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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 7 Jun 2022 10:28:27 -0700
Message-ID: <CANn89iLwEFrb7q2ifqRxc2nvi_-YOwQpdFf0X1N5AH_wSSZ48Q@mail.gmail.com>
Subject: Re: [PATCH 2/2] l2tp: fix possible use-after-free
To:     Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>
Cc:     "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Xiyu Yang <xiyuyang19@fudan.edu.cn>,
        Xin Tan <tanxin.ctf@gmail.com>,
        Xin Xiong <xiongx18@fudan.edu.cn>,
        Tom Parkin <tparkin@katalix.com>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Jun 7, 2022 at 9:16 AM Xiaohui Zhang <xiaohuizhang@ruc.edu.cn> wrote:
>
> We detected a suspected bug with our code clone detection tool.
>
> Similar to the handling of l2tp_tunnel_get in commit a622b40035d1
> ("l2ip: fix possible use-after-free"), we thought a patch might
> be needed here as well.
>
> Before taking a refcount on a rcu protected structure,
> we need to make sure the refcount is not zero.
>
> Signed-off-by: Xiaohui Zhang <xiaohuizhang@ruc.edu.cn>
> ---
>  net/l2tp/l2tp_core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> index b759fbd09b65..c5de6d4e0818 100644
> --- a/net/l2tp/l2tp_core.c
> +++ b/net/l2tp/l2tp_core.c
> @@ -273,8 +273,8 @@ struct l2tp_session *l2tp_session_get(const struct net *net, u32 session_id)
>
>         rcu_read_lock_bh();
>         hlist_for_each_entry_rcu(session, session_list, global_hlist)
> -               if (session->session_id == session_id) {
> -                       l2tp_session_inc_refcount(session);
> +               if (session->session_id == session_id &&
> +                   refcount_inc_not_zero(&session->ref_count)) {
>                         rcu_read_unlock_bh();
>
>                         return session;
> --
> 2.17.1
>

Please fix all bugs in a single patch.

net/l2tp/l2tp_core.c contains four suspect calls to l2tp_session_inc_refcount()