[alsa-devel] [PATCH v2 2/3] ALSA: usb-audio: More strict sanity checks for clock parsers

[Ruslan Bilovol]

Hi Takashi,

On Wed, Apr 4, 2018 at 8:36 AM, Takashi Iwai <tiwai at suse.de> wrote:
> The sanity checks introduced for malformed descriptors loosely check
> the given descriptor size, although the size greater than the defined
> description is invalid.  It was due to a concern of any funky firmware
> in the actual products.  But this doesn't look hitting, and any sane
> products must have the defined descriptors.
>
> So in this patch, we make the validators more strict, allowing only
> with the defined descriptor sizes.
>
> Suggested-by: Ruslan Bilovol <ruslan.bilovol at gmail.com>
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> ---
>  sound/usb/clock.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/sound/usb/clock.c b/sound/usb/clock.c
> index 27c2275a2505..5e533edfb092 100644
> --- a/sound/usb/clock.c
> +++ b/sound/usb/clock.c
> @@ -52,7 +52,7 @@ static void *find_uac_clock_desc(struct usb_host_interface *iface, int id,
>  static bool validate_clock_source_v2(void *p, int id)
>  {
>         struct uac_clock_source_descriptor *cs = p;
> -       return cs->bLength >= sizeof(*cs) && cs->bClockID == id;
> +       return cs->bLength == sizeof(*cs) && cs->bClockID == id;

This one looks good

>  }
>
>  static bool validate_clock_source_v3(void *p, int id)
> @@ -64,8 +64,8 @@ static bool validate_clock_source_v3(void *p, int id)
>  static bool validate_clock_selector_v2(void *p, int id)
>  {
>         struct uac_clock_selector_descriptor *cs = p;
> -       return cs->bLength >= sizeof(*cs) && cs->bClockID == id &&
> -               cs->bLength >= 5 + cs->bNrInPins;
> +       return cs->bLength == sizeof(*cs) && cs->bClockID == id &&
> +               cs->bLength == 5 + cs->bNrInPins;

This one is wrong. uac_clock_selector_descriptor is of variable size, so
original first bLength check was correct, but last one can be more strict.
However, as per UAC2 spec, bLength should be "7 + bNrInPins", so
finally we shoud have here something like this:

       return cs->bLength >= sizeof(*cs) && cs->bClockID == id &&
               cs->bLength == 7 + cs->bNrInPins;


>  }
>
>  static bool validate_clock_selector_v3(void *p, int id)
> @@ -77,7 +77,7 @@ static bool validate_clock_selector_v3(void *p, int id)
>  static bool validate_clock_multiplier_v2(void *p, int id)
>  {
>         struct uac_clock_multiplier_descriptor *cs = p;
> -       return cs->bLength >= sizeof(*cs) && cs->bClockID == id;
> +       return cs->bLength == sizeof(*cs) && cs->bClockID == id;

This one looks good too

Thanks,
Ruslan