[PATCH net v3] usbnet: cap max_mtu for drivers without bind callback

[PATCH net v3] usbnet: cap max_mtu for drivers without bind callback

[Laurent Vivier]

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@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>
---

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

Re: [PATCH net v3] usbnet: cap max_mtu for drivers without bind callback

[patchwork-bot+netdevbpf]

Hello:

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

On Fri, 31 Jul 2026 11:27:11 +0200 you wrote:
> 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.
> 
> [...]

Here is the summary with links:
  - [net,v3] usbnet: cap max_mtu for drivers without bind callback
    https://git.kernel.org/netdev/net/c/1505b2cb6ae1

You are awesome, thank you!
-- 
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