KVM: Write protect memory after slot swap

I have observed the following bug trigger:

1. userspace calls GET_DIRTY_LOG
2. kvm_mmu_slot_remove_write_access is called and makes a page ro
3. page fault happens and makes the page writeable
   fault is logged in the bitmap appropriately
4. kvm_vm_ioctl_get_dirty_log swaps slot pointers

a lot of time passes

5. guest writes into the page
6. userspace calls GET_DIRTY_LOG

At point (5), bitmap is clean and page is writeable,
thus, guest modification of memory is not logged
and GET_DIRTY_LOG returns an empty bitmap.

The rule is that all pages are either dirty in the current bitmap,
or write-protected, which is violated here.

It seems that just moving kvm_mmu_slot_remove_write_access down
to after the slot pointer swap should fix this bug.

KVM-Stable-Tag.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -3169,10 +3169,6 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		struct kvm_memslots *slots, *old_slots;
 		unsigned long *dirty_bitmap;
 
-		spin_lock(&kvm->mmu_lock);
-		kvm_mmu_slot_remove_write_access(kvm, log->slot);
-		spin_unlock(&kvm->mmu_lock);
-
 		r = -ENOMEM;
 		dirty_bitmap = vmalloc(n);
 		if (!dirty_bitmap)
@@ -3194,6 +3190,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
 		dirty_bitmap = old_slots->memslots[log->slot].dirty_bitmap;
 		kfree(old_slots);
 
+		spin_lock(&kvm->mmu_lock);
+		kvm_mmu_slot_remove_write_access(kvm, log->slot);
+		spin_unlock(&kvm->mmu_lock);
+
 		r = -EFAULT;
 		if (copy_to_user(log->dirty_bitmap, dirty_bitmap, n)) {
 			vfree(dirty_bitmap);