linux-kernel - Re: [PATCH v3 06/24] drm/rockchip: dw-mipi-dsi: avoid out-of-bounds read on tx_buf

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Date:   Tue, 31 Jan 2017 09:48:18 -0500
From:   Sean Paul <seanpaul@...omium.org>
To:     John Keeping <john@...anate.com>
Cc:     Chris Zhong <zyw@...k-chips.com>,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 06/24] drm/rockchip: dw-mipi-dsi: avoid out-of-bounds
 read on tx_buf

On Tue, Jan 31, 2017 at 11:45:48AM +0000, John Keeping wrote:
> On Mon, 30 Jan 2017 15:09:55 -0500, Sean Paul wrote:
> 
> > On Mon, Jan 30, 2017 at 06:16:36PM +0000, John Keeping wrote:
> > > On Mon, 30 Jan 2017 13:01:46 -0500, Sean Paul wrote:
> > >   
> > > > On Sun, Jan 29, 2017 at 01:24:26PM +0000, John Keeping wrote:  
> > > > > As a side-effect of this, encode the endianness explicitly rather than
> > > > > casting a u16.
> > > > > 
> > > > > Signed-off-by: John Keeping <john@...anate.com>
> > > > > Reviewed-by: Chris Zhong <zyw@...k-chips.com>
> > > > > ---
> > > > > v3:
> > > > > - Add Chris' Reviewed-by
> > > > > Unchanged in v2
> > > > > 
> > > > >  drivers/gpu/drm/rockchip/dw-mipi-dsi.c | 9 +++++++--
> > > > >  1 file changed, 7 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > > > > index 4be1ff3a42bb..2e6ad4591ebf 100644
> > > > > --- a/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > > > > +++ b/drivers/gpu/drm/rockchip/dw-mipi-dsi.c
> > > > > @@ -572,8 +572,13 @@ static int dw_mipi_dsi_gen_pkt_hdr_write(struct dw_mipi_dsi *dsi, u32 hdr_val)
> > > > >  static int dw_mipi_dsi_dcs_short_write(struct dw_mipi_dsi *dsi,
> > > > >  				       const struct mipi_dsi_msg *msg)
> > > > >  {
> > > > > -	const u16 *tx_buf = msg->tx_buf;
> > > > > -	u32 val = GEN_HDATA(*tx_buf) | GEN_HTYPE(msg->type);
> > > > > +	const u8 *tx_buf = msg->tx_buf;
> > > > > +	u32 val = GEN_HTYPE(msg->type);
> > > > > +
> > > > > +	if (msg->tx_len > 0)
> > > > > +		val |= GEN_HDATA(tx_buf[0]);
> > > > > +	if (msg->tx_len > 1)
> > > > > +		val |= GEN_HDATA(tx_buf[1] << 8);    
> > > > 
> > > > You should probably update the mask inside GEN_HDATA to mask off 8 bits instead of
> > > > 16.  
> > > 
> > > Won't that mask off the data written by "tx_buf[1] << 8"?  
> > 
> > I would move the shift outside the mask, ie:
> > 
> > val |= GEN_HDATA(tx_buf[1]) << 8;
> 
> I can do that, but that doesn't seem to match the intention of the
> macros which are about encoding the placement and size of fields within
> registers.
> 
> Maybe it would be clearer to do:
> 
>     u16 data = 0;
> 
>     if (msg->tx_len > 0)
>         data |= tx_buf[0];
>     if (msg->tx_len > 1)
>         data |= tx_buf[1];

Yep, this looks good to me, but I think you need:

data |= tx_buf[1] << 8;

Sean

> 
>     val = GEN_HDATA(data) | GEN_HTYPE(msg->type);
> 
> ?
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-- 
Sean Paul, Software Engineer, Google / Chromium OS

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