linux-kernel - Re: [bug report] net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

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Message-ID: <aYYE0SBR6V-HWcPr@linaro.org>
Date: Fri, 6 Feb 2026 16:12:17 +0100
From: Stephan Gerhold <stephan.gerhold@...aro.org>
To: Dan Carpenter <dan.carpenter@...aro.org>
Cc: Stephan Gerhold <stephan@...hold.net>,
	Johannes Berg <johannes@...solutions.net>, netdev@...r.kernel.org,
	linux-arm-msm@...r.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [bug report] net: wwan: Add Qualcomm BAM-DMUX WWAN network driver

Hi Dan,

On Fri, Feb 06, 2026 at 04:38:30PM +0300, Dan Carpenter wrote:
> [ Smatch checking is paused while we raise funding.  #SadFace
>   https://lore.kernel.org/all/aTaiGSbWZ9DJaGo7@stanley.mountain/ -dan ]
> 
> Hello Stephan Gerhold,
> 
> Commit 21a0ffd9b38c ("net: wwan: Add Qualcomm BAM-DMUX WWAN network
> driver") from Nov 27, 2021 (linux-next), leads to the following
> Smatch static checker warning:
> 
> 	drivers/net/wwan/qcom_bam_dmux.c:505 bam_dmux_cmd_data()
> 	error: buffer overflow 'dmux->netdevs' 8 <= 255 user_rl='0-255' uncapped
> 
> drivers/net/wwan/qcom_bam_dmux.c
>     500 static void bam_dmux_cmd_data(struct bam_dmux_skb_dma *skb_dma)
>     501 {
>     502         struct bam_dmux *dmux = skb_dma->dmux;
>     503         struct sk_buff *skb = skb_dma->skb;
>     504         struct bam_dmux_hdr *hdr = (struct bam_dmux_hdr *)skb->data;
> --> 505         struct net_device *netdev = dmux->netdevs[hdr->ch];
>                                                           ^^^^^^^
> Smatch thinks skb->data is untrusted.  This is the rx path.
> 

Thanks a lot for the report!

I believe this is not a problem in practice, since there is an existing
check for this in bam_dmux_rx_callback() (which is the only function
that calls bam_dmux_cmd_data()):

	if (hdr->ch >= BAM_DMUX_NUM_CH) {
		dev_dbg(dmux->dev, "Unsupported channel: %u\n", hdr->ch);
		goto out;
	}

	switch (hdr->cmd) {
	case BAM_DMUX_CMD_DATA:
		bam_dmux_cmd_data(skb_dma);
		break;

Is that something Smatch should be able to detect?

Thanks,
Stephan