Subject: Re: [PATCH] usbnet: dereference after null check in usbnet_start_xmit() and __usbnet_read_cmd()

From: Bjørn Mork <[email-masked]>
Date: Wed, 19 Aug 2015 13:51:20 +0200

Vivek Kumar Bhagat <[email-masked]> writes:

> usbnet_start_xmit() - If info->tx_fixup is not defined by class driver,
> NULL check does not happen for skb pointer and leads to NULL dereference.
> __usbnet_read_cmd() - if data pointer is passed as NULL, memcpy will
> dereference NULL pointer.

That's two completely different issues.  Mixing them in a single patch
is only confusing things.

> Signed-off-by: Vivek Kumar Bhagat <[email-masked]>
> ---
>  drivers/net/usb/usbnet.c |    5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index 3c86b10..ec4d224 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -1294,6 +1294,8 @@ netdev_tx_t usbnet_start_xmit (struct sk_buff *skb,
>  
>  	if (skb)
>  		skb_tx_timestamp(skb);
> +	else
> +		goto drop;
>  
>  	// some devices want funky USB-level framing, for
>  	// win32 driver (usually) and/or hardware quirks

This is wrong.  There are usbnet minidrivers depending on info->tx_fixup
being called with a NULL skb.

> @@ -1906,7 +1908,8 @@ static int __usbnet_read_cmd(struct usbnet *dev, u8 cmd, u8 reqtype,
>  		buf = kmalloc(size, GFP_KERNEL);
>  		if (!buf)
>  			goto out;
> -	}
> +	} else
> +		goto out;
>  
>  	err = usb_control_msg(dev->udev, usb_rcvctrlpipe(dev->udev, 0),
>  			      cmd, reqtype, value, index, buf, size,

This is also wrong.  It makes __usbnet_read_cmd() return -ENOMEM if
called with a NULL data pointer.  I don't know if it is used, but it's
perfectly valid to call __usbnet_read_cmd() with data == NULL if
size == 0. No memcpy will happen in this case because usb_control_msg
can only return 0 or an error

Please don't submit any more such patches without proper justification.
You cannot trust that someone will actually take the time to sanity
check your changes.  Patches claiming to fix a NULL dereference should
at least provide an oops.

Bjørn