[PATCH net] tcp: fix a possible lockdep splat in tcp_done()

[PATCH net] tcp: fix a possible lockdep splat in tcp_done()

[Eric Dumazet]

syzbot found that if __inet_inherit_port() returns an error,
we call tcp_done() after inet_csk_prepare_forced_close(),
meaning the socket lock is no longer held.

We might fix this in a different way in net-next, but
for 5.4 it seems safer to relax the lockdep check.

Fixes: d983ea6f16b8 ("tcp: add rcu protection around tp->fastopen_rsk")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
---
 net/ipv4/tcp.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b2ac4f074e2da21db57923fda722b6d23f170de9..42187a3b82f4f6280d831c34d9ca6b7bcffc191f 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -3842,8 +3842,12 @@ void tcp_done(struct sock *sk)
 {
 	struct request_sock *req;
 
-	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk,
-					lockdep_sock_is_held(sk));
+	/* We might be called with a new socket, after
+	 * inet_csk_prepare_forced_close() has been called
+	 * so we can not use lockdep_sock_is_held(sk)
+	 */
+	req = rcu_dereference_protected(tcp_sk(sk)->fastopen_rsk, 1);
+
 	if (sk->sk_state == TCP_SYN_SENT || sk->sk_state == TCP_SYN_RECV)
 		TCP_INC_STATS(sock_net(sk), TCP_MIB_ATTEMPTFAILS);
 
-- 
2.23.0.700.g56cf767bdb-goog

Re: [PATCH net] tcp: fix a possible lockdep splat in tcp_done()

[David Miller]

From: Eric Dumazet <edumazet@google.com>
Date: Mon, 14 Oct 2019 06:47:57 -0700

> syzbot found that if __inet_inherit_port() returns an error,
> we call tcp_done() after inet_csk_prepare_forced_close(),
> meaning the socket lock is no longer held.
> 
> We might fix this in a different way in net-next, but
> for 5.4 it seems safer to relax the lockdep check.
> 
> Fixes: d983ea6f16b8 ("tcp: add rcu protection around tp->fastopen_rsk")
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Reported-by: syzbot <syzkaller@googlegroups.com>

Applied.