[U-Boot] usb: hub enumeration and potential NULL ptr dereference in usb_device_info()

[Simon Glass]

Hi Benhard,

On 22 June 2016 at 03:05, Bernhard Nortmann <bernhard.nortmann at web.de> wrote:
> Starting with commit b19236fd1c1ef289bab9e243ee5b50d658fcac3f I am observing
> a breakage of the "usb info" command on my BananaPi (Allwinner A20, sun7i),
> while "usb tree" and dm commands ("dm tree", "dm uclass") are fine.
> See attached usb-info-breakage.log
>
> Tracing back the error positon from pc and lr registers pointed me to the
> device_get_uclass_id() call within usb_device_info(), and suggested the
> problem is caused by trying to dereference a NULL struct udevice *hub.
>
> Therefore I added a diagnostic printf and a safeguard to this routine:
>
> diff --git a/cmd/usb.c b/cmd/usb.c
> index b83d323..a5e2463 100644
> --- a/cmd/usb.c
> +++ b/cmd/usb.c
> @@ -610,6 +610,12 @@ static int usb_device_info(void)
>          struct udevice *hub;
>
>          device_find_first_child(bus, &hub);
> +        printf("bus %p, uclass %d, hub %p: ",
> +            bus, device_get_uclass_id(bus), hub);
> +        if (!hub) {
> +            printf("<no hub>\n");
> +            continue;
> +        }
>          if (device_get_uclass_id(hub) == UCLASS_USB_HUB &&
>              device_active(hub)) {
>              show_info(hub);
>
> And it became apparent that hub actually receives NULL values during the
> device enumeration. The safeguard prevented the "data abort" error and got
> "usb info" working again - see attached usb-info-fixed.log
>
> I'm not sure why this particular problem didn't manifest earlier and
> only now became apparent with the change in SPL header size /
> CONFIG_SPL_TEXT_BASE. But it seems that accessing a NULL hub with
> device_get_uclass_id() is clearly wrong and should be avoided.
> Which brings me to two questions:
>
> 1. Is getting a NULL hub value legit when iterating UCLASS_USB devices
>    the way that usb_device_info() does? If yes, the code seems to be
>    lacking protection against passing it to device_get_uclass_id().

Well if there are no child devices of a USB controller, then yes.
Normally there is at least one (a USB hub). But this code is wrong -
it should not assume that.

In fact. now, the new uclass_first_device_err() function is probably a
better choice, since it returns an error if nothing is found.

>
> 2. Why does usb_device_info() choose to enumerate hubs this way at all?
>    If the routine is aiming at UCLASS_USB_HUB - which seems to be the
>    purpose of the subsequent device_get_uclass_id(hub) test - and the
>    device tree already provides this information (as suggested by the
>    output of "dm uclass"), then why not enumerate UCLASS_USB_HUB directly?

It was probably trying to duplicate the operation of the old code:

if (strncmp(argv[1], "inf", 3) == 0) {
if (argc == 2) {
#ifdef CONFIG_DM_USB
usb_device_info();
#else
int d;
for (d = 0; d < USB_MAX_DEVICE; d++) {
udev = usb_get_dev_index(d);
if (udev == NULL)
break;
usb_display_desc(udev);
usb_display_config(udev);
}
#endif

But I'm not sure that the ordering would change in any case, or even
if it matters. Feel free to change it to enumerate USB_HUB.

Another explanation is that originally I was not sure if USB hubs
should have their own uclass. With PCI bridges we don't do it that
way. But I decided in the end to go ahead, and I think it has worked
ouit. So perhaps the code was converted mid-stream.

>
> Regards, B. Nortmann
>

Regards,
Simon