From: Jakub Sitnicki jakub@cloudflare.com
[ Upstream commit af295e854a4e3813ffbdef26dbb6a4d6226c3ea1 ]
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 Signed-off-by: David S. Miller davem@davemloft.net Stable-dep-of: 0b2c59720e65 ("l2tp: close all race conditions in l2tp_tunnel_register()") Signed-off-by: Sasha Levin sashal@kernel.org --- net/l2tp/l2tp_core.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c index c77032638a06..d22dbd92b709 100644 --- a/net/l2tp/l2tp_core.c +++ b/net/l2tp/l2tp_core.c @@ -1476,11 +1476,12 @@ 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); @@ -1509,8 +1510,6 @@ 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; @@ -1524,16 +1523,18 @@ 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; }