Subject: Re: [PATCH] net: mcs7830: handle usb read errors properly

From: Greg KH <[email-masked]>
Date: Fri, 7 Jan 2022 11:06:49 +0100

On Fri, Jan 07, 2022 at 01:57:16AM +0300, Pavel Skripkin 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 <[email-masked]>
> Reported-and-tested-by: [email-masked]
> Fixes: 2a36d7083438 ("USB: driver for mcs7830 (aka DeLOCK) USB ethernet adapter")
> Signed-off-by: Pavel Skripkin <[email-masked]>
> ---
> 
> @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.  
> 
> Thanks
> 
> 
> ---
>  drivers/net/usb/mcs7830.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/usb/mcs7830.c b/drivers/net/usb/mcs7830.c
> index 326cc4e749d8..fdda0616704e 100644
> --- a/drivers/net/usb/mcs7830.c
> +++ b/drivers/net/usb/mcs7830.c
> @@ -108,8 +108,16 @@ static const char driver_name[] = "MOSCHIP usb-ethernet driver";
>  
>  static int mcs7830_get_reg(struct usbnet *dev, u16 index, u16 size, void *data)
>  {
> -	return usbnet_read_cmd(dev, MCS7830_RD_BREQ, MCS7830_RD_BMREQ,
> -				0x0000, index, data, size);
> +	int ret;
> +
> +	ret = usbnet_read_cmd(dev, MCS7830_RD_BREQ, MCS7830_RD_BMREQ,
> +			      0x0000, index, data, size);
> +	if (ret < 0)
> +		return ret;
> +	else if (ret < size)
> +		return -ENODATA;

We have a usb core function that handles these "short reads are an
error" issue.  Perhaps usbnet_read_cmd() should be converted to use it
instead?

thanks,

greg k-h