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

[彭辉]

Hi, Takashi:
Here the problem is that `desc->bLength` is controlled by the device side,
so  `desc->bLength` may not represent the real length of the descriptor.
That is why I use pointer arithmetic operations to derive the real size of
the buffer
in my patch.

On Wed, Aug 14, 2019 at 2:36 AM Takashi Iwai <tiwai at suse.de> wrote:

> On Wed, 14 Aug 2019 04:36:24 +0200,
> Hui Peng wrote:
> >
> > The `uac_mixer_unit_descriptor` shown as below is read from the
> > device side. In `parse_audio_mixer_unit`, `baSourceID` field is
> > accessed from index 0 to `bNrInPins` - 1, the current implementation
> > assumes that descriptor is always valid (the length  of descriptor
> > is no shorter than 5 + `bNrInPins`). If a descriptor read from
> > the device side is invalid, it may trigger out-of-bound memory
> > access.
> >
> > ```
> > struct uac_mixer_unit_descriptor {
> >       __u8 bLength;
> >       __u8 bDescriptorType;
> >       __u8 bDescriptorSubtype;
> >       __u8 bUnitID;
> >       __u8 bNrInPins;
> >       __u8 baSourceID[];
> > }
> > ```
> >
> > This patch fixes the bug by add a sanity check on the length of
> > the descriptor.
> >
> > Signed-off-by: Hui Peng <benquike at gmail.com>
> > Reported-by: Hui Peng <benquike at gmail.com>
> > Reported-by: Mathias Payer <mathias.payer at nebelwelt.net>
> > ---
> >  sound/usb/mixer.c | 9 +++++++++
> >  1 file changed, 9 insertions(+)
> >
> > diff --git a/sound/usb/mixer.c b/sound/usb/mixer.c
> > index 7498b5191b68..38202ce67237 100644
> > --- a/sound/usb/mixer.c
> > +++ b/sound/usb/mixer.c
> > @@ -2091,6 +2091,15 @@ static int parse_audio_mixer_unit(struct
> mixer_build *state, int unitid,
> >       struct usb_audio_term iterm;
> >       int input_pins, num_ins, num_outs;
> >       int pin, ich, err;
> > +     int desc_len = (int) ((unsigned long) state->buffer +
> > +                     state->buflen - (unsigned long) raw_desc);
> > +
> > +     if (desc_len < sizeof(*desc) + desc->bNrInPins) {
> > +             usb_audio_err(state->chip,
> > +                           "descriptor %d too short\n",
> > +                           unitid);
> > +             return -EINVAL;
> > +     }
> >
> >       err = uac_mixer_unit_get_channels(state, desc);
> >       if (err < 0) {
>
> Hm, what is the desc->bLength value in the error case?
>
> Basically the buffer boundary is already checked against bLength in
> snd_usb_find_desc() which is called from obtaining the raw_desc in the
> caller of this function (parse_audio_unit()).
>
> So, if any, we need to check bLength for the possible overflow like
> below.
>
>
> thanks,
>
> Takashi
>
> --- a/sound/usb/mixer.c
> +++ b/sound/usb/mixer.c
> @@ -744,6 +744,8 @@ static int uac_mixer_unit_get_channels(struct
> mixer_build *state,
>                 return -EINVAL;
>         if (!desc->bNrInPins)
>                 return -EINVAL;
> +       if (desc->bLength < sizeof(*desc) + desc->bNrInPins)
> +               return -EINVAL;
>
>         switch (state->mixer->protocol) {
>         case UAC_VERSION_1:
>


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