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 14:03:19 +0200

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

Just for the record - a simple grep for usbnet_read_cmd shows that at
least drivers/net/usb/plusb.c depends on the current behaviour:

static inline int
pl_vendor_req(struct usbnet *dev, u8 req, u8 val, u8 index)
{
        return usbnet_read_cmd(dev, req,
                                USB_DIR_IN | USB_TYPE_VENDOR |
                                USB_RECIP_DEVICE,
                                val, index, NULL, 0);
}

Bjørn