Re: [PATCH] Input: zforce_ts - fix playload length check

Dmitry Torokhov
2015-07-27T21:44:48Z

On Mon, Jul 27, 2015 at 11:35:23PM +0200, Heiko Stübner wrote:
> Hi Dmitry,
>
> Am Montag, 27. Juli 2015, 14:06:19 schrieb Dmitry Torokhov:
> > Commit 7d01cd261c76f95913c81554a751968a1d282d3a ("Input: zforce - don't
> > overwrite the stack") attempted to add a check for payload size being too
> > large for the supplied buffer. Unfortunately with the currently selected
> > buffer size the comparison is always false as buffer size is larger than
> > the value a single byte can hold, and that results in compiler warnings.
> > Additionally the check was incorrect as it was not accounting for the
> > already read 2 bytes of data stored in the buffer.
> >
> > Fixes: 7d01cd261c76f95913c81554a751968a1d282d3a
> > Reported-by: kbuild test robot <fengguang.wu@intel.com>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
> > ---
> >
> > This seems to shut up my GCC, I wonder if it is going to work gfor
> > everyone or we better add BUILD_BUG_ON(FRAME_MAXSIZE < 257) and a
> > comment and remove check.
>
> needed a bit to get to know my old zforce driver again ;-)
>
>
> I may be blind, but currently I fail to see what problem the original patch
> actually tries to fix.
>
> buf[PAYLOAD_LENGTH] is an u8, so the max value it can contain is 255. The
> i2c_master_recv reads buf[PAYLOAD_LENGTH]-bytes into the buffer starting at
> buf[PAYLOAD_BODY] (= buf[2]). So it reads at max 255 bytes into a 257 byte big
> buffer starting at index 2.
>
> zforce_read_packet, also is an internal function used only by the interrupt
> handler, which always only calls it with a buffer of FRAME_MAXSIZE size.
>
>
> The original patch said "If we get a corrupted packet with PAYLOAD_LENGTH >
> FRAME_MAXSIZE, we will silently overwrite the stack." but payload_length can
> never actually be greater than the buffer size?

Right, not unless we for some reason decide to adjust FRAME_MAXSIZE to
make it smaller than 257 and then fail to add the check to make sure we
do not go past the buffer.

So everything is fine now, but I guess we'd like to be more safe in the
future...

Thanks.

--
Dmitry