tracing: add trace_event_read_lock()
I found that there is nothing to protect event_hash in
ftrace_find_event(). Rcu protects the event hashlist
but not the event itself while we use it after its extraction
through ftrace_find_event().
This lack of a proper locking in this spot opens a race
window between any event dereferencing and module removal.
Eg:
--Task A--
print_trace_line(trace) {
event = find_ftrace_event(trace)
--Task B--
trace_module_remove_events(mod) {
list_trace_events_module(ev, mod) {
unregister_ftrace_event(ev->event) {
hlist_del(ev->event->node)
list_del(....)
}
}
}
|--> module removed, the event has been dropped
--Task A--
event->print(trace); // Dereferencing freed memory
If the event retrieved belongs to a module and this module
is concurrently removed, we may end up dereferencing a data
from a freed module.
RCU could solve this, but it would add latency to the kernel and
forbid tracers output callbacks to call any sleepable code.
So this fix converts 'trace_event_mutex' to a read/write semaphore,
and adds trace_event_read_lock() to protect ftrace_find_event().
[ Impact: fix possible freed memory dereference in ftrace ]
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4A114806.7090302@cn.fujitsu.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index 489c0e8..7136420 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -14,7 +14,7 @@
/* must be a power of 2 */
#define EVENT_HASHSIZE 128
-static DEFINE_MUTEX(trace_event_mutex);
+static DECLARE_RWSEM(trace_event_mutex);
static struct hlist_head event_hash[EVENT_HASHSIZE] __read_mostly;
static int next_event_type = __TRACE_LAST_TYPE + 1;
@@ -466,6 +466,7 @@
* @type: the type of event to look for
*
* Returns an event of type @type otherwise NULL
+ * Called with trace_event_read_lock() held.
*/
struct trace_event *ftrace_find_event(int type)
{
@@ -475,7 +476,7 @@
key = type & (EVENT_HASHSIZE - 1);
- hlist_for_each_entry_rcu(event, n, &event_hash[key], node) {
+ hlist_for_each_entry(event, n, &event_hash[key], node) {
if (event->type == type)
return event;
}
@@ -513,6 +514,16 @@
return last + 1;
}
+void trace_event_read_lock(void)
+{
+ down_read(&trace_event_mutex);
+}
+
+void trace_event_read_unlock(void)
+{
+ up_read(&trace_event_mutex);
+}
+
/**
* register_ftrace_event - register output for an event type
* @event: the event type to register
@@ -533,7 +544,7 @@
unsigned key;
int ret = 0;
- mutex_lock(&trace_event_mutex);
+ down_write(&trace_event_mutex);
if (WARN_ON(!event))
goto out;
@@ -581,11 +592,11 @@
key = event->type & (EVENT_HASHSIZE - 1);
- hlist_add_head_rcu(&event->node, &event_hash[key]);
+ hlist_add_head(&event->node, &event_hash[key]);
ret = event->type;
out:
- mutex_unlock(&trace_event_mutex);
+ up_write(&trace_event_mutex);
return ret;
}
@@ -597,10 +608,10 @@
*/
int unregister_ftrace_event(struct trace_event *event)
{
- mutex_lock(&trace_event_mutex);
+ down_write(&trace_event_mutex);
hlist_del(&event->node);
list_del(&event->list);
- mutex_unlock(&trace_event_mutex);
+ up_write(&trace_event_mutex);
return 0;
}
