Re: [PATCH] net: Fix for packets being rejected in the xHCI controller's ring buffer

[Greg KH]

On Tue, Dec 13, 2022 at 12:14:28PM -0500, Seija K. wrote:
> When a packet larger than MTU arrives in Linux from the modem,
> it is discarded with -EOVERFLOW error (Babble error).
> 
> This is seen on USB3.0 and USB2.0 buses.
> 
> This is because the MRU (Max Receive Size) is not a separate entity
> from the MTU (Max Transmit Size),
> and the received packets can be larger than those transmitted.
> 
> Following the babble error, there was an endless supply of zero-length URBs,
> which are rejected with -EPROTO (increasing the rx input error counter
> each time).
> 
> This is only seen on USB3.0.
> These continue to come ad infinitum until the modem is shut down.
> 
> There appears to be a bug in the core USB handling code in Linux
> that doesn't deal well with network MTUs smaller than 1500 bytes.
> 
> By default, the dev->hard_mtu (the real MTU)
> is in lockstep with dev->rx_urb_size (essentially an MRU),
> and the latter is causing trouble.
> 
> This has nothing to do with the modems,
> as the issue can be reproduced by getting a USB-Ethernet dongle,
> setting the MTU to 1430, and pinging with size greater than 1406.
> 
> Signed-off-by: Seija Kijin <doremylover123@xxxxxxxxx>
> Co-Authored-By: TarAldarion <gildeap@xxxxxx>
> 
> diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
> index 554d4e2a84a4..39db53a74b5a 100644
> --- a/drivers/net/usb/qmi_wwan.c
> +++ b/drivers/net/usb/qmi_wwan.c
> @@ -842,6 +842,13 @@ static int qmi_wwan_bind(struct usbnet *dev,
> struct usb_interface *intf)
> }
> dev->net->netdev_ops = &qmi_wwan_netdev_ops;
> dev->net->sysfs_groups[0] = &qmi_wwan_sysfs_attr_group;
> + /* LTE Networks don't always respect their own MTU on receive side;
> + * e.g. AT&T pushes 1430 MTU but still allows 1500 byte packets from
> + * far-end network. Make the receive buffer large enough to accommodate
> + * them, and add four bytes so MTU does not equal MRU on network
> + * with 1500 MTU otherwise usbnet_change_mtu() will change both.
> + */
> + dev->rx_urb_size = ETH_DATA_LEN + 4;
> err:
> return status;
> }

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
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