From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 14B4CC4332F for ; Sat, 19 Nov 2022 14:28:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230387AbiKSO16 (ORCPT ); Sat, 19 Nov 2022 09:27:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40002 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229592AbiKSO15 (ORCPT ); Sat, 19 Nov 2022 09:27:57 -0500 Received: from www262.sakura.ne.jp (www262.sakura.ne.jp [202.181.97.72]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C956A84818 for ; Sat, 19 Nov 2022 06:27:56 -0800 (PST) Received: from fsav413.sakura.ne.jp (fsav413.sakura.ne.jp [133.242.250.112]) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTP id 2AJERsNJ007758; Sat, 19 Nov 2022 23:27:54 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Received: from www262.sakura.ne.jp (202.181.97.72) by fsav413.sakura.ne.jp (F-Secure/fsigk_smtp/550/fsav413.sakura.ne.jp); Sat, 19 Nov 2022 23:27:54 +0900 (JST) X-Virus-Status: clean(F-Secure/fsigk_smtp/550/fsav413.sakura.ne.jp) Received: from [192.168.1.9] (M106072142033.v4.enabler.ne.jp [106.72.142.33]) (authenticated bits=0) by www262.sakura.ne.jp (8.15.2/8.15.2) with ESMTPSA id 2AJERsTw007755 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NO); Sat, 19 Nov 2022 23:27:54 +0900 (JST) (envelope-from penguin-kernel@I-love.SAKURA.ne.jp) Message-ID: Date: Sat, 19 Nov 2022 23:27:54 +0900 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 6.3; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.5.0 Subject: Re: [PATCH net] l2tp: Don't sleep and disable BH under writer-side sk_callback_lock Content-Language: en-US From: Tetsuo Handa To: Jakub Sitnicki , netdev@vger.kernel.org Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Tom Parkin , syzbot+703d9e154b3b58277261@syzkaller.appspotmail.com, syzbot+50680ced9e98a61f7698@syzkaller.appspotmail.com, syzbot+de987172bb74a381879b@syzkaller.appspotmail.com References: <20221119130317.39158-1-jakub@cloudflare.com> In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 2022/11/19 22:52, Tetsuo Handa wrote: > On 2022/11/19 22:03, Jakub Sitnicki wrote: >> When holding a reader-writer spin lock we cannot sleep. Calling >> setup_udp_tunnel_sock() with write lock held violates this rule, because we >> end up calling percpu_down_read(), which might sleep, as syzbot reports >> [1]: >> >> __might_resched.cold+0x222/0x26b kernel/sched/core.c:9890 >> percpu_down_read include/linux/percpu-rwsem.h:49 [inline] >> cpus_read_lock+0x1b/0x140 kernel/cpu.c:310 >> static_key_slow_inc+0x12/0x20 kernel/jump_label.c:158 >> udp_tunnel_encap_enable include/net/udp_tunnel.h:187 [inline] >> setup_udp_tunnel_sock+0x43d/0x550 net/ipv4/udp_tunnel_core.c:81 >> l2tp_tunnel_register+0xc51/0x1210 net/l2tp/l2tp_core.c:1509 >> pppol2tp_connect+0xcdc/0x1a10 net/l2tp/l2tp_ppp.c:723 >> >> Trim the writer-side critical section for sk_callback_lock down to the >> minimum, so that it covers only operations on sk_user_data. > > This patch does not look correct. > > Since l2tp_validate_socket() checks that sk->sk_user_data == NULL with > sk->sk_callback_lock held, you need to call rcu_assign_sk_user_data(sk, tunnel) > before releasing sk->sk_callback_lock. > Is it safe to temporarily set a dummy pointer like below? If it is not safe, what makes assignments done by setup_udp_tunnel_sock() safe? diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index 754fdda8a5f5..198d38d8fceb 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -1474,11 +1474,12 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net, } sk = sock->sk; - write_lock(&sk->sk_callback_lock); - + write_lock_bh(&sk->sk_callback_lock); ret = l2tp_validate_socket(sk, net, tunnel->encap); if (ret < 0) goto err_sock; + rcu_assign_sk_user_data(sk, (void *) 1); + write_unlock_bh(&sk->sk_callback_lock); tunnel->l2tp_net = net; pn = l2tp_pernet(net); @@ -1492,6 +1493,8 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net, spin_unlock_bh(&pn->l2tp_tunnel_list_lock); sock_put(sk); ret = -EEXIST; + write_lock_bh(&sk->sk_callback_lock); + rcu_assign_sk_user_data(sk, NULL); goto err_sock; } } @@ -1522,16 +1525,15 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net, if (tunnel->fd >= 0) sockfd_put(sock); - write_unlock(&sk->sk_callback_lock); return 0; err_sock: + write_unlock_bh(&sk->sk_callback_lock); + if (tunnel->fd < 0) sock_release(sock); else sockfd_put(sock); - - write_unlock(&sk->sk_callback_lock); err: return ret; }