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

[weiyongjun (A)]

> Network Developers <netdev@xxxxxxxxxxxxxxx>
> Subject: Re: [PATCH] tun: Fix use-after-free in tun_net_xmit
> 
> On Mon, Apr 29, 2019 at 7:55 AM Michael S. Tsirkin <mst@xxxxxxxxxx>
> wrote:
> > The problem seems real enough, but an extra synchronize_net on
> tun_attach
> > might be a problem, slowing guest startup significantly.
> > Better ideas?
> 
> Yes, I proposed the following patch in the other thread.
> 
> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> index e9ca1c088d0b..31c3210288cb 100644
> --- a/drivers/net/tun.c
> +++ b/drivers/net/tun.c
> @@ -3431,6 +3431,7 @@ static int tun_chr_open(struct inode *inode,
> struct file * file)
>         file->private_data = tfile;
>         INIT_LIST_HEAD(&tfile->next);
> 
> +       sock_set_flag(&tfile->sk, SOCK_RCU_FREE);
>         sock_set_flag(&tfile->sk, SOCK_ZEROCOPY);
> 
>         return 0;


This patch should not work. The key point is that when detach the queue
with index is equal to tun->numqueues - 1, we do not clear the point
in tun->tfiles:

static void __tun_detach(...)
{
...
        **** if index == tun->numqueues - 1, nothing changed ****
        rcu_assign_pointer(tun->tfiles[index],
                                                tun->tfiles[tun->numqueues - 1]);
....
}

And after tfile free, xmit have change to get and use the freed file point.

Regards