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

[Dmitry Torokhov]

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@xxxxxxxxx>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> > ---
> > 
> > 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
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