svn commit: r310180 - head/sys/net

[Mateusz Guzik]

On Fri, Dec 16, 2016 at 10:39:31PM +0000, Alan Somers wrote:
> Author: asomers
> Date: Fri Dec 16 22:39:30 2016
> New Revision: 310180
> URL: https://svnweb.freebsd.org/changeset/base/310180
> 
> Log:
>   Fix panic during lagg destruction with simultaneous status check
>   
>   If you run "ifconfig lagg0 destroy" and "ifconfig lagg0" at the same time a
>   page fault may result. The first process will destroy ifp->if_lagg in
>   lagg_clone_destroy (called by if_clone_destroy). Then the second process
>   will observe that ifp->if_lagg is NULL at the top of lagg_port_ioctl and
>   goto fallback: where it will promptly dereference ifp->if_lagg anyway.
>   
>   The solution is to repeat the NULL check for ifp->if_lagg
>   

I don't understand how this solves the problem. What prevents the object
from getting freed after the pointer got NULLified? That is, it seems
the patch turns a null pointer deref into a use-after-free.

There seems to be a refcounting issue here.

That said, I only did cursory reading.
-- 
Mateusz Guzik <mjguzik gmail.com>