netdev - Re: [RFC] CDC-NCM: avoid overflow in sanity checking

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Message-ID: <a9143724-51ca-08ea-588c-b849a4ba7011@selasky.org>
Date:   Thu, 10 Feb 2022 17:38:44 +0100
From:   Hans Petter Selasky <hps@...asky.org>
To:     Oliver Neukum <oneukum@...e.com>, bjorn@...k.no,
        linux-usb@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [RFC] CDC-NCM: avoid overflow in sanity checking

On 2/10/22 16:54, Oliver Neukum wrote:
> A broken device may give an extreme offset like 0xFFF0
> and a reasonable length for a fragment. In the sanity
> check as formulated now, this will create an integer
> overflow, defeating the sanity check. It needs to be
> rewritten as a subtraction and the variables should be
> unsigned.
> 

Hi Oliver,

First of all I'd like to update:

Hans Petter Selasky <hans.petter.selasky@...ricsson.com>

To:

Hans Petter Selasky <hselasky@...ebsd.org>

Secondly,

"int" variables are 32-bit, so 0xFFF0 won't overflow.

The initial driver code written by me did only support 16-bit lengths 
and offset. Then integer overflow is not possible.

It looks like somebody else introduced this integer overflow :-(

commit 0fa81b304a7973a499f844176ca031109487dd31
Author: Alexander Bersenev <bay@...kerdom.ru>
Date:   Fri Mar 6 01:33:16 2020 +0500

     cdc_ncm: Implement the 32-bit version of NCM Transfer Block

     The NCM specification defines two formats of transfer blocks: with 
16-bit
     fields (NTB-16) and with 32-bit fields (NTB-32). Currently only 
NTB-16 is
     implemented.

....

With NCM 32, both "len" and "offset" must be checked, because these are 
now 32-bit and stored into regular "int".

The fix you propose is not fully correct!

--HPS

> Signed-off-by: Oliver Neukum <oneukum@...e.com>
> ---
>   drivers/net/usb/cdc_ncm.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/usb/cdc_ncm.c b/drivers/net/usb/cdc_ncm.c
> index e303b522efb5..f78fccbc4b93 100644
> --- a/drivers/net/usb/cdc_ncm.c
> +++ b/drivers/net/usb/cdc_ncm.c
> @@ -1715,10 +1715,10 @@ int cdc_ncm_rx_fixup(struct usbnet *dev, struct sk_buff *skb_in)
>   {
>   	struct sk_buff *skb;
>   	struct cdc_ncm_ctx *ctx = (struct cdc_ncm_ctx *)dev->data[0];
> -	int len;
> +	unsigned int len;
>   	int nframes;
>   	int x;
> -	int offset;
> +	unsigned int offset;
>   	union {
>   		struct usb_cdc_ncm_ndp16 *ndp16;
>   		struct usb_cdc_ncm_ndp32 *ndp32;
> @@ -1791,7 +1791,7 @@ int cdc_ncm_rx_fixup(struct usbnet *dev, struct sk_buff *skb_in)
>   		}
>   
>   		/* sanity checking */
> -		if (((offset + len) > skb_in->len) ||
> +		if ((offset > skb_in->len - len) ||
>   				(len > ctx->rx_max) || (len < ETH_HLEN)) {
>   			netif_dbg(dev, rx_err, dev->net,
>   				  "invalid frame detected (ignored) offset[%u]=%u, length=%u, skb=%p\n",