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

[Laurent Vivier]

Commit c7159e960f14 ("usbnet: limit max_mtu based on device's hard_mtu")
caps max_mtu inside the if (info->bind) block in usbnet_probe(). 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.

Move the max_mtu capping after the if/else block so it applies to all
usbnet drivers.

Fixes: c7159e960f14 ("usbnet: limit max_mtu based on device's hard_mtu")
Cc: stable@xxxxxxxxxxxxxxx
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@xxxxxxxxxx>
---
 drivers/net/usb/usbnet.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 25518635b7b7..d87299025552 100644
--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1825,10 +1825,6 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod)
 		if ((dev->driver_info->flags & FLAG_NOARP) != 0)
 			net->flags |= IFF_NOARP;
 
-		if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 &&
-		    net->max_mtu > (dev->hard_mtu - net->hard_header_len))
-			net->max_mtu = dev->hard_mtu - net->hard_header_len;
-
 		if (net->mtu > (dev->hard_mtu - net->hard_header_len))
 			net->mtu = dev->hard_mtu - net->hard_header_len;
 
@@ -1851,6 +1847,10 @@ usbnet_probe(struct usb_interface *udev, const struct usb_device_id *prod)
 		if (status == 0 && !usb_check_bulk_endpoints(udev, ep_addrs))
 			status = -EINVAL;
 	}
+	if ((dev->driver_info->flags & FLAG_NOMAXMTU) == 0 &&
+	    net->max_mtu > (dev->hard_mtu - net->hard_header_len))
+		net->max_mtu = dev->hard_mtu - net->hard_header_len;
+
 	if (status >= 0 && dev->status)
 		status = init_status(dev, udev);
 	if (status < 0)
-- 
2.54.0