Re: KASAN: use-after-free Write in hiddev_disconnect

[Alan Stern]

On Tue, 14 Jan 2020, Dan Carpenter wrote:

> I'm trying to take a stab at diagnosing these bugs.  (But I'm seldom
> smart enough to actually fix anything).  These hiddev_disconnect() bugs
> are a race condition:
> 
> devel/drivers/hid/usbhid/hiddev.c
>    924  void hiddev_disconnect(struct hid_device *hid)
>    925  {
>    926          struct hiddev *hiddev = hid->hiddev;
>    927          struct usbhid_device *usbhid = hid->driver_data;
>    928  
>    929          usb_deregister_dev(usbhid->intf, &hiddev_class);
>    930  
>    931          mutex_lock(&hiddev->existancelock);
>    932          hiddev->exist = 0;
>                 ^^^^^^^^^^^^^^^^^^
> We set "exist = 0;"
> 
>    933  
>    934          if (hiddev->open) {
>    935                  mutex_unlock(&hiddev->existancelock);
>                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Then we drop the lock.
> 
>    936                  hid_hw_close(hiddev->hid);
>    937                  wake_up_interruptible(&hiddev->wait);
>                                                ^^^^^^
> The other thread frees hiddev and it crashes here.
> 
>    938          } else {
>    939                  mutex_unlock(&hiddev->existancelock);
>    940                  kfree(hiddev);
>    941          }
>    942  }
> 
> The other thread is doing:
> 
> drivers/hid/usbhid/hiddev.c
>    216  static int hiddev_release(struct inode * inode, struct file * file)
>    217  {
>    218          struct hiddev_list *list = file->private_data;
>    219          unsigned long flags;
>    220  
>    221          spin_lock_irqsave(&list->hiddev->list_lock, flags);
>    222          list_del(&list->node);
>    223          spin_unlock_irqrestore(&list->hiddev->list_lock, flags);
>    224  
>    225          mutex_lock(&list->hiddev->existancelock);
>                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
> Takes the lock.
> 
>    226          if (!--list->hiddev->open) {
>                      ^^^^^^^^^^^^^^^^^^^^
> Decrements open.
> 
>    227                  if (list->hiddev->exist) {
>                             ^^^^^^^^^^^^^^^^^^^
> This is false.
> 
>    228                          hid_hw_close(list->hiddev->hid);
>    229                          hid_hw_power(list->hiddev->hid, PM_HINT_NORMAL);
>    230                  } else {
>    231                          mutex_unlock(&list->hiddev->existancelock);
>    232                          kfree(list->hiddev);
>                                       ^^^^^^^^^^^^
> Freed here.
> 
>    233                          vfree(list);
>    234                          return 0;
>    235                  }
>    236          }
>    237  
>    238          mutex_unlock(&list->hiddev->existancelock);
>    239          vfree(list);
>    240  
>    241          return 0;
>    242  }
> 
> I'm not sure what the fix should be though.

Good sleuthing!

It certainly looks as though hiddev_disconnect() should not try to
reference hiddev after dropping the existance_lock -- at least, not in
the case that the device file is still open.  So I suggest reordering
the statements in that routine: First call hid_hw_close() and
wake_up_interruptible(), then drop the mutex.

Too bad syzbot doesn't have a reproducer to test this, though.

Alan Stern