[PATCH net v2] usbnet: cap max_mtu for drivers without bind callback

[Laurent Vivier <lvivier@redhat.com>]

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 based on the device's hard_mtu.

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:
    v2:
    - Set max_mtu to net->mtu at init, update inside bind
    - Fix Fixes tag to f77f0aee4da4

 drivers/net/usb/usbnet.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
index 25518635b7b7..9f11ca449aad 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;
@@ -1825,8 +1825,9 @@ 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))
+		if (dev->driver_info->flags & FLAG_NOMAXMTU)
+			net->max_mtu = ETH_MAX_MTU;
+		else
 			net->max_mtu = dev->hard_mtu - net->hard_header_len;
 
 		if (net->mtu > (dev->hard_mtu - net->hard_header_len))
-- 
2.54.0