[PATCH net v2] usbnet: limit max_mtu based on device's hard_mtu

[PATCH net v2] usbnet: limit max_mtu based on device's hard_mtu

[Laurent Vivier]

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: f77f0aee4da4 ("net: use core MTU range checking in USB NIC drivers")
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://bugs.passt.top/show_bug.cgi?id=189
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
---
 drivers/net/usb/usbnet.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 36742e64cff7..1093c2a412d9 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1821,9 +1821,12 @@ 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;
+		if (net->max_mtu > (dev->hard_mtu - net->hard_header_len))
+			net->max_mtu = dev->hard_mtu - net->hard_header_len;
+
+		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

Re: [PATCH net v2] usbnet: limit max_mtu based on device's hard_mtu

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Mon, 19 Jan 2026 08:55:18 +0100 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net,v2] usbnet: limit max_mtu based on device's hard_mtu
    https://git.kernel.org/netdev/net/c/c7159e960f14

You are awesome, thank you!
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