[SRU][Bionic][Cosmic][PATCH 1/1] tcp: do not release socket ownership in tcp_close()

[Ioanna Alifieraki]

From: Eric Dumazet <edumazet at google.com>

BugLink: https://bugs.launchpad.net/bugs/1830813

syzkaller was able to hit the WARN_ON(sock_owned_by_user(sk));
in tcp_close()

While a socket is being closed, it is very possible other
threads find it in rtnetlink dump.

tcp_get_info() will acquire the socket lock for a short amount
of time (slow = lock_sock_fast(sk)/unlock_sock_fast(sk, slow);),
enough to trigger the warning.

Fixes: 67db3e4bfbc9 ("tcp: no longer hold ehash lock while calling tcp_get_info()")
Signed-off-by: Eric Dumazet <edumazet at google.com>
Reported-by: syzbot <syzkaller at googlegroups.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit 8873c064d1de579ea23412a6d3eee972593f142b)
Signed-off-by: Ioanna Alifieraki <ioanna-maria.alifieraki at canonical.com>
---
 include/net/sock.h |  1 +
 net/core/sock.c    |  2 +-
 net/ipv4/tcp.c     | 11 +++--------
 3 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 7a7b14e9628a..5fcdd5b7b47c 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1461,6 +1461,7 @@ static inline void lock_sock(struct sock *sk)
 	lock_sock_nested(sk, 0);
 }
 
+void __release_sock(struct sock *sk);
 void release_sock(struct sock *sk);
 
 /* BH context may only use the following locking interface. */
diff --git a/net/core/sock.c b/net/core/sock.c
index 1039cb1fdcff..050dba776f75 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -2242,7 +2242,7 @@ static void __lock_sock(struct sock *sk)
 	finish_wait(&sk->sk_lock.wq, &wait);
 }
 
-static void __release_sock(struct sock *sk)
+void __release_sock(struct sock *sk)
 	__releases(&sk->sk_lock.slock)
 	__acquires(&sk->sk_lock.slock)
 {
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ba6255d934a4..8a9ea72c82fa 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2242,16 +2242,10 @@ void tcp_close(struct sock *sk, long timeout)
 	sock_hold(sk);
 	sock_orphan(sk);
 
-	/* It is the last release_sock in its life. It will remove backlog. */
-	release_sock(sk);
-
-
-	/* Now socket is owned by kernel and we acquire BH lock
-	 *  to finish close. No need to check for user refs.
-	 */
 	local_bh_disable();
 	bh_lock_sock(sk);
-	WARN_ON(sock_owned_by_user(sk));
+	/* remove backlog if any, without releasing ownership. */
+	__release_sock(sk);
 
 	percpu_counter_inc(sk->sk_prot->orphan_count);
 
@@ -2320,6 +2314,7 @@ void tcp_close(struct sock *sk, long timeout)
 out:
 	bh_unlock_sock(sk);
 	local_bh_enable();
+	release_sock(sk);
 	sock_put(sk);
 }
 EXPORT_SYMBOL(tcp_close);
-- 
2.17.1