Re: [PATCH] tun: Fix use-after-free in tun_net_xmit

[Cong Wang]

On Sun, Apr 28, 2019 at 12:51 AM Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > tun_net_xmit() doesn't have the chance to
> > access the change because it holding the rcu_read_lock().
>
>
> The problem is the following codes:
>
>
>          --tun->numqueues;
>
>          ...
>
>          synchronize_net();
>
> We need make sure the decrement of tun->numqueues be visible to readers
> after synchronize_net(). And in tun_net_xmit():

It doesn't matter at all. Readers are okay to read it even they still use the
stale tun->numqueues, as long as the tfile is not freed readers can read
whatever they want...

The decrement of tun->numqueues is just how we unpublish the old
tfile, it is still valid for readers to read it _after_ unpublish, we only need
to worry about free, not about unpublish. This is the whole spirit of RCU.

You need to rethink about my SOCK_RCU_FREE patch.

Thanks.