[PATCH 22/22] usb: document that URB transfer_buffer should be aligned

[Alan Stern]

On Thu, 30 Mar 2017, Mauro Carvalho Chehab wrote:

> Em Thu, 30 Mar 2017 10:26:32 -0400 (EDT)
> Alan Stern <stern at rowland.harvard.edu> escreveu:
> 
> > On Thu, 30 Mar 2017, Oliver Neukum wrote:
> > 
> > > > Btw, I'm a lot more concerned about USB storage drivers. When I was
> > > > discussing about this issue at the #raspberrypi-devel IRC channel,
> > > > someone complained that, after switching from the RPi downstream Kernel
> > > > to upstream, his USB data storage got corrupted. Well, if the USB
> > > > storage drivers also assume that the buffer can be continuous,
> > > > that can corrupt data.  
> > 
> > > 
> > > They do assume that.  
> > 
> > Wait a minute.  Where does that assumption occur?
> > 
> > And exactly what is the assumption?  Mauro wrote "the buffer can be 
> > continuous", but that is certainly not what he meant.
> 
> What I meant to say is that drivers like the uvcdriver (and maybe network and
> usb-storage drivers) may allocate a big buffer and get data there on some
> random order, e. g.: 
> 
> int get_from_buf_pos(char *buf, int pos, int size)
> {
> 	/* or an equivalent call to usb_submit_urb() */
> 	usb_control_msg(..., buf + pos, size, ...);
> }
> 
> some_function ()
> {
> 	...
> 
> 	chr *buf = kzalloc(4, GFP_KERNEL);
> 
> 	/* 
> 	 * Access the bytes at the array on a random order, with random size,
> 	 * Like:
> 	 */
> 	get_from_buf_pos(buf, 2, 2);	/* should read 0x56, 0x78 */
> 	get_from_buf_pos(buf, 0, 2);	/* should read 0x12, 0x34 */
> 
> 	/*
> 	 * the expected value for the buffer would be:
> 	 * 	{ 0x12, 0x34, 0x56, 0x78 }
> 	 */
> 
> E. g. they assume that the transfer URB can work with any arbitrary
> pointer and size, without needing of pre-align them.
> 
> This doesn't work with HCD drivers like dwc2, as each USB_IN operation will
> actually write 4 bytes to the buffer.
> 
> So, what happens, instead, is that each data transfer will get four
> bytes. Due to a hack inside dwc2, with checks if the transfer_buffer
> is DWORD aligned. So, the first transfer will do what's expected: it will
> read 4 bytes to a temporary buffer, allocated inside the driver,
> copying just two bytes to buf. So, after the first read, the
> buffer content will be:
> 
> 	buf = { 0x00, x00, 0x56, 0x78 }
> 
> But, on the second read, it won't be using any temporary
> buffer. So, instead of reading a 16-bits word (0x5678),
> it will actually read 32 bits, with 16-bits with some random value,
> causing a buffer overflow. E. g. buffer content will now be:
> 
> 	buf = { 0x12, x34, 0xde, 0xad }
> 
> In other words, the second transfer corrupted the data from the
> first transfer.

I'm pretty sure that usb-storage does not do this, at least, not when 
operating in its normal Bulk-Only-Transport mode.  It never tries to 
read the results of an earlier transfer after carrying out a later 
transfer to any part of the same buffer.

Alan Stern