netlink: GPF in sock_sndtimeo

[Cong Wang]

On Thu, Dec 8, 2016 at 10:02 PM, Richard Guy Briggs <rgb at redhat.com> wrote:
> I also tried to extend Cong Wang's idea to attempt to proactively respond to a
> NETLINK_URELEASE on the audit_sock and reset it, but ran into a locking error
> stack dump using mutex_lock(&audit_cmd_mutex) in the notifier callback.
> Eliminating the lock since the sock is dead anways eliminates the error.
>
> Is it safe?  I'll resubmit if this looks remotely sane.  Meanwhile I'll try to
> get the test case to compile.

It doesn't look safe, because 'audit_sock', 'audit_nlk_portid' and 'audit_pid'
are updated as a whole and race between audit_receive_msg() and
NETLINK_URELEASE.


> @@ -1167,10 +1190,14 @@ static void __net_exit audit_net_exit(struct net *net)
>  {
>         struct audit_net *aunet = net_generic(net, audit_net_id);
>         struct sock *sock = aunet->nlsk;
> +
> +       mutex_lock(&audit_cmd_mutex);
>         if (sock == audit_sock) {
>                 audit_pid = 0;
> +               audit_nlk_portid = 0;
>                 audit_sock = NULL;
>         }
> +       mutex_unlock(&audit_cmd_mutex);
>

If you decide to use NETLINK_URELEASE notifier, the above piece is no
longer needed, the net_exit path simply releases a refcnt.