tracing: Change event_filter_read/write to verify i_private != NULL

event_filter_read/write() are racy, ftrace_event_call can be already
freed by trace_remove_event_call() callers.

1. Shift mutex_lock(event_mutex) from print/apply_event_filter to
   the callers.

2. Change the callers, event_filter_read() and event_filter_write()
   to read i_private under this mutex and abort if it is NULL.

This fixes nothing, but now we can change debugfs_remove("filter")
callers to nullify ->i_private and fix the the problem.

Link: http://lkml.kernel.org/r/20130726172540.GA3619@redhat.com

Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

2 files changed, 25 insertions, 18 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 3dfa8419d0dc..1d7b6d03cd51 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -980,21 +980,28 @@ static ssize_t
 event_filter_read(struct file *filp, char __user *ubuf, size_t cnt,
                   loff_t *ppos)
 {
-        struct ftrace_event_call *call = filp->private_data;
+        struct ftrace_event_call *call;
         struct trace_seq *s;
-        int r;
+        int r = -ENODEV;
 
         if (*ppos)
                 return 0;
 
         s = kmalloc(sizeof(*s), GFP_KERNEL);
+
         if (!s)
                 return -ENOMEM;
 
         trace_seq_init(s);
 
-        print_event_filter(call, s);
-        r = simple_read_from_buffer(ubuf, cnt, ppos, s->buffer, s->len);
+        mutex_lock(&event_mutex);
+        call = event_file_data(filp);
+        if (call)
+                print_event_filter(call, s);
+        mutex_unlock(&event_mutex);
+
+        if (call)
+                r = simple_read_from_buffer(ubuf, cnt, ppos, s->buffer, s->len);
 
         kfree(s);
 
@@ -1005,9 +1012,9 @@ static ssize_t
 event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
                    loff_t *ppos)
 {
-        struct ftrace_event_call *call = filp->private_data;
+        struct ftrace_event_call *call;
         char *buf;
-        int err;
+        int err = -ENODEV;
 
         if (cnt >= PAGE_SIZE)
                 return -EINVAL;
@@ -1022,7 +1029,12 @@ event_filter_write(struct file *filp, const char __user *ubuf, size_t cnt,
         }
         buf[cnt] = '\0';
 
-        err = apply_event_filter(call, buf);
+        mutex_lock(&event_mutex);
+        call = event_file_data(filp);
+        if (call)
+                err = apply_event_filter(call, buf);
+        mutex_unlock(&event_mutex);
+
         free_page((unsigned long) buf);
         if (err < 0)
                 return err;
diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
index 0c7b75a8acc8..97daa8cf958d 100644
--- a/kernel/trace/trace_events_filter.c
+++ b/kernel/trace/trace_events_filter.c
@@ -637,17 +637,15 @@ static void append_filter_err(struct filter_parse_state *ps,
         free_page((unsigned long) buf);
 }
 
+/* caller must hold event_mutex */
 void print_event_filter(struct ftrace_event_call *call, struct trace_seq *s)
 {
-        struct event_filter *filter;
+        struct event_filter *filter = call->filter;
 
-        mutex_lock(&event_mutex);
-        filter = call->filter;
         if (filter && filter->filter_string)
                 trace_seq_printf(s, "%s\n", filter->filter_string);
         else
                 trace_seq_puts(s, "none\n");
-        mutex_unlock(&event_mutex);
 }
 
 void print_subsystem_event_filter(struct event_subsystem *system,
@@ -1841,23 +1839,22 @@ static int create_system_filter(struct event_subsystem *system,
         return err;
 }
 
+/* caller must hold event_mutex */
 int apply_event_filter(struct ftrace_event_call *call, char *filter_string)
 {
         struct event_filter *filter;
-        int err = 0;
-
-        mutex_lock(&event_mutex);
+        int err;
 
         if (!strcmp(strstrip(filter_string), "0")) {
                 filter_disable(call);
                 filter = call->filter;
                 if (!filter)
-                        goto out_unlock;
+                        return 0;
                 RCU_INIT_POINTER(call->filter, NULL);
                 /* Make sure the filter is not being used */
                 synchronize_sched();
                 __free_filter(filter);
-                goto out_unlock;
+                return 0;
         }
 
         err = create_filter(call, filter_string, true, &filter);
@@ -1884,8 +1881,6 @@ int apply_event_filter(struct ftrace_event_call *call, char *filter_string)
                         __free_filter(tmp);
                 }
         }
-out_unlock:
-        mutex_unlock(&event_mutex);
 
         return err;
 }