On Sun, 31 Jan 2016, Clemens Ladisch wrote:
> Alan Stern wrote:
> > On Sun, 31 Jan 2016, Clemens Ladisch wrote:
> >> Alan Stern wrote:
> >>> On Fri, 29 Jan 2016, Dan Carpenter wrote:
> >>>> The patch c6994e6f067c: "USB: gadget: add USB Audio Gadget driver"
> >>>> from Jun 3, 2009, leads to the following static checker warning:
> >>>>
> >>>> drivers/usb/gadget/function/f_uac1.c:371 f_audio_complete()
> >>>> error: __memcpy() '&data' too small (4 vs u32max)
> >>>>
> >>>> drivers/usb/gadget/function/f_uac1.c
> >>>> 370 else if (audio->set_con) {
> >>>> 371 memcpy(&data, req->buf, req->length);
> >>>> ^^^^^^^^^^^
> >>>> Back in 2014 Kees ran a USB fuzzer on the kernel. My take on that was
> >>>> that req->length could not be trusted and had to be capped so I changed
> >>>> Smatch to complain about these. But after a while I decided I was not
> >>>> sure enough of the rules so I just ignore those bugs these days... What
> >>>> are the rules?
> >>>
> >>> req->length is set by the driver and not changed anywhere else in the
> >>> kernel. (Or rather, changing it would be a bug.) Of course, it's
> >>> possible that the driver could be fooled into setting req->length to
> >>> something larger than 4 -- I haven't looked through the code.
> >>
> >> That value comes from the control setup packet's wLength field. Neither
> >> audio_set_intf_req() nor f_audio_setup() check it.
> >
> > Maybe those routines don't check wLength, but it has to get set
> > somewhere in the driver.
>
> In a USB gadget driver, these packets are received from the host PC.
> Which, for the purposes of this discussion, must be considered evil.
Of course -- I feel for stupid for forgetting that we're talking about
a gadget driver.
The normal thing for gadget drivers to do is to set req->length to be
the minimum of wLength and the amount of data they want to send (which
in this case would be 4 or smaller). If the gadget driver doesn't do
that min() calculation, it's probably a bug.
Alan Stern
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