netdev - [PATCH] usbnet: limit max_mtu based on device's hard

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Message-ID: <20260114090317.3214026-1-lvivier@redhat.com>
Date: Wed, 14 Jan 2026 10:03:17 +0100
From: Laurent Vivier <lvivier@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: netdev@...r.kernel.org,
	Oliver Neukum <oneukum@...e.com>,
	linux-usb@...r.kernel.org,
	Laurent Vivier <lvivier@...hat.com>
Subject: [PATCH] usbnet: limit max_mtu based on device's hard_mtu

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

Signed-off-by: Laurent Vivier <lvivier@...hat.com>
---
 drivers/net/usb/usbnet.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 36742e64cff7..8dbbeb8ce3f8 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1821,9 +1821,14 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod)
 		if ((dev->driver_info->flags & FLAG_NOARP) != 0)
 			net->flags |= IFF_NOARP;
 
-		/* maybe the remote can't receive an Ethernet MTU */
-		if (net->mtu > (dev->hard_mtu - net->hard_header_len))
-			net->mtu = dev->hard_mtu - net->hard_header_len;
+		/* limit max_mtu to the device's hard_mtu */
+		if (net->max_mtu > (dev->hard_mtu - net->hard_header_len))
+			net->max_mtu = dev->hard_mtu - net->hard_header_len;
+
+		/* limit mtu to max_mtu */
+		if (net->mtu > net->max_mtu)
+			net->mtu = net->max_mtu;
+
 	} else if (!info->in || !info->out)
 		status = usbnet_get_endpoints(dev, udev);
 	else {
-- 
2.52.0