Add a new VM feature 'KVM_ARM_CAP_MTE' which enables memory tagging
for a VM. This will expose the feature to the guest and automatically
tag memory pages touched by the VM as PG_mte_tagged (and clear the tag
storage) to ensure that the guest cannot see stale tags, and so that
the tags are correctly saved/restored across swap.

Actually exposing the new capability to user space happens in a later
patch.

Signed-off-by: Steven Price <steven.price@arm.com>
---
 arch/arm64/include/asm/kvm_emulate.h |  3 ++
 arch/arm64/include/asm/kvm_host.h    |  3 ++
 arch/arm64/kvm/hyp/exception.c       |  3 +-
 arch/arm64/kvm/mmu.c                 | 62 +++++++++++++++++++++++++++-
 arch/arm64/kvm/sys_regs.c            |  7 ++++
 include/uapi/linux/kvm.h             |  1 +
 6 files changed, 77 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_emulate.h b/arch/arm64/include/asm/kvm_emulate.h
index f612c090f2e4..6bf776c2399c 100644
--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h

@@ -84,6 +84,9 @@ static inline void vcpu_reset_hcr(struct kvm_vcpu *vcpu)
 	if (cpus_have_const_cap(ARM64_MISMATCHED_CACHE_TYPE) ||
 	    vcpu_el1_is_32bit(vcpu))
 		vcpu->arch.hcr_el2 |= HCR_TID2;
+
+	if (kvm_has_mte(vcpu->kvm))
+		vcpu->arch.hcr_el2 |= HCR_ATA;
 }
 
 static inline unsigned long *vcpu_hcr(struct kvm_vcpu *vcpu)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 7cd7d5c8c4bc..afaa5333f0e4 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h

@@ -132,6 +132,8 @@ struct kvm_arch {
 
 	u8 pfr0_csv2;
 	u8 pfr0_csv3;
+	/* Memory Tagging Extension enabled for the guest */
+	bool mte_enabled;
 };
 
 struct kvm_vcpu_fault_info {

@@ -769,6 +771,7 @@ bool kvm_arm_vcpu_is_finalized(struct kvm_vcpu *vcpu);
 #define kvm_arm_vcpu_sve_finalized(vcpu) \
 	((vcpu)->arch.flags & KVM_ARM64_VCPU_SVE_FINALIZED)
 
+#define kvm_has_mte(kvm) (system_supports_mte() && (kvm)->arch.mte_enabled)
 #define kvm_vcpu_has_pmu(vcpu)					\
 	(test_bit(KVM_ARM_VCPU_PMU_V3, (vcpu)->arch.features))
 

diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
index 73629094f903..56426565600c 100644
--- a/arch/arm64/kvm/hyp/exception.c
+++ b/arch/arm64/kvm/hyp/exception.c

@@ -112,7 +112,8 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
 	new |= (old & PSR_C_BIT);
 	new |= (old & PSR_V_BIT);
 
-	// TODO: TCO (if/when ARMv8.5-MemTag is exposed to guests)
+	if (kvm_has_mte(vcpu->kvm))
+		new |= PSR_TCO_BIT;
 
 	new |= (old & PSR_DIT_BIT);
 

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index c5d1f3c87dbd..f5305b7561ad 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c

@@ -822,6 +822,45 @@ transparent_hugepage_adjust(struct kvm_memory_slot *memslot,
 	return PAGE_SIZE;
 }
 
+/*
+ * The page will be mapped in stage 2 as Normal Cacheable, so the VM will be
+ * able to see the page's tags and therefore they must be initialised first. If
+ * PG_mte_tagged is set, tags have already been initialised.
+ *
+ * The race in the test/set of the PG_mte_tagged flag is handled by:
+ * - preventing VM_SHARED mappings in a memslot with MTE preventing two VMs
+ *   racing to santise the same page
+ * - mmap_lock protects between a VM faulting a page in and the VMM performing
+ *   an mprotect() to add VM_MTE
+ */
+static int sanitise_mte_tags(struct kvm *kvm, kvm_pfn_t pfn,
+			     unsigned long size)
+{
+	unsigned long i, nr_pages = size >> PAGE_SHIFT;
+	struct page *page;
+
+	if (!kvm_has_mte(kvm))
+		return 0;
+
+	/*
+	 * pfn_to_online_page() is used to reject ZONE_DEVICE pages
+	 * that may not support tags.
+	 */
+	page = pfn_to_online_page(pfn);
+
+	if (!page)
+		return -EFAULT;
+
+	for (i = 0; i < nr_pages; i++, page++) {
+		if (!test_bit(PG_mte_tagged, &page->flags)) {
+			mte_clear_page_tags(page_address(page));
+			set_bit(PG_mte_tagged, &page->flags);
+		}
+	}
+
+	return 0;
+}
+
 static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 			  struct kvm_memory_slot *memslot, unsigned long hva,
 			  unsigned long fault_status)

@@ -971,8 +1010,16 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
 	if (writable)
 		prot |= KVM_PGTABLE_PROT_W;
 
-	if (fault_status != FSC_PERM && !device)
+	if (fault_status != FSC_PERM && !device) {
+		/* Check the VMM hasn't introduced a new VM_SHARED VMA */
+		if (kvm_has_mte(kvm) && vma->vm_flags & VM_SHARED)
+			return -EINVAL;