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

[Amadeusz Sławiński]

On 2/10/2023 8:06 PM, Kees Cook wrote:
> 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.

I did spend some more time on this, apparently struct was missing one 
field according to IPC protocol. I've send fix for this.