Re: [PATCH v2] usb:xhci fix panic in xhci_free_virt_devices_depth_first

[Mathias Nyman]

On 06.11.2017 14:36, Chen Yu wrote:
> On 2017/11/6 19:32, Greg KH wrote:
> > > A simple process is as below:
> > > 	xhci_plat_probe()
> > >                 |
> > > 	usb_add_hcd()					xhci_plat_remove()
> > > 	       |						|
> > > 	find some device				usb_remove_hcd()
> > > 	       |						|
> > > 	hub_port_connect() -> usb_alloc_dev()		usb_disconnect()
> > > 	       |						|
> > > 	before hub_enable_device()			xhci_stop()
> > > 								|
> > > 							xhci_mem_cleanup()
> > > 								|
> > > 							xhci_free_virt_devices_depth_first()
> > > 								|
> > > 							real_port is 0 access xhci->rh_bw[vdev->real_port-1]
> > >
> > > The problem came from https://bugs.96boards.org/show_bug.cgi?id=535
> > > Also look at crbug.com/700041
> >
> > Then the bug needs to be fixed, throwing a huge kernel trace message
> > into the kernel log is not "fixing" the problem at all, right?
> >
> > thanks,
> >
> > greg k-h
> >
> > .
>
> You are right, the way that xhci_plat_remove() to be called needs to be fixed.
> But there is still possibility for this crash.
> What do you think if just add an "xhci_warn" instead of "WARN_ON"?
> +       if (!vdev->real_port) {
> +               xhci_warn(xhci, "Bad vdev->real_port\n");
> +               goto out;
> +       }
> +

This patch solves the issue, just drop all the error messages.

vdev->real_port is not set until the the device enable/address
stage, and we know it won't have any children yet then, so no need to
worry about a child having tt pointers to this device.

The "goto out" to xhci_free_virt_device() you do is fine here.

xhci_plat_remove() is the .remove callback for the xhci platform driver.
It might get called before a device is properly enabled/addressed.
Not really a error. A unlikely but possible situation.

xhci_free_tt_info() already has a similar check

Thanks
-Mathias