linux-kernel - Re: [PATCH v2] ALSA: usb-mixer: us16x08: validate meter packet indices

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Message-ID: <878qf2g0tm.wl-tiwai@suse.de>
Date: Tue, 16 Dec 2025 11:47:49 +0100
From: Takashi Iwai <tiwai@...e.de>
To: Shipei Qu <qu@...knavy.com>
Cc: Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>,
	alsa-devel@...a-project.org,
	linux-kernel@...r.kernel.org,
	DARKNAVY <vr@...knavy.com>
Subject: Re: [PATCH v2] ALSA: usb-mixer: us16x08: validate meter packet indices

On Tue, 16 Dec 2025 07:01:56 +0100,
Shipei Qu wrote:
> 
> Hi,
> 
> resending with a properly formatted diff (the previous email had a malformed
> patch header). The change itself is the same: while fuzzing a USB gadget that
> emulates a Tascam US-16x08 we found that get_meter_levels_from_urb() in
> mixer_us16x08.c uses a channel index taken directly from the 64-byte meter
> packet to index meter_level[], comp_level[] and master_level[] without any
> bounds checking. A malformed packet can therefore cause out-of-bounds writes in
> the snd_us16x08_meter_store.
> 
> A malicious USB audio device (or USB gadget implementation) that pretends to be
> a US-16x08-compatible interface can trigger this by sending crafted meter
> packets. We have a small USB gadget-based PoC for this behaviour and can share
> it if that would be helpful.
> 
> This driver is used by common distributions (e.g. Ubuntu) when a US-16x08 or
> compatible USB audio device is present. The same pattern is present in current
> mainline kernels.
> 
> This issue was first reported via security@...nel.org. The kernel security team
> explained that, in the upstream threat model, USB endpoints are expected to be
> trusted (i.e. only trusted devices should be bound to drivers), so they
> consider this a normal bug rather than a security vulnerability, and asked us
> to send a fix to the development lists. The patch below adds simple range
> checks before updating these arrays.
> 
> Reported-by: DARKNAVY (@DarkNavyOrg) <vr@...knavy.com>
> Signed-off-by: Shipei Qu <qu@...knavy.com>

Thanks, now it looks applicable, and the code change itself looks
good.

But, could you try to rephrase the patch description above?  It'll be
used directly as the commit message.  So it should be well descriptive
about the bug and the fix, while it should concentrate on the
technical issues.

You can check other commits how they look like, and also see
Documentation/process/submitting-patches.rst.


thanks,

Takashi

> ---
>  sound/usb/mixer_us16x08.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/sound/usb/mixer_us16x08.c b/sound/usb/mixer_us16x08.c
> index 1c5712c31..f9df40730 100644
> --- a/sound/usb/mixer_us16x08.c
> +++ b/sound/usb/mixer_us16x08.c
> @@ -655,17 +655,25 @@ static void get_meter_levels_from_urb(int s,
>  	u8 *meter_urb)
>  {
>  	int val = MUC2(meter_urb, s) + (MUC3(meter_urb, s) << 8);
> +	int ch = MUB2(meter_urb, s) - 1;
> +
> +	if (ch < 0)
> +		return;
>  
>  	if (MUA0(meter_urb, s) == 0x61 && MUA1(meter_urb, s) == 0x02 &&
>  		MUA2(meter_urb, s) == 0x04 && MUB0(meter_urb, s) == 0x62) {
> -		if (MUC0(meter_urb, s) == 0x72)
> -			store->meter_level[MUB2(meter_urb, s) - 1] = val;
> -		if (MUC0(meter_urb, s) == 0xb2)
> -			store->comp_level[MUB2(meter_urb, s) - 1] = val;
> +		if (ch < SND_US16X08_MAX_CHANNELS) {
> +			if (MUC0(meter_urb, s) == 0x72)
> +				store->meter_level[ch] = val;
> +			if (MUC0(meter_urb, s) == 0xb2)
> +				store->comp_level[ch] = val;
> +		}
>  	}
>  	if (MUA0(meter_urb, s) == 0x61 && MUA1(meter_urb, s) == 0x02 &&
> -		MUA2(meter_urb, s) == 0x02 && MUB0(meter_urb, s) == 0x62)
> -		store->master_level[MUB2(meter_urb, s) - 1] = val;
> +		MUA2(meter_urb, s) == 0x02 && MUB0(meter_urb, s) == 0x62) {
> +		if (ch < ARRAY_SIZE(store->master_level))
> +			store->master_level[ch] = val;
> +	}
>  }
>  
>  /* Function to retrieve current meter values from the device.
> -- 
> 2.45.1