| Message ID | 20221121085426.21315-1-jakub@cloudflare.com (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | af295e854a4e3813ffbdef26dbb6a4d6226c3ea1 |
| Delegated to: | Netdev Maintainers |
| Headers | show |
| Series | [net,v2] l2tp: Don't sleep and disable BH under writer-side sk_callback_lock | expand |
Hello: This patch was applied to netdev/net.git (master) by David S. Miller <davem@davemloft.net>: On Mon, 21 Nov 2022 09:54:26 +0100 you 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 > > [...] Here is the summary with links: - [net,v2] l2tp: Don't sleep and disable BH under writer-side sk_callback_lock https://git.kernel.org/netdev/net/c/af295e854a4e You are awesome, thank you!
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index 754fdda8a5f5..9a1415fe3fa7 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; + goto err_inval_sock; + rcu_assign_sk_user_data(sk, tunnel); + write_unlock_bh(&sk->sk_callback_lock); tunnel->l2tp_net = net; pn = l2tp_pernet(net); @@ -1507,8 +1508,6 @@ int l2tp_tunnel_register(struct l2tp_tunnel *tunnel, struct net *net, }; setup_udp_tunnel_sock(net, sock, &udp_cfg); - } else { - rcu_assign_sk_user_data(sk, tunnel); } tunnel->old_sk_destruct = sk->sk_destruct; @@ -1522,16 +1521,18 @@ 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_lock_bh(&sk->sk_callback_lock); + rcu_assign_sk_user_data(sk, NULL); +err_inval_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; }
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. Also, when grabbing the sk_callback_lock, we always need to disable BH, as Eric points out. Failing to do so leads to deadlocks because we acquire sk_callback_lock in softirq context, which can get stuck waiting on us if: 1) it runs on the same CPU, or CPU0 ---- lock(clock-AF_INET6); <Interrupt> lock(clock-AF_INET6); 2) lock ordering leads to priority inversion CPU0 CPU1 ---- ---- lock(clock-AF_INET6); local_irq_disable(); lock(&tcp_hashinfo.bhash[i].lock); lock(clock-AF_INET6); <Interrupt> lock(&tcp_hashinfo.bhash[i].lock); ... as syzbot reports [2,3]. Use the _bh variants for write_(un)lock. [1] https://lore.kernel.org/netdev/0000000000004e78ec05eda79749@google.com/ [2] https://lore.kernel.org/netdev/000000000000e38b6605eda76f98@google.com/ [3] https://lore.kernel.org/netdev/000000000000dfa31e05eda76f75@google.com/ v2: - Check and set sk_user_data while holding sk_callback_lock for both L2TP encapsulation types (IP and UDP) (Tetsuo) Cc: Tom Parkin <tparkin@katalix.com> Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> Fixes: b68777d54fac ("l2tp: Serialize access to sk_user_data with sk_callback_lock") Reported-by: Eric Dumazet <edumazet@google.com> Reported-by: syzbot+703d9e154b3b58277261@syzkaller.appspotmail.com Reported-by: syzbot+50680ced9e98a61f7698@syzkaller.appspotmail.com Reported-by: syzbot+de987172bb74a381879b@syzkaller.appspotmail.com Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com> --- net/l2tp/l2tp_core.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)