Re: [PATCH] tracing: Fix uaf issue in tracing_open_file_tr

[Steven Rostedt]

On Thu, 2 May 2024 03:10:24 +0000
Tze-nan Wu (吳澤南) <Tze-nan.Wu@xxxxxxxxxxxx> wrote:

> >   
> Sorry for my late reply, I'm testing the patch on my machine now. 
> Test will be done in four hours.
> 
> There's something I'm worrying about in the patch,
> what I'm worrying about is commented in the code below.
> 
> /kernel/trace/trace_events.c:
>   static int
>   event_create_dir(struct eventfs_inode *parent, 
>   struct trace_event_file *file) 
>   {
>         ...
>         ...
>         ...
>         nr_entries = ARRAY_SIZE(event_entries);
> 
>         name = trace_event_name(call);
> 
>         +event_file_get(file);        // Line A
>             ^^^^^^^^^^^^^
>         // Should we move the "event_file_get" to here, instead  
>         // of calling it at line C?
>         // Due to Line B could eventually invoke "event_file_put".
>         //   eventfs_create_dir -> free_ei ->put_ei -> kref_put 
>         //  -> release_ei -> event_release -> event_file_put
>         // Not sure if this is a potential risk? If Line B do call   
>         // event_file_put,"event_file_put" will be called prior to
>         // "event_file_get", could corrupt the reference of the file.

No, but you do bring up a good point. The release should not be called on
error, but it looks like it possibly can be.

> 
>         ei = eventfs_create_dir(name, e_events,    // Line B 
>              event_entries, nr_entries, file);
>         if (IS_ERR(ei)) {
>                 pr_warn("Could not create tracefs '%s' directory\n", 
>                 name);
>                 return -1;
>         }
>         file->ei = ei;
> 
>         ret = event_define_fields(call);
>         if (ret < 0) {
>                 pr_warn("Could not initialize trace point events/%s\n",
> name);
>                 return ret;
>                    ^^^^^^^^^          
>        // Maybe we chould have similar concern if we return here.
>        // Due to the event_inode had been created, but we did not call 
>        // event_file_get. 
>        // Could it lead to some issues in the future while freeing 
>        // event_indoe?
>         }
> 
> 
>         -event_file_get(file);       //Line C
>         return 0;
>   }

This prevents the release() function from being called on failure of
creating the ei.

Can you try this patch instead?

-- Steve

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index 894c6ca1e500..f5510e26f0f6 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -84,10 +84,17 @@ enum {
 static void release_ei(struct kref *ref)
 {
 	struct eventfs_inode *ei = container_of(ref, struct eventfs_inode, kref);
+	const struct eventfs_entry *entry;
 	struct eventfs_root_inode *rei;
 
 	WARN_ON_ONCE(!ei->is_freed);
 
+	for (int i = 0; i < ei->nr_entries; i++) {
+		entry = &ei->entries[i];
+		if (entry->release)
+			entry->release(entry->name, ei->data);
+	}
+
 	kfree(ei->entry_attrs);
 	kfree_const(ei->name);
 	if (ei->is_events) {
@@ -112,6 +119,18 @@ static inline void free_ei(struct eventfs_inode *ei)
 	}
 }
 
+/*
+ * Called when creation of an ei fails, do not call release() functions.
+ */
+static inline void cleanup_ei(struct eventfs_inode *ei)
+{
+	if (ei) {
+		/* Set nr_entries to 0 to prevent release() function being called */
+		ei->nr_entries = 0;
+		free_ei(ei);
+	}
+}
+
 static inline struct eventfs_inode *get_ei(struct eventfs_inode *ei)
 {
 	if (ei)
@@ -734,7 +753,7 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 
 	/* Was the parent freed? */
 	if (list_empty(&ei->list)) {
-		free_ei(ei);
+		cleanup_ei(ei);
 		ei = NULL;
 	}
 	return ei;
@@ -835,7 +854,7 @@ struct eventfs_inode *eventfs_create_events_dir(const char *name, struct dentry
 	return ei;
 
  fail:
-	free_ei(ei);
+	cleanup_ei(ei);
 	tracefs_failed_creating(dentry);
 	return ERR_PTR(-ENOMEM);
 }
diff --git a/include/linux/tracefs.h b/include/linux/tracefs.h
index 7a5fe17b6bf9..d03f74658716 100644
--- a/include/linux/tracefs.h
+++ b/include/linux/tracefs.h
@@ -62,6 +62,8 @@ struct eventfs_file;
 typedef int (*eventfs_callback)(const char *name, umode_t *mode, void **data,
 				const struct file_operations **fops);
 
+typedef void (*eventfs_release)(const char *name, void *data);
+
 /**
  * struct eventfs_entry - dynamically created eventfs file call back handler
  * @name:	Then name of the dynamic file in an eventfs directory
@@ -72,6 +74,7 @@ typedef int (*eventfs_callback)(const char *name, umode_t *mode, void **data,
 struct eventfs_entry {
 	const char			*name;
 	eventfs_callback		callback;
+	eventfs_release			release;
 };
 
 struct eventfs_inode;
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 52f75c36bbca..6ef29eba90ce 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -2552,6 +2552,14 @@ static int event_callback(const char *name, umode_t *mode, void **data,
 	return 0;
 }
 
+/* The file is incremented on creation and freeing the enable file decrements it */
+static void event_release(const char *name, void *data)
+{
+	struct trace_event_file *file = data;
+
+	event_file_put(file);
+}
+
 static int
 event_create_dir(struct eventfs_inode *parent, struct trace_event_file *file)
 {
@@ -2566,6 +2574,7 @@ event_create_dir(struct eventfs_inode *parent, struct trace_event_file *file)
 		{
 			.name		= "enable",
 			.callback	= event_callback,
+			.release	= event_release,
 		},
 		{
 			.name		= "filter",
@@ -2634,6 +2643,9 @@ event_create_dir(struct eventfs_inode *parent, struct trace_event_file *file)
 		return ret;
 	}
 
+	/* Gets decremented on freeing of the "enable" file */
+	event_file_get(file);
+
 	return 0;
 }