[RFC PATCH v7 3/3] KVM: arm64: ioctl to fetch/store tags in a guest

[Steven Price]

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.

Signed-off-by: Steven Price <steven.price at arm.com>
---
 arch/arm64/include/uapi/asm/kvm.h | 13 +++++++
 arch/arm64/kvm/arm.c              | 59 +++++++++++++++++++++++++++++++
 include/uapi/linux/kvm.h          |  1 +
 3 files changed, 73 insertions(+)

diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
index 24223adae150..5fc2534ac5df 100644
--- a/arch/arm64/include/uapi/asm/kvm.h
+++ b/arch/arm64/include/uapi/asm/kvm.h
@@ -184,6 +184,19 @@ struct kvm_vcpu_events {
 	__u32 reserved[12];
 };
 
+struct kvm_arm_copy_mte_tags {
+	__u64 guest_ipa;
+	__u64 length;
+	union {
+		void __user *addr;
+		__u64 padding;
+	};
+	__u64 flags;
+};
+
+#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 f4c2fd2e7c49..d6dd6b79bb77 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -1303,6 +1303,55 @@ static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
 	}
 }
 
+static int 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;
+	size_t pages;
+	bool write = !(copy_tags->flags & KVM_ARM_TAGS_FROM_GUEST);
+
+	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);
+	pages = length >> PAGE_SHIFT;
+
+	while (length > 0) {
+		kvm_pfn_t pfn = gfn_to_pfn_prot(kvm, gfn, write, NULL);
+		void *maddr;
+		unsigned long num_tags = PAGE_SIZE / MTE_GRANULE_SIZE;
+
+		if (is_error_noslot_pfn(pfn))
+			return -ENOENT;
+
+		maddr = page_address(pfn_to_page(pfn));
+
+		if (!write) {
+			num_tags = mte_copy_tags_to_user(tags, maddr, num_tags);
+			kvm_release_pfn_clean(pfn);
+		} else {
+			num_tags = mte_copy_tags_from_user(maddr, tags,
+							   num_tags);
+			kvm_release_pfn_dirty(pfn);
+		}
+
+		if (num_tags != PAGE_SIZE / MTE_GRANULE_SIZE)
+			return -EFAULT;
+
+		gfn++;
+		tags += num_tags;
+		length -= PAGE_SIZE;
+	}
+
+	return 0;
+}
+
 long kvm_arch_vm_ioctl(struct file *filp,
 		       unsigned int ioctl, unsigned long arg)
 {
@@ -1339,6 +1388,16 @@ 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 (!kvm_has_mte(kvm))
+			return -EINVAL;
+
+		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/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index de737d5102ca..76fccb33d025 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1397,6 +1397,7 @@ struct kvm_s390_ucas_mapping {
 /* Available with KVM_CAP_PMU_EVENT_FILTER */
 #define KVM_SET_PMU_EVENT_FILTER  _IOW(KVMIO,  0xb2, struct kvm_pmu_event_filter)
 #define KVM_PPC_SVM_OFF		  _IO(KVMIO,  0xb3)
+#define KVM_ARM_MTE_COPY_TAGS	  _IOR(KVMIO,  0xb4, struct kvm_arm_copy_mte_tags)
 
 /* ioctl for vm fd */
 #define KVM_CREATE_DEVICE	  _IOWR(KVMIO,  0xe0, struct kvm_create_device)
-- 
2.20.1