[PATCH net v3] usbnet: cap max_mtu for drivers without bind callback
From: Laurent Vivier
Date: Fri Jul 31 2026 - 05:30:53 EST
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@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Link: https://gitlab.com/qemu-project/qemu/-/issues/3268
Link: https://bugs.passt.top/show_bug.cgi?id=189
Signed-off-by: Laurent Vivier <lvivier@xxxxxxxxxx>
---
Notes:
v3:
- set max_mtu to ETH_MAX_MTU under if (info->bind) before calling
info->bind()
v2:
- Set max_mtu to net->mtu at init, update inside bind
- Fix Fixes tag to f77f0aee4da4
drivers/net/usb/usbnet.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
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;
--
2.54.0