[alsa-devel] [PATCH] Fix an OOB bug in uac_mixer_unit_bmControls

[Takashi Iwai]

On Sat, 17 Aug 2019 17:57:38 +0200,
Hui Peng wrote:
> 
> Looking around, there are other suspicious codes. E.g., in the following
> function, it seems to be the same as `uac_mixer_unit_bmControls`, but it is
> accessing `desc->bNrInPins + 5`, in case of UAC_VERSION_1.
> Is this intended?

Yes, this isn't for the mixer unit but for the processing unit.
They have different definitions.

Now back to the original report: I read the code again but fail to see
where is OOB access.  Let's see the function:

static int uac_mixer_unit_get_channels(struct mixer_build *state,
				       struct uac_mixer_unit_descriptor *desc)
{
	int mu_channels;
	void *c;

	if (desc->bLength < sizeof(*desc))
		return -EINVAL;
	if (!desc->bNrInPins)
		return -EINVAL;
	if (desc->bLength < sizeof(*desc) + desc->bNrInPins)
		return -EINVAL;

	switch (state->mixer->protocol) {
	case UAC_VERSION_1:
	case UAC_VERSION_2:
	default:
		if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 1)
			return 0; /* no bmControls -> skip */
		mu_channels = uac_mixer_unit_bNrChannels(desc);
		break;
	case UAC_VERSION_3:
		mu_channels = get_cluster_channels_v3(state,
				uac3_mixer_unit_wClusterDescrID(desc));
		break;
	}

	if (!mu_channels)
		return 0;

... until this point, mu_channels is calculated but no actual access
happens.  Then:

	c = uac_mixer_unit_bmControls(desc, state->mixer->protocol);

... this returns the *address* of the bmControls bitmap.  At this
point, it's not accessed yet.  Now:

	if (c - (void *)desc + (mu_channels - 1) / 8 >= desc->bLength)
		return 0; /* no bmControls -> skip */

... here we check whether the actual bitmap address plus the max
bitmap size overflows bLength.  And if it overflows, returns 0,
indicating no bitmap available.

So the code doesn't access but checks properly beforehand as far as I
understand.  Is the actual OOB access triggered by some program?


thanks,

Takashi

> 
> static inline __u8 *uac_processing_unit_bmControls(struct uac_processing_unit_descriptor *desc,
>                                                    int protocol)
> {
>         switch (protocol) {
>         case UAC_VERSION_1:
>                 return &desc->baSourceID[desc->bNrInPins + 5];
>         case UAC_VERSION_2:
>                 return &desc->baSourceID[desc->bNrInPins + 6];
>         case UAC_VERSION_3:
>                 return &desc->baSourceID[desc->bNrInPins + 2];
>         default:
>                 return NULL;
>         }
> }
> 
> On Sat, Aug 17, 2019 at 2:55 AM Takashi Iwai <tiwai at suse.de> wrote:
> 
>     On Sat, 17 Aug 2019 06:32:07 +0200,
>     Hui Peng wrote:
>     >
>     > `uac_mixer_unit_get_channels` calls `uac_mixer_unit_bmControls`
>     > to get pointer to bmControls field. The current implementation of
>     > `uac_mixer_unit_get_channels` does properly check the size of
>     > uac_mixer_unit_descriptor descriptor and may allow OOB access
>     > in `uac_mixer_unit_bmControls`.
>     >
>     > Reported-by: Hui Peng <benquike at gmail.com>
>     > Reported-by: Mathias Payer <mathias.payer at nebelwelt.net>
>     > Signed-off-by: Hui Peng <benquike at gmail.com>
>    
>     Ah a good catch.
>    
>     One easier fix in this case would be to get the offset from
>     uac_mixer_unit_bmControls(), e.g.
>    
>             /* calculate the offset of bmControls field */
>             size_t bmc_offset = uac_mixer_unit_bmControls(NULL, protocol) -
>     NULL;
>             ....
>             if (desc->bLength < bmc_offset)
>                     return 0;
>    
>     thanks,
>    
>     Takashi
> 
>     > ---
>     >  sound/usb/mixer.c | 25 ++++++++++++++++++-------
>     >  1 file changed, 18 insertions(+), 7 deletions(-)
>     >
>     > diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
>     > index b5927c3d5bc0..00e6274a63c3 100644
>     > --- a/sound/usb/mixer.c
>     > +++ b/sound/usb/mixer.c
>     > @@ -738,28 +738,39 @@ static int get_cluster_channels_v3(struct
>     mixer_build *state, unsigned int clust
>     >  static int uac_mixer_unit_get_channels(struct mixer_build *state,
>     >                                      struct uac_mixer_unit_descriptor
>     *desc)
>     >  {
>     > -     int mu_channels;
>     > +     int mu_channels = 0;
>     >       void *c;
>     > 
>     > -     if (desc->bLength < sizeof(*desc))
>     > -             return -EINVAL;
>     >       if (!desc->bNrInPins)
>     >               return -EINVAL;
>     > -     if (desc->bLength < sizeof(*desc) + desc->bNrInPins)
>     > -             return -EINVAL;
>     > 
>     >       switch (state->mixer->protocol) {
>     >       case UAC_VERSION_1:
>     > +             // limit derived from uac_mixer_unit_bmControls
>     > +             if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 4)
>     > +                     return 0;
>     > +
>     > +             mu_channels = uac_mixer_unit_bNrChannels(desc);
>     > +             break;
>     > +
>     >       case UAC_VERSION_2:
>     > -     default:
>     > -             if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 1)
>     > +             // limit derived from uac_mixer_unit_bmControls
>     > +             if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 6)
>     >                       return 0; /* no bmControls -> skip */
>     > +
>     >               mu_channels = uac_mixer_unit_bNrChannels(desc);
>     >               break;
>     >       case UAC_VERSION_3:
>     > +             // limit derived from uac_mixer_unit_bmControls
>     > +             if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 2)
>     > +                     return 0; /* no bmControls -> skip */
>     > +
>     >               mu_channels = get_cluster_channels_v3(state,
>     >                               uac3_mixer_unit_wClusterDescrID(desc));
>     >               break;
>     > +
>     > +     default:
>     > +             break;
>     >       }
>     > 
>     >       if (!mu_channels)
>     > --
>     > 2.22.1
>     >
>     >
> 
>