net: Fix for packets being rejected in the xHCI controller's ring buffer

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@gmail.com>
Co-Authored-By: TarAldarion <gildeap@tcd.ie>

}

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@gmail.com>
> Co-Authored-By: TarAldarion <gildeap@tcd.ie>
> 
> 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;
> }

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