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SubjectRe: [PATCH] Fix an OOB bug in parse_audio_mixer_unit
On Wed, Aug 14, 2019 at 08:36:42AM +0200, Takashi Iwai 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@gmail.com>
> > Reported-by: Hui Peng <benquike@gmail.com>
> > Reported-by: Mathias Payer <mathias.payer@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;

VERSION 1 and 2 already have a different check:

if (desc->bLength < sizeof(*desc) + desc->bNrInPins + 1)
return 0; /* no bmControls -> skip */

So something is possibly off by one. It's just version 3 which doesn't
have a check.

regards,
dan carpenter

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