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

[Bernhard Nortmann]

Hi Simon, thanks for replying!

Am 23.06.2016 um 15:12 schrieb Simon Glass:
> Hi Benhard,
>
> On 22 June 2016 at 03:05, Bernhard Nortmann <bernhard.nortmann at web.de> wrote:
>> [...]
>>
>> 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.

uclass_first_device_err() won't help here. It's not the outer UCLASS_USB
enumeration that is at fault - getting a NULL "bus" would simply terminate
the loop. In fact this loop correctly processes all four 'top level'
devices available on sun7i-a20:

uclass 60: usb
- * usb at 01c14000 @ 7af37148, seq 0, (req -1)
- * usb at 01c14400 @ 7af371b0, seq 1, (req -1)
- * usb at 01c1c000 @ 7af37218, seq 2, (req -1)
- * usb at 01c1c400 @ 7af37280, seq 3, (req -1)

The root cause of the usb_device_info() problem here is that two of these
controllers (ohci0: usb at 01c14400, ohci1: usb at 01c1c400) remain dormant /
'invisible' to DM as long as no actual USB1-only peripheral is attached, and
device_find_first_child(bus, &hub) will return a NULL hub subsequently.
("usb tree" doesn't show them either in this state.)

I have a suspicion that this might easily happen with other EHCI-OHCI
controller combinations too.

>
>> 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:
> [...]
>
> 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.

Yes, I figured it might be something along those "historical" lines. But if
I understand you correctly, then any "usb_hub" (uclass 62) child of 
UCLASS_USB
should also show up when iterating UCLASS_USB_HUB with uclass_first_device()
and uclass_next_device(). If that's guaranteed, my preferred solution would
actually be to do away with the "bus" enumeration and replace it with a more
straightforward "show_info(hub) over all the hubs" solution.

I'll submit a patch for that shortly.

Regards, B. Nortmann