Commit c7159e960f14 ("usbnet: limit max_mtu based on device's hard_mtu") caps max_mtu inside the if (info->bind) block in usbnet_probe(). Drivers without a bind callback never enter this block, so max_mtu stays at ETH_MAX_MTU. QEMU's usb-net device (0x0525/0xa4a2) is claimed by the cdc_subset driver which has no bind callback. The guest accepts any MTU from DHCP (e.g. 65520 from passt), leading to TCP segments that exceed the device's 2048-byte receive buffer and are silently dropped. Move the max_mtu capping after the if/else block so it applies to all usbnet drivers. Fixes: c7159e960f14 ("usbnet: limit max_mtu based on device's hard_mtu") Cc: stable@vger.kernel.org Link: https://gitlab.com/qemu-project/qemu/-/issues/3268 Link: https://bugs.passt.top/show_bug.cgi?id=189 Signed-off-by: Laurent Vivier --- drivers/net/usb/usbnet.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c index 25518635b7b7..d87299025552 100644 --- a/drivers/net/usb/usbnet.c +++ b/drivers/net/usb/usbnet.c @@ -1825,10 +1825,6 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod) if ((dev->driver_info->flags & FLAG_NOARP) != 0) net->flags |= IFF_NOARP; - if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 && - net->max_mtu > (dev->hard_mtu - net->hard_header_len)) - net->max_mtu = dev->hard_mtu - net->hard_header_len; - if (net->mtu > (dev->hard_mtu - net->hard_header_len)) net->mtu = dev->hard_mtu - net->hard_header_len; @@ -1851,6 +1847,10 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod) if (status == 0 && !usb_check_bulk_endpoints(udev, ep_addrs)) status = -EINVAL; } + if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 && + net->max_mtu > (dev->hard_mtu - net->hard_header_len)) + net->max_mtu = dev->hard_mtu - net->hard_header_len; + if (status >= 0 && dev->status) status = init_status(dev, udev); if (status < 0) -- 2.54.0