[net-next] tipc: fix a deadlock when flushing scheduled work

[Hoang Huu Le <hoang.h.le@dektech.com.au>]

In the commit fdeba99b1e58
("tipc: fix use-after-free in tipc_bcast_get_mode"), we're trying
to make sure the tipc_net_finalize_work work item finished if it
enqueued. But calling flush_scheduled_work() is not just affecting
above work item but either any scheduled work. This has turned out
to be overkill and caused to deadlock as syzbot reported:

======================================================
WARNING: possible circular locking dependency detected
5.9.0-rc2-next-20200828-syzkaller #0 Not tainted
------------------------------------------------------
kworker/u4:6/349 is trying to acquire lock:
ffff8880aa063d38 ((wq_completion)events){+.+.}-{0:0}, at: flush_workqueue+0xe1/0x13e0 kernel/workqueue.c:2777

but task is already holding lock:
ffffffff8a879430 (pernet_ops_rwsem){++++}-{3:3}, at: cleanup_net+0x9b/0xb10 net/core/net_namespace.c:565

[...]
 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(pernet_ops_rwsem);
                               lock(&sb->s_type->i_mutex_key#13);
                               lock(pernet_ops_rwsem);
  lock((wq_completion)events);

 *** DEADLOCK ***
[...]

To fix the original issue, we replace above calling by introducing
a bit flag. When a namespace cleaned-up, bit flag is set to zero and:
- tipc_net_finalize functionial just does return immediately.
- tipc_net_finalize_work does not enqueue into the scheduled work queue.

Reported-by: syzbot+d5aa7e0385f6a5d0f4fd@syzkaller.appspotmail.com
Fixes: fdeba99b1e58 ("tipc: fix use-after-free in tipc_bcast_get_mode")
Acked-by: Jon Maloy <jmaloy@redhat.com>
Signed-off-by: Hoang Huu Le <hoang.h.le@dektech.com.au>
---
 net/tipc/core.c |  8 ++++----
 net/tipc/core.h |  1 +
 net/tipc/net.c  | 10 +++++++++-
 3 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/net/tipc/core.c b/net/tipc/core.c
index 37d8695548cf..5e7bb768f45c 100644
--- a/net/tipc/core.c
+++ b/net/tipc/core.c
@@ -60,6 +60,7 @@ static int __net_init tipc_init_net(struct net *net)
 	tn->trial_addr = 0;
 	tn->addr_trial_end = 0;
 	tn->capabilities = TIPC_NODE_CAPABILITIES;
+	test_and_set_bit_lock(0, &tn->net_exit_flag);
 	memset(tn->node_id, 0, sizeof(tn->node_id));
 	memset(tn->node_id_string, 0, sizeof(tn->node_id_string));
 	tn->mon_threshold = TIPC_DEF_MON_THRESHOLD;
@@ -110,10 +111,6 @@ static void __net_exit tipc_exit_net(struct net *net)
 	tipc_detach_loopback(net);
 	tipc_net_stop(net);
 
-	/* Make sure the tipc_net_finalize_work stopped
-	 * before releasing the resources.
-	 */
-	flush_scheduled_work();
 	tipc_bcast_stop(net);
 	tipc_nametbl_stop(net);
 	tipc_sk_rht_destroy(net);
@@ -124,6 +121,9 @@ static void __net_exit tipc_exit_net(struct net *net)
 
 static void __net_exit tipc_pernet_pre_exit(struct net *net)
 {
+	struct tipc_net *tn = tipc_net(net);
+
+	clear_bit_unlock(0, &tn->net_exit_flag);
 	tipc_node_pre_cleanup_net(net);
 }
 
diff --git a/net/tipc/core.h b/net/tipc/core.h
index 631d83c9705f..aa75882dd932 100644
--- a/net/tipc/core.h
+++ b/net/tipc/core.h
@@ -143,6 +143,7 @@ struct tipc_net {
 	/* TX crypto handler */
 	struct tipc_crypto *crypto_tx;
 #endif
+	unsigned long net_exit_flag;
 };
 
 static inline struct tipc_net *tipc_net(struct net *net)
diff --git a/net/tipc/net.c b/net/tipc/net.c
index 85400e4242de..8ad5b9ad89c0 100644
--- a/net/tipc/net.c
+++ b/net/tipc/net.c
@@ -132,6 +132,9 @@ static void tipc_net_finalize(struct net *net, u32 addr)
 {
 	struct tipc_net *tn = tipc_net(net);
 
+	if (unlikely(!test_bit(0, &tn->net_exit_flag)))
+		return;
+
 	if (cmpxchg(&tn->node_addr, 0, addr))
 		return;
 	tipc_set_node_addr(net, addr);
@@ -153,8 +156,13 @@ static void tipc_net_finalize_work(struct work_struct *work)
 
 void tipc_sched_net_finalize(struct net *net, u32 addr)
 {
-	struct tipc_net_work *fwork = kzalloc(sizeof(*fwork), GFP_ATOMIC);
+	struct tipc_net *tn = tipc_net(net);
+	struct tipc_net_work *fwork;
+
+	if (unlikely(!test_bit(0, &tn->net_exit_flag)))
+		return;
 
+	fwork = kzalloc(sizeof(*fwork), GFP_ATOMIC);
 	if (!fwork)
 		return;
 	INIT_WORK(&fwork->work, tipc_net_finalize_work);
-- 
2.25.1