drm/i915/gvt: Protect gfn hash table with vgpu_lock

Use vgpu_lock instead of KVM's mmu_lock to protect accesses to the hash
table used to track which gfns are write-protected when shadowing the
guest's GTT, and hoist the acquisition of vgpu_lock from
intel_vgpu_page_track_handler() out to its sole caller,
kvmgt_page_track_write().

This fixes a bug where kvmgt_page_track_write(), which doesn't hold
kvm->mmu_lock, could race with intel_gvt_page_track_remove() and trigger
a use-after-free.

Fixing kvmgt_page_track_write() by taking kvm->mmu_lock is not an option
as mmu_lock is a r/w spinlock, and intel_vgpu_page_track_handler() might
sleep when acquiring vgpu->cache_lock deep down the callstack:

  intel_vgpu_page_track_handler()
  |
  |->  page_track->handler / ppgtt_write_protection_handler()
       |
       |-> ppgtt_handle_guest_write_page_table_bytes()
           |
           |->  ppgtt_handle_guest_write_page_table()
                |
                |-> ppgtt_handle_guest_entry_removal()
                    |
                    |-> ppgtt_invalidate_pte()
                        |
                        |-> intel_gvt_dma_unmap_guest_page()
                            |
                            |-> mutex_lock(&vgpu->cache_lock);

Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yongwei Ma <yongwei.ma@intel.com>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Link: https://lore.kernel.org/r/20230729013535.1070024-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

drivers/gpu/drm/i915/gvt/kvmgt.c

@@ -352,6 +352,8 @@ __kvmgt_protect_table_find(struct intel_vgpu *info, gfn_t gfn)
 {
 	struct kvmgt_pgfn *p, *res = NULL;
 
+	lockdep_assert_held(&info->vgpu_lock);
+
 	hash_for_each_possible(info->ptable, p, hnode, gfn) {
 		if (gfn == p->gfn) {
 			res = p;
@@ -1553,6 +1555,9 @@ int intel_gvt_page_track_add(struct intel_vgpu *info, u64 gfn)
 	if (!test_bit(INTEL_VGPU_STATUS_ATTACHED, info->status))
 		return -ESRCH;
 
+	if (kvmgt_gfn_is_write_protected(info, gfn))
+		return 0;
+
 	idx = srcu_read_lock(&kvm->srcu);
 	slot = gfn_to_memslot(kvm, gfn);
 	if (!slot) {
@@ -1561,16 +1566,12 @@ int intel_gvt_page_track_add(struct intel_vgpu *info, u64 gfn)
 	}
 
 	write_lock(&kvm->mmu_lock);
-
-	if (kvmgt_gfn_is_write_protected(info, gfn))
-		goto out;
-
 	kvm_slot_page_track_add_page(kvm, slot, gfn, KVM_PAGE_TRACK_WRITE);
-	kvmgt_protect_table_add(info, gfn);
-
-out:
 	write_unlock(&kvm->mmu_lock);
+
 	srcu_read_unlock(&kvm->srcu, idx);
+
+	kvmgt_protect_table_add(info, gfn);
 	return 0;
 }
 
@@ -1583,6 +1584,9 @@ int intel_gvt_page_track_remove(struct intel_vgpu *info, u64 gfn)
 	if (!test_bit(INTEL_VGPU_STATUS_ATTACHED, info->status))
 		return -ESRCH;
 
+	if (!kvmgt_gfn_is_write_protected(info, gfn))
+		return 0;
+
 	idx = srcu_read_lock(&kvm->srcu);
 	slot = gfn_to_memslot(kvm, gfn);
 	if (!slot) {
@@ -1591,16 +1595,11 @@ int intel_gvt_page_track_remove(struct intel_vgpu *info, u64 gfn)
 	}
 
 	write_lock(&kvm->mmu_lock);
-
-	if (!kvmgt_gfn_is_write_protected(info, gfn))
-		goto out;
-
 	kvm_slot_page_track_remove_page(kvm, slot, gfn, KVM_PAGE_TRACK_WRITE);
-	kvmgt_protect_table_del(info, gfn);
-
-out:
 	write_unlock(&kvm->mmu_lock);
 	srcu_read_unlock(&kvm->srcu, idx);
+
+	kvmgt_protect_table_del(info, gfn);
 	return 0;
 }
 
@@ -1611,9 +1610,13 @@ static void kvmgt_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa,
 	struct intel_vgpu *info =
 		container_of(node, struct intel_vgpu, track_node);
 
+	mutex_lock(&info->vgpu_lock);
+
 	if (kvmgt_gfn_is_write_protected(info, gpa_to_gfn(gpa)))
 		intel_vgpu_page_track_handler(info, gpa,
 						     (void *)val, len);
+
+	mutex_unlock(&info->vgpu_lock);
 }
 
 static void kvmgt_page_track_flush_slot(struct kvm *kvm,
@@ -1625,16 +1628,20 @@ static void kvmgt_page_track_flush_slot(struct kvm *kvm,
 	struct intel_vgpu *info =
 		container_of(node, struct intel_vgpu, track_node);
 
-	write_lock(&kvm->mmu_lock);
+	mutex_lock(&info->vgpu_lock);
+
 	for (i = 0; i < slot->npages; i++) {
 		gfn = slot->base_gfn + i;
 		if (kvmgt_gfn_is_write_protected(info, gfn)) {
+			write_lock(&kvm->mmu_lock);
 			kvm_slot_page_track_remove_page(kvm, slot, gfn,
 						KVM_PAGE_TRACK_WRITE);
+			write_unlock(&kvm->mmu_lock);
+
 			kvmgt_protect_table_del(info, gfn);
 		}
 	}
-	write_unlock(&kvm->mmu_lock);
+	mutex_unlock(&info->vgpu_lock);
 }
 
 void intel_vgpu_detach_regions(struct intel_vgpu *vgpu)

drivers/gpu/drm/i915/gvt/page_track.c

@@ -162,13 +162,9 @@ int intel_vgpu_page_track_handler(struct intel_vgpu *vgpu, u64 gpa,
 	struct intel_vgpu_page_track *page_track;
 	int ret = 0;
 
-	mutex_lock(&vgpu->vgpu_lock);
-
 	page_track = intel_vgpu_find_page_track(vgpu, gpa >> PAGE_SHIFT);
-	if (!page_track) {
-		ret = -ENXIO;
-		goto out;
-	}
+	if (!page_track)
+		return -ENXIO;
 
 	if (unlikely(vgpu->failsafe)) {
 		/* Remove write protection to prevent furture traps. */
@@ -179,7 +175,5 @@ int intel_vgpu_page_track_handler(struct intel_vgpu *vgpu, u64 gpa,
 			gvt_err("guest page write error, gpa %llx\n", gpa);
 	}
 
-out:
-	mutex_unlock(&vgpu->vgpu_lock);
 	return ret;
 }