usb-serial: fix mos_parport refcount imbalance on error path

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Mar 20, 2019 at 04:59:02PM +0800, Jian Liu/Gmail wrote:
> Hi all,
> 
> We might find a security issue in function
> 'write_parport_reg_nonbloc'(for source codes, please refer to
> https://github.com/torvalds/linux/blob/master/drivers/usb/serial/mos7720.c),
> which is logically an unbalanced reference count vulnerability.
> 
> Let us check ref_count 'mos_parport->ref_count' of kernel object
> 'mos_parport' in the function. In 'Line 369:
> kref_get(&mos_parport->ref_count)' and 'Line 370:
> urbtrack->mos_parport = mos_parport', the ref_count has been increased
> because of a new variable 'urbtrack->mos_parport' pointing to
> 'mos_parport', that make sense to balance ref_count. However, at 'Line
> 373 and Line 379: kfree(urbtrack)', variable 'urbtrack' has been freed
> before a return, which will make 'urbtrack->mos_parport' freed, too.
> But, a decreasing of 'mos_parport->ref_count' is missing in the
> current kernel version, which will bring an unbalanced reference
> count.
> 
> We have wrote a patch showed in Lin's email. The idea is simple. We
> added a calling of 'kref_put(&mos_parport->ref_count,
> destroy_mos_parport)' below Line 373 and Line 379. Currently, we are
> not sure if this is really a vulnerability. If you are familiar with
> codes, please help to verify it.

Yes, this is a bugfix, but not really a "vulnerability" as it is almost
impossible to keep hitting a memory allocation failure enough times to
cause any problems here.

The most that can happen is that the reference really is not dropped,
and memory leaks.  Not a big deal, if memory is failing to be allocated,
your system has bigger issues at the moment :)

So it's just a nice bugfix, for a failure mode that is pretty much
impossible to ever hit in the "wild".

thanks,

greg k-h