Re: [PATCH v4 07/10] can: peak_usb: fix RX cursor desync for self-received RTR frames with SRR

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- [Medium] This patch adds an error-path stats regression in
  pcan_usb_decode_data() (drivers/net/can/usb/peak_usb/pcan_usb.c).

--

can: peak_usb: fix RX cursor desync for self-received RTR frames with SRR

This commit moves the SRR private id byte skip in pcan_usb_decode_data()
out of the non-RTR branch. The byte is now consumed whenever
PCAN_USB_TX_SRR is set, which matches the layout used by
pcan_usb_encode_msg() and stops the RX cursor desync for self-received
RTR frames.

> diff --git a/drivers/net/can/usb/peak_usb/pcan_usb.c b/drivers/net/can/usb/peak_usb/pcan_usb.c
> index 93da0fd4f10e7..4c8cfed31634a 100644
> --- a/drivers/net/can/usb/peak_usb/pcan_usb.c
> +++ b/drivers/net/can/usb/peak_usb/pcan_usb.c
> @@ -697,16 +697,19 @@ static int pcan_usb_decode_data(struct pcan_usb_msg_context *mc, u8 status_len)
>  		memcpy(cf->data, mc->ptr, cf->len);
>  		mc->ptr += rec_len;
>  
> -		/* Ignore next byte (client private id) if SRR bit is set */
> -		if (can_id_flags & PCAN_USB_TX_SRR) {
> -			if ((mc->ptr + 1) > mc->end)
> -				goto decode_failed;
> -			mc->ptr++;
> -		}
> -
>  		/* update statistics */
>  		mc->netdev->stats.rx_bytes += cf->len;
>  	}
> +
> +	/* Ignore next byte (client private id) if SRR bit is set.
> +	 * Note: this test must be done even when the frame is an RTR frame.
> +	 */
> +	if (can_id_flags & PCAN_USB_TX_SRR) {
> +		if ((mc->ptr + 1) > mc->end)
> +			goto decode_failed;
> +		mc->ptr++;
> +	}
> +
>  	mc->netdev->stats.rx_packets++;

[Severity: Medium]
Can this leave rx_bytes counting a frame that gets dropped?

Before this patch, the SRR bounds check ran inside the non-RTR branch,
ahead of the rx_bytes update. Now rx_bytes is updated first, and the
check comes after it.

Take a non-RTR record that has PCAN_USB_TX_SRR set and a non-zero
payload ending exactly at mc->end, so the private id byte is missing.
In pcan_usb_decode_data():

    mc->ptr += rec_len;
    ...
    mc->netdev->stats.rx_bytes += cf->len;
    }
    ...
    if (can_id_flags & PCAN_USB_TX_SRR) {
        if ((mc->ptr + 1) > mc->end)
            goto decode_failed;

decode_failed frees the skb and returns -EINVAL. No frame is delivered
and rx_packets is not incremented, but the rx_bytes increment of up to
8 bytes is kept.

The device firmware controls this data, so a faulty or malicious USB
device could trigger it through:

peak_usb_read_bulk_callback()->pcan_usb_decode_buf()->
    pcan_usb_decode_msg()->pcan_usb_decode_data()

Would it work to do the non-RTR rx_bytes update after the shared SRR
check succeeds? For example, next to rx_packets++ and guarded by
!(status_len & PCAN_USB_STATUSLEN_RTR).

>  
>  	/* convert timestamp into kernel time */

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20261020-canfd_check_channel_idx-v4-0-bfbde81f8bab%40peak-system.fr