Hi Steven,
On Mon, 21 Jun 2021 12:17:15 +0100,
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(©_tags, argp, sizeof(copy_tags)))
> + return -EFAULT;
> + return kvm_vm_ioctl_mte_copy_tags(kvm, ©_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@xxxxxxxxxx>
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@xxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
---
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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