[PATCH net-next] tcp, ulp: remove socket lock assertion on ULP cleanup

[Daniel Borkmann <daniel@iogearbox.net>]

Eric reported that syzkaller triggered a splat in tcp_cleanup_ulp()
where assertion sock_owned_by_me() failed. This happened through
inet_csk_prepare_forced_close() first releasing the socket lock,
then calling into tcp_done(newsk) which is called after the
inet_csk_prepare_forced_close() and therefore without the socket
lock held. The sock_owned_by_me() assertion can generally be
removed as the only place where tcp_cleanup_ulp() is called from
now is out of inet_csk_destroy_sock() -> sk->sk_prot->destroy()
where socket is in dead state and unreachable. Therefore, add a
comment why the check is not needed instead.

Fixes: 8b9088f806e1 ("tcp, ulp: enforce sock_owned_by_me upon ulp init and cleanup")
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 net/ipv4/tcp_ulp.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_ulp.c b/net/ipv4/tcp_ulp.c
index a9162aa..95df7f7 100644
--- a/net/ipv4/tcp_ulp.c
+++ b/net/ipv4/tcp_ulp.c
@@ -99,8 +99,10 @@ void tcp_cleanup_ulp(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 
-	sock_owned_by_me(sk);
-
+	/* No sock_owned_by_me() check here as at the time the
+	 * stack calls this function, the socket is dead and
+	 * about to be destroyed.
+	 */
 	if (!icsk->icsk_ulp_ops)
 		return;
 
-- 
2.9.5