Re: [PATCH] ASoC: Intel: Skylake: Replace 1-element array with flex-array

[Kees Cook]

On Fri, Feb 10, 2023 at 02:10:56PM +0100, Amadeusz Sławiński wrote:
> On 2/10/2023 6:14 AM, Kees Cook wrote:
> > The kernel is globally removing the ambiguous 0-length and 1-element
> > arrays in favor of flexible arrays, so that we can gain both compile-time
> > and run-time array bounds checking[1]. In this instance, struct
> > skl_cpr_cfg contains struct skl_cpr_gtw_cfg, which defined "config_data"
> > as a 1-element array.
> > 
> > Normally when switching from a 1-element array to a flex-array, any
> > related size calculations must be adjusted too. However, it seems the
> > original code was over-allocating space, since 1 extra u32 would be
> > included by the sizeof():
> > 
> >                  param_size = sizeof(struct skl_cpr_cfg);
> >                  param_size += mconfig->formats_config[SKL_PARAM_INIT].caps_size;
> > 
> > But the copy uses caps_size bytes, and cap_size / 4 (i.e. sizeof(u32))
> > for the length tracking:
> > 
> >          memcpy(cpr_mconfig->gtw_cfg.config_data,
> >                          mconfig->formats_config[SKL_PARAM_INIT].caps,
> >                          mconfig->formats_config[SKL_PARAM_INIT].caps_size);
> > 
> >          cpr_mconfig->gtw_cfg.config_length =
> >                          (mconfig->formats_config[SKL_PARAM_INIT].caps_size) / 4;
> > 
> > Therefore, no size calculations need adjusting. Change the struct
> > skl_cpr_gtw_cfg config_data member to be a true flexible array, which
> > also fixes the over-allocation, and silences this memcpy run-time false
> > positive:
> > 
> >    memcpy: detected field-spanning write (size 100) of single field "cpr_mconfig->gtw_cfg.config_data" at sound/soc/intel/skylake/skl-messages.c:554 (size 4)
> > 
> > [1] For lots of details, see both:
> >      https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays
> >      https://people.kernel.org/kees/bounded-flexible-arrays-in-c
> > 
> > Reported-by: Sasa Ostrouska <casaxa@xxxxxxxxx>
> > Link: https://lore.kernel.org/all/CALFERdwvq5day_sbDfiUsMSZCQu9HG8-SBpOZDNPeMdZGog6XA@xxxxxxxxxxxxxx/
> > Cc: Cezary Rojewski <cezary.rojewski@xxxxxxxxx>
> > Cc: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
> > Cc: Liam Girdwood <liam.r.girdwood@xxxxxxxxxxxxxxx>
> > Cc: Peter Ujfalusi <peter.ujfalusi@xxxxxxxxxxxxxxx>
> > Cc: Bard Liao <yung-chuan.liao@xxxxxxxxxxxxxxx>
> > Cc: Ranjani Sridharan <ranjani.sridharan@xxxxxxxxxxxxxxx>
> > Cc: Kai Vehmanen <kai.vehmanen@xxxxxxxxxxxxxxx>
> > Cc: Mark Brown <broonie@xxxxxxxxxx>
> > Cc: Jaroslav Kysela <perex@xxxxxxxx>
> > Cc: Takashi Iwai <tiwai@xxxxxxxx>
> > Cc: "Gustavo A. R. Silva" <gustavoars@xxxxxxxxxx>
> > Cc: "Amadeusz Sławiński" <amadeuszx.slawinski@xxxxxxxxxxxxxxx>
> > Cc: alsa-devel@xxxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >   sound/soc/intel/skylake/skl-topology.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/sound/soc/intel/skylake/skl-topology.h b/sound/soc/intel/skylake/skl-topology.h
> > index 6db0fd7bad49..ad94f8020c27 100644
> > --- a/sound/soc/intel/skylake/skl-topology.h
> > +++ b/sound/soc/intel/skylake/skl-topology.h
> > @@ -115,7 +115,7 @@ struct skl_cpr_gtw_cfg {
> >   	u32 dma_buffer_size;
> >   	u32 config_length;
> >   	/* not mandatory; required only for DMIC/I2S */
> > -	u32 config_data[1];
> > +	u32 config_data[];
> >   } __packed;
> >   struct skl_dma_control {
> 
> This fails in our validation.

Ah, okay. Thanks for checking!

> Maybe we can use the union workaround, to
> leave the size as is?
> 
> Following seems to work in manual test:
> diff --git a/sound/soc/intel/skylake/skl-topology.h
> b/sound/soc/intel/skylake/skl-topology.h
> index 6db0fd7bad49..ffbd2e60fede 100644
> --- a/sound/soc/intel/skylake/skl-topology.h
> +++ b/sound/soc/intel/skylake/skl-topology.h
> @@ -115,7 +115,10 @@ struct skl_cpr_gtw_cfg {
>         u32 dma_buffer_size;
>         u32 config_length;
>         /* not mandatory; required only for DMIC/I2S */
> -       u32 config_data[1];
> +       union {
> +               u32 x;
> +               u32 config_data[0];
> +       };

Yeah, that could work, though the last member would be:
	DECLARE_FLEX_ARRAY(u32, config_data);
otherwise the array is 0 length (rather than a proper flex array).

But before that, let me see if I can track down where the size is being
used, in case we can avoid adding the padding.

-- 
Kees Cook