usbnet: limit max_mtu based on device's hard_mtu

[dilyanpalauzov]

This backport of https://git.kernel.org/netdev/net/c/c7159e960f14 will allow running Raspberry Pi OS kernel in QEMU using passt usb networking, without requiring throttling.

The initial problem is described at https://gitlab.com/qemu-project/qemu/-/issues/3268 and https://bugs.passt.top/attachment.cgi?bugid=189.

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From: Laurent Vivier <lvivier working in redhat.com>
Date: Mon, 19 Jan 2026 08:55:18 +0100

The usbnet driver initializes net->max_mtu to ETH_MAX_MTU before calling the device's bind() callback. When the bind() callback sets dev->hard_mtu based the device's actual capability (from CDC Ethernet's wMaxSegmentSize descriptor), max_mtu is never updated to reflect this hardware limitation).

This allows userspace (DHCP or IPv6 RA) to configure MTU larger than the device can handle, leading to silent packet drops when the backend sends packet exceeding the device's buffer size.

Fix this by limiting net->max_mtu to the device's hard_mtu after the bind callback returns.

See https://gitlab.com/qemu-project/qemu/-/issues/3268 and
https://bugs.passt.top/attachment.cgi?bugid=189

Fixes: f77f0ae ("net: use core MTU range checking in USB NIC drivers")

Link: https://bugs.passt.top/show_bug.cgi?id=189
Reviewed-by: Stefano Brivio <sbrivio working in redhat.com>
Link: https://patch.msgid.link/20260119075518.2774373-1-lvivier@redhat.com

[pelwell]

You're keen - this commit hasn't even hit linux-next yet. Fortunately for you it has a Fixes tag, so I expect it to be automatically back-ported to the LTS branches, including 6.12. If nothing has appeared after a week or two, feel free to add a comment.