[PATCH 7.1 137/271] usbnet: cap max_mtu for drivers without bind callback

7.1-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Laurent Vivier <[email protected]>

commit 1505b2cb6ae1c7e8ac0c6e4590a204ffc3ab2b24 upstream.

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: [email protected]
Cc: [email protected]
Link: https://gitlab.com/qemu-project/qemu/-/issues/3268
Link: https://bugs.passt.top/show_bug.cgi?id=189
Signed-off-by: Laurent Vivier <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
 drivers/net/usb/usbnet.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/net/usb/usbnet.c
+++ b/drivers/net/usb/usbnet.c
@@ -1798,7 +1798,7 @@ usbnet_probe(struct usb_interface *udev,
 	 */
 	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;
@@ -1808,6 +1808,7 @@ usbnet_probe(struct usb_interface *udev,
 	// 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;