From: Laurent Vivier Date: Fri, 31 Jul 2026 09:27:11 +0000 (+0200) Subject: usbnet: cap max_mtu for drivers without bind callback X-Git-Url: http://git.ipfire.org/gitweb/?a=commitdiff_plain;h=1505b2cb6ae1c7e8ac0c6e4590a204ffc3ab2b24;p=thirdparty%2Fkernel%2Flinux.git usbnet: cap max_mtu for drivers without bind callback usbnet_probe() initializes max_mtu to ETH_MAX_MTU and only caps it inside the if (info->bind) block. 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. Initialize max_mtu to net->mtu at probe time and update it inside the bind block. Fixes: f77f0aee4da4 ("net: use core MTU range checking in USB NIC drivers") Cc: jarod@redhat.com 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 Link: https://patch.msgid.link/20260731092711.857684-1-lvivier@redhat.com Signed-off-by: Jakub Kicinski --- diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c index 25518635b7b7..a19ecf718f36 100644 --- a/drivers/net/usb/usbnet.c +++ b/drivers/net/usb/usbnet.c @@ -1794,7 +1794,7 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod) */ dev->hard_mtu = net->mtu + net->hard_header_len; net->min_mtu = 0; - net->max_mtu = ETH_MAX_MTU; + net->max_mtu = net->mtu; net->netdev_ops = &usbnet_netdev_ops; net->watchdog_timeo = TX_TIMEOUT_JIFFIES; @@ -1804,6 +1804,7 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod) // allow device-specific bind/init procedures // NOTE net->name still not usable ... if (info->bind) { + net->max_mtu = ETH_MAX_MTU; status = info->bind(dev, udev); if (status < 0) goto out1;