cc: Steven Price <steven.price@arm.com>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Marc Zyngier <maz@kernel.org>
    ---
    arch/arm64/kvm/guest.c | 12 ++++++++----
    1 file changed, 8 insertions(+), 4 deletions(-)

    diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
    index 4ddb20017b2f..60815ae477cf 100644
    --- a/arch/arm64/kvm/guest.c
    +++ b/arch/arm64/kvm/guest.c
    @@ -1053,6 +1053,14 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
    } else {
    num_tags = mte_copy_tags_from_user(maddr, tags,
    MTE_GRANULES_PER_PAGE);
    +
    + /*
    + * Set the flag after checking the write
    + * completed fully
    + */
    + if (num_tags == MTE_GRANULES_PER_PAGE)
    + set_bit(PG_mte_tagged, &page->flags);
    +
    kvm_release_pfn_dirty(pfn);
    }

    @@ -1061,10 +1069,6 @@ long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
    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;
    --
    2.30.2

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

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