Subject: Re: [PATCH] net: usb: smsc95xx: Limit packet length to skb->len

From: Jakub Kicinski <[email-masked]>
Date: Wed, 15 Mar 2023 21:24:25 -0700

On Mon, 13 Mar 2023 23:01:24 +0100 Szymon Heidrich wrote:
> Packet length retrieved from skb data may be larger than

nit: s/skb data/descriptor/ may be better in terms of terminology

> the actual socket buffer length (up to 1526 bytes). In such

nit: the "up to 1526 bytes" is a bit confusing, I'd remove it.

> case the cloned skb passed up the network stack will leak
> kernel memory contents.

> Fixes: 2f7ca802bdae ("net: Add SMSC LAN9500 USB2.0 10/100 ethernet adapter driver")
> Signed-off-by: Szymon Heidrich <[email-masked]>
> ---
>  drivers/net/usb/smsc95xx.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/usb/smsc95xx.c b/drivers/net/usb/smsc95xx.c
> index 32d2c60d3..ba766bdb2 100644
> --- a/drivers/net/usb/smsc95xx.c
> +++ b/drivers/net/usb/smsc95xx.c
> @@ -1851,7 +1851,8 @@ static int smsc95xx_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
>  			}
>  		} else {
>  			/* ETH_FRAME_LEN + 4(CRC) + 2(COE) + 4(Vlan) */
> -			if (unlikely(size > (ETH_FRAME_LEN + 12))) {
> +			if (unlikely(size > (ETH_FRAME_LEN + 12) ||
> +				     size > skb->len)) {

We need this check on both sides of the big if {} statement.

In case the error bit is set and we drop the packet we still
end up in skb_pull() which if size > skb->len will panic the
kernel.

So let's do this check right after size and align are calculated?
The patch for smsc75xx has sadly already been applied so you'll
need to prepare a fix to the fix :(

>  				netif_dbg(dev, rx_err, dev->net,
>  					  "size err header=0x%08x\n", header);
>  				return 0;