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

[Takashi Iwai]

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: