On Thu, 30 Mar 2017, Mauro Carvalho Chehab wrote:
> Em Thu, 30 Mar 2017 10:26:32 -0400 (EDT)
> Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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
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