[Linux-kernel-mentees] [PATCH] net: can: prevent potential access of uninitialized value in canfd_rcv()

[Marc Kleine-Budde]

On 11/2/20 4:13 AM, Anant Thazhemadam wrote:
> In canfd_rcv(), cfd->len is uninitialized when skb->len = 0, and this
> uninitialized cfd->len is accessed nonetheless by pr_warn_once().
> 
> Fix this uninitialized variable access by checking cfd->len's validity
> condition (cfd->len > CANFD_MAX_DLEN) separately after the skb->len's
> condition is checked, and appropriately modify the log messages that
> are generated as well.
> In case either of the required conditions fail, the skb is freed and
> NET_RX_DROP is returned, same as before.
> 
> Reported-by: syzbot+9bcb0c9409066696d3aa at syzkaller.appspotmail.com
> Tested-by: Anant Thazhemadam <anant.thazhemadam at gmail.com>
> Signed-off-by: Anant Thazhemadam <anant.thazhemadam at gmail.com>
> ---
> This patch was locally tested using the reproducer and .config file 
> generated by syzbot.
> 
>  net/can/af_can.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
> 
> diff --git a/net/can/af_can.c b/net/can/af_can.c
> index ea29a6d97ef5..1b9f2e50f065 100644
> --- a/net/can/af_can.c
> +++ b/net/can/af_can.c
> @@ -694,16 +694,25 @@ static int canfd_rcv(struct sk_buff *skb, struct net_device *dev,

Can you create a similar patch for "can_rcv()"?

>  {
>  	struct canfd_frame *cfd = (struct canfd_frame *)skb->data;
>  
> -	if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU ||
> -		     cfd->len > CANFD_MAX_DLEN)) {
> -		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuf: dev type %d, len %d, datalen %d\n",
> +	if (unlikely(dev->type != ARPHRD_CAN || skb->len != CANFD_MTU)) {
> +		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d\n",
> +			     dev->type, skb->len);
> +		goto free_skb;
> +	}
> +
> +	// This check is made separately since cfd->len would be uninitialized if skb->len = 0.

Please don't use C++ comment style in the kernel.

> +	else if (unlikely(cfd->len > CANFD_MAX_DLEN)) {

Please move the "else" right after the closing curly bracket: "} else if () {"
or convert it into an "if () {"

> +		pr_warn_once("PF_CAN: dropped non conform CAN FD skbuff: dev type %d, len %d, datalen %d\n",
>  			     dev->type, skb->len, cfd->len);
> -		kfree_skb(skb);
> -		return NET_RX_DROP;
> +		goto free_skb;
>  	}
>  
>  	can_receive(skb, dev);
>  	return NET_RX_SUCCESS;
> +
> +free_skb:
> +	kfree_skb(skb);
> +	return NET_RX_DROP;
>  }
>  
>  /* af_can protocol functions */
> 

regards,
Marc

-- 
Pengutronix e.K.                 | Marc Kleine-Budde           |
Embedded Linux                   | https://www.pengutronix.de  |
Vertretung West/Dortmund         | Phone: +49-231-2826-924     |
Amtsgericht Hildesheim, HRA 2686 | Fax:   +49-5121-206917-5555 |

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