This is a note to let you know that I've just added the patch titled
l2tp: Don't sleep and disable BH under writer-side sk_callback_lock
to the 5.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
l2tp-don-t-sleep-and-disable-bh-under-writer-side-sk.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 58bb12c176c9c3a79792d2de604658e095056be5
Author: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
Date: Mon Nov 21 09:54:26 2022 +0100
l2tp: Don't sleep and disable BH under writer-side sk_callback_lock
[ 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@xxxxxxxxxx/
[2] https://lore.kernel.org/netdev/000000000000e38b6605eda76f98@xxxxxxxxxx/
[3] https://lore.kernel.org/netdev/000000000000dfa31e05eda76f75@xxxxxxxxxx/
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@xxxxxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Fixes: b68777d54fac ("l2tp: Serialize access to sk_user_data with sk_callback_lock")
Reported-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: syzbot+703d9e154b3b58277261@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+50680ced9e98a61f7698@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+de987172bb74a381879b@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
index 0e0f3e96b80e..d001e254bada 100644
--- a/net/l2tp/l2tp_core.c
+++ b/net/l2tp/l2tp_core.c
@@ -1496,11 +1496,12 @@ 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);
@@ -1529,8 +1530,6 @@ 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;
@@ -1542,16 +1541,18 @@ 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;
}
