Subject: [PATCH 22/24] drop_monitor: Do not cancel uninitialized work item

From: Ido Schimmel <[email-masked]>
Date: Fri, 7 Feb 2020 19:29:28 +0200

[ Upstream commit dfa7f709596be5ca46c070d4f8acbb344322056a ]

Drop monitor uses a work item that takes care of constructing and
sending netlink notifications to user space. In case drop monitor never
started to monitor, then the work item is uninitialized and not
associated with a function.

Therefore, a stop command from user space results in canceling an
uninitialized work item which leads to the following warning [1].

Fix this by not processing a stop command if drop monitor is not
currently monitoring.

[1]
[   31.735402] ------------[ cut here ]------------
[   31.736470] WARNING: CPU: 0 PID: 143 at kernel/workqueue.c:3032 __flush_work+0x89f/0x9f0
...
[   31.738120] CPU: 0 PID: 143 Comm: dwdump Not tainted 5.5.0-custom-09491-g16d4077796b8 #727
[   31.741968] RIP: 0010:__flush_work+0x89f/0x9f0
...
[   31.760526] Call Trace:
[   31.771689]  __cancel_work_timer+0x2a6/0x3b0
[   31.776809]  net_dm_cmd_trace+0x300/0xef0
[   31.777549]  genl_rcv_msg+0x5c6/0xd50
[   31.781005]  netlink_rcv_skb+0x13b/0x3a0
[   31.784114]  genl_rcv+0x29/0x40
[   31.784720]  netlink_unicast+0x49f/0x6a0
[   31.787148]  netlink_sendmsg+0x7cf/0xc80
[   31.790426]  ____sys_sendmsg+0x620/0x770
[   31.793458]  ___sys_sendmsg+0xfd/0x170
[   31.802216]  __sys_sendmsg+0xdf/0x1a0
[   31.806195]  do_syscall_64+0xa0/0x540
[   31.806885]  entry_SYSCALL_64_after_hwframe+0x49/0xbe

Fixes: 8e94c3bc922e ("drop_monitor: Allow user to start monitoring hardware drops")
Signed-off-by: Ido Schimmel <[email-masked]>
Reviewed-by: Jiri Pirko <[email-masked]>
Signed-off-by: David S. Miller <[email-masked]>
---
 net/core/drop_monitor.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index 536e032d95c8..246a258b1fac 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c
@@ -1004,8 +1004,10 @@ static void net_dm_hw_monitor_stop(struct netlink_ext_ack *extack)
 {
 	int cpu;
 
-	if (!monitor_hw)
+	if (!monitor_hw) {
 		NL_SET_ERR_MSG_MOD(extack, "Hardware monitoring already disabled");
+		return;
+	}
 
 	monitor_hw = false;
 
-- 
2.21.1