From: Heiko =?ISO-8859-1?Q?St=FCbner?= <heiko.stuebner@bq.com>
To: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org, Dirk Behme <dirk.behme@de.bosch.com>,
        Oleksij Rempel <external.Oleksij.Rempel@de.bosch.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Input: zforce_ts - fix playload length check
Date: Mon, 27 Jul 2015 23:53:27 +0200
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Am Montag, 27. Juli 2015, 14:44:42 schrieb Dmitry Torokhov:
> On Mon, Jul 27, 2015 at 11:35:23PM +0200, Heiko St?bner 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...

I would argue that FRAME_MAXSIZE already indicates that it should not be 
changed. It's the maximum size a single frame can be. And this size is a 
property of the hardware itself, because of the format, 257 bytes is always 
the maximum you could get (2 bytes header + at max 255 bytes payload).

So this second check (while only taking up a minimal amount of time) actually 
only checks against kernel-developer making errors in the future and not 
something the hardware can cause.


But your change itself looks correct, so if you prefer to keep that check you 
can also add my
Reviewed-by: Heiko Stuebner <heiko.stuebner@bq.com>


Heiko
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