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SubjectRe: [PATCH] Fix an OOB bug in parse_audio_mixer_unit
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;

switch (state->mixer->protocol) {
case UAC_VERSION_1:
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