Re: [PATCH] net: fix a concurrency bug in l2tp_tunnel_register()

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Apr 14, 2021 at 2:14 PM Sishuai Gong <sishuai@purdue.edu> wrote:
>
> l2tp_tunnel_register() registers a tunnel without fully
> initializing its attribute. This can allow another kernel thread
> running l2tp_xmit_core() to access the uninitialized data and
> then cause a kernel NULL pointer dereference error, as shown below.
>
> Thread 1    Thread 2
> //l2tp_tunnel_register()
> list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
>             //pppol2tp_connect()
>             tunnel = l2tp_tunnel_get(sock_net(sk), info.tunnel_id);
>             // Fetch the new tunnel
>             ...
>             //l2tp_xmit_core()
>             struct sock *sk = tunnel->sock;
>             ...
>             bh_lock_sock(sk);
>             //Bug happens
> tunnel->sock = sk;
>
> Fix this bug by initializing tunnel->sock before adding the
> tunnel into l2tp_tunnel_list.
>
> Signed-off-by: Sishuai Gong <sishuai@purdue.edu>
> Reported-by: Sishuai Gong <sishuai@purdue.edu>
> ---
>  net/l2tp/l2tp_core.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index 203890e378cb..d61c9879076e 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -1478,6 +1478,10 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net,
>         tunnel->l2tp_net = net;
>         pn = l2tp_pernet(net);
>
> +       sk = sock->sk;
> +       sock_hold(sk);
> +       tunnel->sock = sk;
> +

I think you have to put this sock refcnt on the -EEXIST path
after moving it up.

Thanks.