linux-kernel - Re: [PATCH] net: mcs7830: handle usb read errors properly

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Date:   Thu, 6 Jan 2022 21:31:15 -0500
From:   Arnd Bergmann <arnd@...db.de>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, tanghui20@...wei.com,
        Andrew Lunn <andrew@...n.ch>, Oliver Neukum <oneukum@...e.com>,
        Arnd Bergmann <arnd@...db.de>,
        USB list <linux-usb@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        syzbot+003c0a286b9af5412510@...kaller.appspotmail.com
Subject: Re: [PATCH] net: mcs7830: handle usb read errors properly

On Thu, Jan 6, 2022 at 5:57 PM Pavel Skripkin <paskripkin@...il.com> wrote:
>
> Syzbot reported uninit value in mcs7830_bind(). The problem was in
> missing validation check for bytes read via usbnet_read_cmd().
>
> usbnet_read_cmd() internally calls usb_control_msg(), that returns
> number of bytes read. Code should validate that requested number of bytes
> was actually read.
>
> So, this patch adds missing size validation check inside
> mcs7830_get_reg() to prevent uninit value bugs
>
> CC: Arnd Bergmann <arnd@...db.de>
> Reported-and-tested-by: syzbot+003c0a286b9af5412510@...kaller.appspotmail.com
> Fixes: 2a36d7083438 ("USB: driver for mcs7830 (aka DeLOCK) USB ethernet adapter")

Looks good to me.

Reviewed-by: Arnd Bergmann <arnd@...db.de>

> Signed-off-by: Pavel Skripkin <paskripkin@...il.com>
> ---
>
> @Arnd, I am not sure about mcs7830_get_rev() function.
>
> Is get_reg(22, 2) == 1 valid read? If so, I think, we should call
> usbnet_read_cmd() directly here, since other callers care only about
> negative error values.

I have no idea, I never had a datasheet for this device, only
the hardware I bought cheaply and vendor source code I
found somewhere on the net, and that was 16 years ago.

I would not expect the hardware to ever return less data than
was asked for, so any length checking would only have to
account for attackers that fake this device.

         Arnd