KVM: arm64: Reject guest_memfd memslots when the VM has MTE

The user cannot use MTE on VMAs created by mapping a guest_memfd file,
as arch_calc_vm_flag_bits() does not set VM_MTE_ALLOWED.

When creating a guest_memfd backed memslot,
kvm_arch_prepare_memory_region() rejects the memslot if MTE is enabled for
the VM and if guest_memfd has been mapped in a VMA that intersects the
memslot.

However, the documentation for KVM_SET_USER_MEMORY_REGION2 explicitly
states that the only condition for userspace_addr is for it to be a legal
userspace address, but the mapping is not required to be valid nor
populated at memslot creation.

If userspace sets userspace_addr to an address that hasn't been mapped, or
if userspace_addr belongs to a VMA that isn't backed by the guest_memfd
file, or if the VMA doesn't intersect the memslot, memslot creation is
successful and KVM ends up with a VM with MTE and guest_memfd-backed
memslots.

The same happens if the order is reversed: when userspace enables MTE, KVM
does not check if memslots backed by guest_memfd are already present.

Fix both issues by rejecting guest_memfd-backed memslots when MTE is
enabled, and by rejecting MTE when guest_memfd-backed memslots are already
present.

Fixes: 32e200bd6e ("KVM: arm64: Enable support for guest_memfd backed memory")
Tested-by: Fuad Tabba <fuad.tabba@linux.dev>
Reviewed-by: Fuad Tabba <fuad.tabba@linux.dev>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://patch.msgid.link/20260722090354.94245-1-alexandru.elisei@arm.com
Signed-off-by: Marc Zyngier <maz@kernel.org>

Documentation/virt/kvm/api.rst

@@ -8414,6 +8414,12 @@ When this capability is enabled all memory in memslots must be mapped as
 attempts to create a memslot with an invalid mmap will result in an
 -EINVAL return.
 
+``guest_memfd``, even though it is an anonymous file, is not supported with MTE.
+Attempting to create a memslot backed by ``guest_memfd`` when the MTE capability
+is enabled, or attempting to enable the MTE capability after
+``guest_memfd``-backed memslots have been created, will result in an -EINVAL
+return.
+
 When enabled the VMM may make use of the ``KVM_ARM_MTE_COPY_TAGS`` ioctl to
 perform a bulk copy of tags to/from the guest.
 

arch/arm64/kvm/arm.c

@@ -149,14 +149,27 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		set_bit(KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER,
 			&kvm->arch.flags);
 		break;
-	case KVM_CAP_ARM_MTE:
-		mutex_lock(&kvm->lock);
-		if (system_supports_mte() && !kvm->created_vcpus) {
-			r = 0;
-			set_bit(KVM_ARCH_FLAG_MTE_ENABLED, &kvm->arch.flags);
+	case KVM_CAP_ARM_MTE: {
+		struct kvm_memory_slot *memslot;
+		int bkt;
+
+		guard(mutex)(&kvm->lock);
+		if (!system_supports_mte() || kvm->created_vcpus)
+			break;
+
+		r = 0;
+		guard(mutex)(&kvm->slots_lock);
+		kvm_for_each_memslot(memslot, bkt, kvm_memslots(kvm)) {
+			if (kvm_slot_has_gmem(memslot)) {
+				r = -EINVAL;
+				break;
+			}
 		}
-		mutex_unlock(&kvm->lock);
+		if (r == 0)
+			set_bit(KVM_ARCH_FLAG_MTE_ENABLED, &kvm->arch.flags);
 		break;
+
+	}
 	case KVM_CAP_ARM_SYSTEM_SUSPEND:
 		r = 0;
 		set_bit(KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED, &kvm->arch.flags);

arch/arm64/kvm/mmu.c

@@ -2652,6 +2652,10 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 	if (kvm_slot_has_gmem(new) && !kvm_memslot_is_gmem_only(new))
 		return -EINVAL;
 
+	/* guest_memfd is incompatible with MTE. */
+	if (kvm_slot_has_gmem(new) && kvm_has_mte(kvm))
+		return -EINVAL;
+
 	hva = new->userspace_addr;
 	reg_end = hva + (new->npages << PAGE_SHIFT);