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[2a10:fc81:a806:d6a9::1]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-434af6535ddsm1648131f8f.15.2026.01.14.14.19.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Jan 2026 14:19:23 -0800 (PST) Date: Wed, 14 Jan 2026 23:19:22 +0100 From: Stefano Brivio To: Laurent Vivier Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, Oliver Neukum , linux-usb@vger.kernel.org Subject: Re: [PATCH] usbnet: limit max_mtu based on device's hard_mtu Message-ID: <20260114231922.6b41e9ed@elisabeth> In-Reply-To: <20260114090317.3214026-1-lvivier@redhat.com> References: <20260114090317.3214026-1-lvivier@redhat.com> Organization: Red Hat X-Mailer: Claws Mail 4.2.0 (GTK 3.24.49; x86_64-pc-linux-gnu) Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Wed, 14 Jan 2026 10:03:17 +0100 Laurent Vivier 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 Thanks for fixing this! Link: https://bugs.passt.top/show_bug.cgi?id=189 Reviewed-by: Stefano Brivio -- Stefano