tracing: Have "enable" file use refcounts like the "filter" file

The "enable" file for the event system can be removed when a module
is unloaded and the event system only has events from that module.
As the event system nr_events count goes to zero, it may be freed
if its ref_count is also set to zero.

Like the "filter" file, the "enable" file may be opened by a task and
referenced later, after a module has been unloaded and the events for
that event system have been removed.

Although the "filter" file referenced the event system structure,
the "enable" file only references a pointer to the event system
name. Since the name is freed when the event system is removed,
it is possible that an access to the "enable" file may reference
a freed pointer.

Update the "enable" file to use the subsystem_open() routine that
the "filter" file uses, to keep a reference to the event system
structure while the "enable" file is opened.

Cc: <stable@kernel.org>
Reported-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

1 files changed, 22 insertions, 9 deletions

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index ffc5b2884af1..3e2a7c91c548 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -557,7 +557,7 @@ system_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
                    loff_t *ppos)
 {
         const char set_to_char[4] = { '?', '0', '1', 'X' };
-        const char *system = filp->private_data;
+        struct event_subsystem *system = filp->private_data;
         struct ftrace_event_call *call;
         char buf[2];
         int set = 0;
@@ -568,7 +568,7 @@ system_enable_read(struct file *filp, char __user *ubuf, size_t cnt,
                 if (!call->name || !call->class || !call->class->reg)
                         continue;
 
-                if (system && strcmp(call->class->system, system) != 0)
+                if (system && strcmp(call->class->system, system->name) != 0)
                         continue;
 
                 /*
@@ -598,7 +598,8 @@ static ssize_t
 system_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
                     loff_t *ppos)
 {
-        const char *system = filp->private_data;
+        struct event_subsystem *system = filp->private_data;
+        const char *name = NULL;
         unsigned long val;
         char buf[64];
         ssize_t ret;
@@ -622,7 +623,14 @@ system_enable_write(struct file *filp, const char __user *ubuf, size_t cnt,
         if (val != 0 && val != 1)
                 return -EINVAL;
 
-        ret = __ftrace_set_clr_event(NULL, system, NULL, val);
+        /*
+         * Opening of "enable" adds a ref count to system,
+         * so the name is safe to use.
+         */
+        if (system)
+                name = system->name;
+
+        ret = __ftrace_set_clr_event(NULL, name, NULL, val);
         if (ret)
                 goto out;
 
@@ -862,6 +870,9 @@ static int subsystem_open(struct inode *inode, struct file *filp)
         struct event_subsystem *system = NULL;
         int ret;
 
+        if (!inode->i_private)
+                goto skip_search;
+
         /* Make sure the system still exists */
         mutex_lock(&event_mutex);
         list_for_each_entry(system, &event_subsystems, list) {
@@ -880,8 +891,9 @@ static int subsystem_open(struct inode *inode, struct file *filp)
         if (system != inode->i_private)
                 return -ENODEV;
 
+ skip_search:
         ret = tracing_open_generic(inode, filp);
-        if (ret < 0)
+        if (ret < 0 && system)
                 put_system(system);
 
         return ret;
@@ -891,7 +903,8 @@ static int subsystem_release(struct inode *inode, struct file *file)
 {
         struct event_subsystem *system = inode->i_private;
 
-        put_system(system);
+        if (system)
+                put_system(system);
 
         return 0;
 }
@@ -1041,10 +1054,11 @@ static const struct file_operations ftrace_subsystem_filter_fops = {
 };
 
 static const struct file_operations ftrace_system_enable_fops = {
-        .open = tracing_open_generic,
+        .open = subsystem_open,
         .read = system_enable_read,
         .write = system_enable_write,
         .llseek = default_llseek,
+        .release = subsystem_release,
 };
 
 static const struct file_operations ftrace_show_header_fops = {
@@ -1133,8 +1147,7 @@ event_subsystem_dir(const char *name, struct dentry *d_events)
                            "'%s/filter' entry\n", name);
         }
 
-        trace_create_file("enable", 0644, system->entry,
-                          (void *)system->name,
+        trace_create_file("enable", 0644, system->entry, system,
                           &ftrace_system_enable_fops);
 
         return system->entry;