[PATCH v2 14/27] KVM: x86: Reject memslot MOVE operations if KVMGT is attached

[Sean Christopherson]

Disallow moving memslots if the VM has external page-track users, i.e. if
KVMGT is being used to expose a virtual GPU to the guest, as KVM doesn't
correctly handle moving memory regions.

Note, this is potential ABI breakage!  E.g. userspace could move regions
that aren't shadowed by KVMGT without harming the guest.  However, the
only known user of KVMGT is QEMU, and QEMU doesn't move generic memory
regions.  KVM's own support for moving memory regions was also broken for
multiple years (albeit for an edge case, but arguably moving RAM is
itself an edge case), e.g. see commit edd4fa37baa6 ("KVM: x86: Allocate
new rmap and large page tracking when moving memslot").

Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_page_track.h | 3 +++
 arch/x86/kvm/mmu/page_track.c         | 5 +++++
 arch/x86/kvm/x86.c                    | 7 +++++++
 3 files changed, 15 insertions(+)

diff --git a/arch/x86/include/asm/kvm_page_track.h b/arch/x86/include/asm/kvm_page_track.h
index 0d65ae203fd6..6a287bcbe8a9 100644
--- a/arch/x86/include/asm/kvm_page_track.h
+++ b/arch/x86/include/asm/kvm_page_track.h
@@ -77,4 +77,7 @@ kvm_page_track_unregister_notifier(struct kvm *kvm,
 void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new,
 			  int bytes);
 void kvm_page_track_flush_slot(struct kvm *kvm, struct kvm_memory_slot *slot);
+
+bool kvm_page_track_has_external_user(struct kvm *kvm);
+
 #endif
diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
index 39a0863af8b4..1cfc0a0ccc23 100644
--- a/arch/x86/kvm/mmu/page_track.c
+++ b/arch/x86/kvm/mmu/page_track.c
@@ -321,3 +321,8 @@ enum pg_level kvm_page_track_max_mapping_level(struct kvm *kvm, gfn_t gfn,
 	return max_level;
 }
 EXPORT_SYMBOL_GPL(kvm_page_track_max_mapping_level);
+
+bool kvm_page_track_has_external_user(struct kvm *kvm)
+{
+	return hlist_empty(&kvm->arch.track_notifier_head.track_notifier_list);
+}
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 29dd6c97d145..47ac9291cd43 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -12484,6 +12484,13 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
 				   struct kvm_memory_slot *new,
 				   enum kvm_mr_change change)
 {
+	/*
+	 * KVM doesn't support moving memslots when there are external page
+	 * trackers attached to the VM, i.e. if KVMGT is in use.
+	 */
+	if (change == KVM_MR_MOVE && kvm_page_track_has_external_user(kvm))
+		return -EINVAL;
+
 	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE) {
 		if ((new->base_gfn + new->npages - 1) > kvm_mmu_max_gfn())
 			return -EINVAL;
-- 
2.40.0.rc1.284.g88254d51c5-goog