tcp: properly terminate timers for kernel sockets

We had various syzbot reports about tcp timers firing after
the corresponding netns has been dismantled.

Fortunately Josef Bacik could trigger the issue more often,
and could test a patch I wrote two years ago.

When TCP sockets are closed, we call inet_csk_clear_xmit_timers()
to 'stop' the timers.

inet_csk_clear_xmit_timers() can be called from any context,
including when socket lock is held.
This is the reason it uses sk_stop_timer(), aka del_timer().
This means that ongoing timers might finish much later.

For user sockets, this is fine because each running timer
holds a reference on the socket, and the user socket holds
a reference on the netns.

For kernel sockets, we risk that the netns is freed before
timer can complete, because kernel sockets do not hold
reference on the netns.

This patch adds inet_csk_clear_xmit_timers_sync() function
that using sk_stop_timer_sync() to make sure all timers
are terminated before the kernel socket is released.
Modules using kernel sockets close them in their netns exit()
handler.

Also add sock_not_owned_by_me() helper to get LOCKDEP
support : inet_csk_clear_xmit_timers_sync() must not be called
while socket lock is held.

It is very possible we can revert in the future commit
3a58f13a881e ("net: rds: acquire refcount on TCP sockets")
which attempted to solve the issue in rds only.
(net/smc/af_smc.c and net/mptcp/subflow.c have similar code)

We probably can remove the check_net() tests from
tcp_out_of_resources() and __tcp_close() in the future.

Reported-by: Josef Bacik <[e-mail]>
Closes: https://lore.kernel.org/netdev/20240314210740.GA2823176@perftesting/
Fixes: 26abe14379f8 ("net: Modify sk_alloc to not reference count the netns of kernel sockets.")
Fixes: 8a68173691f0 ("net: sk_clone_lock() should only do get_net() if the parent is not a kernel socket")
Link: https://lore.kernel.org/bpf/[e-mail]/
Signed-off-by: Eric Dumazet <[e-mail]>
Tested-by: Josef Bacik <[e-mail]>
Cc: Tetsuo Handa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/inet_connection_sock.h

···
 			       void (*delack_handler)(struct timer_list *),
 			       void (*keepalive_handler)(struct timer_list *));
 void inet_csk_clear_xmit_timers(struct sock *sk);
+void inet_csk_clear_xmit_timers_sync(struct sock *sk);
 
 static inline void inet_csk_schedule_ack(struct sock *sk)
 {

include/net/sock.h

···
 #endif
 }
 
+static inline void sock_not_owned_by_me(const struct sock *sk)
+{
+#ifdef CONFIG_LOCKDEP
+	WARN_ON_ONCE(lockdep_sock_is_held(sk) && debug_locks);
+#endif
+}
+
 static inline bool sock_owned_by_user(const struct sock *sk)
 {
 	sock_owned_by_me(sk);

net/ipv4/inet_connection_sock.c

···
 }
 EXPORT_SYMBOL(inet_csk_clear_xmit_timers);
 
+void inet_csk_clear_xmit_timers_sync(struct sock *sk)
+{
+	struct inet_connection_sock *icsk = inet_csk(sk);
+
+	/* ongoing timer handlers need to acquire socket lock. */
+	sock_not_owned_by_me(sk);
+
+	icsk->icsk_pending = icsk->icsk_ack.pending = 0;
+
+	sk_stop_timer_sync(sk, &icsk->icsk_retransmit_timer);
+	sk_stop_timer_sync(sk, &icsk->icsk_delack_timer);
+	sk_stop_timer_sync(sk, &sk->sk_timer);
+}
+
 void inet_csk_delete_keepalive_timer(struct sock *sk)
 {
 	sk_stop_timer(sk, &sk->sk_timer);

net/ipv4/tcp.c

···
 	lock_sock(sk);
 	__tcp_close(sk, timeout);
 	release_sock(sk);
+	if (!sk->sk_net_refcnt)
+		inet_csk_clear_xmit_timers_sync(sk);
 	sock_put(sk);
 }
 EXPORT_SYMBOL(tcp_close);