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usbnet: cap max_mtu for drivers without bind callback

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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: f77f0aee4d ("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 <lvivier@redhat.com>
Link: https://patch.msgid.link/20260731092711.857684-1-lvivier@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Laurent Vivier 2026-07-31 11:27:11 +02:00 committed by Jakub Kicinski
parent 2cbd8a4e5e
commit 1505b2cb6a
1 changed files with 2 additions and 1 deletions
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3
drivers/net/usb/usbnet.c
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@ -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;