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Commit 4f535968 authored by Lai Jiangshan's avatar Lai Jiangshan Committed by Frederic Weisbecker
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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: default avatarLai Jiangshan <laijs@cn.fujitsu.com>
Acked-by: default avatarSteven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4A114806.7090302@cn.fujitsu.com>
Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
parent 55379376

kernel/trace/trace.c

+8 −0
Original line number Diff line number Diff line
@@ -1569,12 +1569,14 @@ static void *s_start(struct seq_file *m, loff_t *pos) 
		p = s_next(m, p, &l);

	}



	trace_event_read_lock();

	return p;

}



static void s_stop(struct seq_file *m, void *p)

{

	atomic_dec(&trace_record_cmdline_disabled);

	trace_event_read_unlock();

}



static void print_lat_help_header(struct seq_file *m)
@@ -1817,6 +1819,7 @@ static int trace_empty(struct trace_iterator *iter) 
	return 1;

}



/*  Called with trace_event_read_lock() held. */

static enum print_line_t print_trace_line(struct trace_iterator *iter)

{

	enum print_line_t ret;
@@ -3008,6 +3011,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf, 
	       offsetof(struct trace_iterator, seq));

	iter->pos = -1;



	trace_event_read_lock();

	while (find_next_entry_inc(iter) != NULL) {

		enum print_line_t ret;

		int len = iter->seq.len;
@@ -3024,6 +3028,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf, 
		if (iter->seq.len >= cnt)

			break;

	}

	trace_event_read_unlock();



	/* Now copy what we have to the user */

	sret = trace_seq_to_user(&iter->seq, ubuf, cnt);
@@ -3146,6 +3151,8 @@ static ssize_t tracing_splice_read_pipe(struct file *filp, 
		goto out_err;

	}



	trace_event_read_lock();



	/* Fill as many pages as possible. */

	for (i = 0, rem = len; i < PIPE_BUFFERS && rem; i++) {

		pages[i] = alloc_page(GFP_KERNEL);
@@ -3168,6 +3175,7 @@ static ssize_t tracing_splice_read_pipe(struct file *filp, 
		trace_seq_init(&iter->seq);

	}



	trace_event_read_unlock();

	mutex_unlock(&iter->mutex);



	spd.nr_pages = i;

kernel/trace/trace_output.c

+18 −7
Original line number Diff line number Diff line
@@ -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 @@ static int task_state_char(unsigned long state) 
 * @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 @@ struct trace_event *ftrace_find_event(int type) 


	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 @@ static int trace_search_list(struct list_head **list) 
	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 @@ int register_ftrace_event(struct trace_event *event) 
	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 @@ int register_ftrace_event(struct trace_event *event) 


	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 @@ EXPORT_SYMBOL_GPL(register_ftrace_event); 
 */

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;

}

kernel/trace/trace_output.h

+2 −0
Original line number Diff line number Diff line
@@ -20,6 +20,8 @@ extern int seq_print_user_ip(struct trace_seq *s, struct mm_struct *mm, 
extern int trace_print_context(struct trace_iterator *iter);

extern int trace_print_lat_context(struct trace_iterator *iter);



extern void trace_event_read_lock(void);

extern void trace_event_read_unlock(void);

extern struct trace_event *ftrace_find_event(int type);



extern enum print_line_t trace_nop_print(struct trace_iterator *iter,