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:

quoted

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 <redacted>
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
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