netdev - Re: [PATCH] usbnet: limit max_mtu based on device's hard

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Message-ID: <20260114231922.6b41e9ed@elisabeth>
Date: Wed, 14 Jan 2026 23:19:22 +0100
From: Stefano Brivio <sbrivio@...hat.com>
To: Laurent Vivier <lvivier@...hat.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, Oliver Neukum
 <oneukum@...e.com>, linux-usb@...r.kernel.org
Subject: Re: [PATCH] usbnet: limit max_mtu based on device's hard_mtu

On Wed, 14 Jan 2026 10:03:17 +0100
Laurent Vivier <lvivier@...hat.com> 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.
> 
> 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
> 
> Signed-off-by: Laurent Vivier <lvivier@...hat.com>

Thanks for fixing this!

Link: https://bugs.passt.top/show_bug.cgi?id=189
Reviewed-by: Stefano Brivio <sbrivio@...hat.com>

-- 
Stefano