[PATCH 3.16 359/366] tracing: Fix crash when freeing instances with event triggers

[Ben Hutchings]

3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>

commit 86b389ff22bd6ad8fd3cb98e41cd271886c6d023 upstream.

If a instance has an event trigger enabled when it is freed, it could cause
an access of free memory. Here's the case that crashes:

 # cd /sys/kernel/tracing
 # mkdir instances/foo
 # echo snapshot > instances/foo/events/initcall/initcall_start/trigger
 # rmdir instances/foo

Would produce:

 general protection fault: 0000 [#1] PREEMPT SMP PTI
 Modules linked in: tun bridge ...
 CPU: 5 PID: 6203 Comm: rmdir Tainted: G        W         4.17.0-rc4-test+ #933
 Hardware name: Hewlett-Packard HP Compaq Pro 6300 SFF/339A, BIOS K01 v03.03 07/14/2016
 RIP: 0010:clear_event_triggers+0x3b/0x70
 RSP: 0018:ffffc90003783de0 EFLAGS: 00010286
 RAX: 0000000000000000 RBX: 6b6b6b6b6b6b6b2b RCX: 0000000000000000
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8800c7130ba0
 RBP: ffffc90003783e00 R08: ffff8801131993f8 R09: 0000000100230016
 R10: ffffc90003783d80 R11: 0000000000000000 R12: ffff8800c7130ba0
 R13: ffff8800c7130bd8 R14: ffff8800cc093768 R15: 00000000ffffff9c
 FS:  00007f6f4aa86700(0000) GS:ffff88011eb40000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007f6f4a5aed60 CR3: 00000000cd552001 CR4: 00000000001606e0
 Call Trace:
  event_trace_del_tracer+0x2a/0xc5
  instance_rmdir+0x15c/0x200
  tracefs_syscall_rmdir+0x52/0x90
  vfs_rmdir+0xdb/0x160
  do_rmdir+0x16d/0x1c0
  __x64_sys_rmdir+0x17/0x20
  do_syscall_64+0x55/0x1a0
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

This was due to the call the clears out the triggers when an instance is
being deleted not removing the trigger from the link list.

Fixes: 85f2b08268c01 ("tracing: Add basic event trigger framework")
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 kernel/trace/trace_events_trigger.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -469,9 +469,10 @@ clear_event_triggers(struct trace_array
 	struct ftrace_event_file *file;
 
 	list_for_each_entry(file, &tr->events, list) {
-		struct event_trigger_data *data;
-		list_for_each_entry_rcu(data, &file->triggers, list) {
+		struct event_trigger_data *data, *n;
+		list_for_each_entry_safe(data, n, &file->triggers, list) {
 			trace_event_trigger_enable_disable(file, 0);
+			list_del_rcu(&data->list);
 			if (data->ops->free)
 				data->ops->free(data->ops, data);
 		}