linux-kernel - Re: [PATCH 1/5] comedi: ni_usb6501: fix NULL-deref in command paths

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Date:   Tue, 26 Oct 2021 15:12:28 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     Johan Hovold <johan@...nel.org>,
        H Hartley Sweeten <hsweeten@...ionengravers.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, Luca Ellero <luca.ellero@...ckedbrain.com>
Subject: Re: [PATCH 1/5] comedi: ni_usb6501: fix NULL-deref in command paths

On 25/10/2021 12:45, Johan Hovold wrote:
> The driver uses endpoint-sized USB transfer buffers but had no sanity
> checks on the sizes. This can lead to zero-size-pointer dereferences or
> overflowed transfer buffers in ni6501_port_command() and
> ni6501_counter_command() if a (malicious) device has smaller max-packet
> sizes than expected (or when doing descriptor fuzz testing).
> 
> Add the missing sanity checks to probe().
> 
> Fixes: a03bb00e50ab ("staging: comedi: add NI USB-6501 support")
> Cc: stable@...r.kernel.org      # 3.18
> Cc: Luca Ellero <luca.ellero@...ckedbrain.com>
> Signed-off-by: Johan Hovold <johan@...nel.org>
> ---
>   drivers/comedi/drivers/ni_usb6501.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/comedi/drivers/ni_usb6501.c b/drivers/comedi/drivers/ni_usb6501.c
> index 5b6d9d783b2f..eb2e5c23f25d 100644
> --- a/drivers/comedi/drivers/ni_usb6501.c
> +++ b/drivers/comedi/drivers/ni_usb6501.c
> @@ -144,6 +144,10 @@ static const u8 READ_COUNTER_RESPONSE[]	= {0x00, 0x01, 0x00, 0x10,
>   					   0x00, 0x00, 0x00, 0x02,
>   					   0x00, 0x00, 0x00, 0x00};
>   
> +/* Largest supported packets */
> +static const size_t TX_MAX_SIZE	= sizeof(SET_PORT_DIR_REQUEST);
> +static const size_t RX_MAX_SIZE	= sizeof(READ_PORT_RESPONSE);
> +
>   enum commands {
>   	READ_PORT,
>   	WRITE_PORT,
> @@ -486,12 +490,16 @@ static int ni6501_find_endpoints(struct comedi_device *dev)
>   		ep_desc = &iface_desc->endpoint[i].desc;
>   
>   		if (usb_endpoint_is_bulk_in(ep_desc)) {
> +			if (usb_endpoint_maxp(ep_desc) < RX_MAX_SIZE)
> +				continue;
>   			if (!devpriv->ep_rx)
>   				devpriv->ep_rx = ep_desc;
>   			continue;
>   		}
>   
>   		if (usb_endpoint_is_bulk_out(ep_desc)) {
> +			if (usb_endpoint_maxp(ep_desc) < TX_MAX_SIZE)
> +				continue;
>   			if (!devpriv->ep_tx)
>   				devpriv->ep_tx = ep_desc;
>   			continue;
> 

Perhaps it should return an error if the first encountered bulk-in 
endpoint has the wrong size or the first encountered bulk-out endpoint 
has the wrong size. Something like:

		if (usb_endpoint_is_bulk_in(ep_desc)) {
			if (!devpriv->ep_rx) {
				if (usb_endpoint_maxp(ep_desc) < RX_MAX_SIZE)
					break;
			}
			continue;

(similar for bulk-out with TX_MAX_SIZE)

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