[PATCH net 2/4] drop_monitor: use timer_shutdown_sync() to prevent timer rearming during teardown

In drop_monitor teardown paths (net_dm_trace_off_set(),
net_dm_hw_monitor_stop(), and error unwind paths in net_dm_trace_on_set()
and net_dm_hw_monitor_start()), per-CPU timers are stopped using
timer_delete_sync() followed by cancel_work_sync().

However, there is a circular dependency between send_timer and
dm_alert_work:
1) sched_send_work() (timer callback) schedules dm_alert_work.
2) send_dm_alert() / net_dm_hw_summary_work() calls reset_per_cpu_data()
   or net_dm_hw_reset_per_cpu_data().
3) If memory allocation fails under memory pressure in the reset
   function, it re-arms the timer via mod_timer(&data->send_timer, ...).

If dm_alert_work is running concurrently while timer_delete_sync()
executes on another CPU, an allocation failure in the worker will
re-arm the timer after timer_delete_sync() has already returned.
Once cancel_work_sync() completes and module_put() is called, the timer
remains active in the timer wheel. If the module is then unloaded, the
timer will fire and execute sched_send_work() in freed memory,
triggering a kernel panic / use-after-free.

Switch from timer_delete_sync() to timer_shutdown_sync(). This guarantees
that any in-flight timer handler has finished and prevents subsequent
re-arming attempts from running workers from succeeding. When monitoring
is restarted later, timer_setup() is invoked, which cleanly
re-initializes the timer.

Fixes: 9398e9c0b1d4 ("drop_monitor: Perform cleanup upon probe registration failure")
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 net/core/drop_monitor.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/core/drop_monitor.c b/net/core/drop_monitor.c
index 018d19e3a71de0be50bcc5753ba65f715f678aaa..873155ca72432924322bb7961996dd3430d052bc 100644
--- a/net/core/drop_monitor.c
+++ b/net/core/drop_monitor.c

@@ -1083,7 +1083,7 @@ static int net_dm_hw_monitor_start(struct netlink_ext_ack *extack)
 		struct per_cpu_dm_data *hw_data = &per_cpu(dm_hw_cpu_data, cpu);
 		struct sk_buff *skb;
 
-		timer_delete_sync(&hw_data->send_timer);
+		timer_shutdown_sync(&hw_data->send_timer);
 		cancel_work_sync(&hw_data->dm_alert_work);
 		while ((skb = __skb_dequeue(&hw_data->drop_queue))) {
 			struct devlink_trap_metadata *hw_metadata;

@@ -1117,7 +1117,7 @@ static void net_dm_hw_monitor_stop(struct netlink_ext_ack *extack)
 		struct per_cpu_dm_data *hw_data = &per_cpu(dm_hw_cpu_data, cpu);
 		struct sk_buff *skb;
 
-		timer_delete_sync(&hw_data->send_timer);
+		timer_shutdown_sync(&hw_data->send_timer);
 		cancel_work_sync(&hw_data->dm_alert_work);
 		while ((skb = __skb_dequeue(&hw_data->drop_queue))) {
 			struct devlink_trap_metadata *hw_metadata;

@@ -1179,7 +1179,7 @@ static int net_dm_trace_on_set(struct netlink_ext_ack *extack)
 		struct per_cpu_dm_data *data = &per_cpu(dm_cpu_data, cpu);
 		struct sk_buff *skb;
 
-		timer_delete_sync(&data->send_timer);
+		timer_shutdown_sync(&data->send_timer);
 		cancel_work_sync(&data->dm_alert_work);
 		while ((skb = __skb_dequeue(&data->drop_queue)))
 			consume_skb(skb);

@@ -1207,7 +1207,7 @@ static void net_dm_trace_off_set(void)
 		struct per_cpu_dm_data *data = &per_cpu(dm_cpu_data, cpu);
 		struct sk_buff *skb;
 
-		timer_delete_sync(&data->send_timer);
+		timer_shutdown_sync(&data->send_timer);
 		cancel_work_sync(&data->dm_alert_work);
 		while ((skb = __skb_dequeue(&data->drop_queue)))
 			consume_skb(skb);

-- 
2.55.0.1007.g17ff1f9808-goog