[PATCH] tracing: Have show_event_filters/triggers files take trace array ref

[PATCH] tracing: Have show_event_filters/triggers files take trace array ref

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

The newly added files show_event_filters and show_event_triggers that show
all filters or triggers that are set within the trace array do not take a
reference for the trace array it is showing. Without taking a reference,
the trace_array may be freed via "rmdir" while a task is reading one of
theses files. Those files iterate all the events within an instance
(trace_array) and nothing prevents that instance from being freed while
its data is being read. This causes a use-after-free crash.

Have the open of both those files take the trace_array reference via the
trace_array_get() that prevents the trace_array from being freed while the
files are opened.

Cc: stable@vger.kernel.org
Fixes: 729757b96a662 ("tracing: Add show_event_filters to expose active event filters")
Fixes: 6a80838814eea ("tracing: Add show_event_triggers to expose active event triggers")
Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Closes: https://lore.kernel.org/all/CA+0ovCjerKZJLwXScM9bF2ga2rLi4_XOpUfK41NDbENpeu98jA@mail.gmail.com/
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/trace/trace_events.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index 1d39eaf6a0f7..9dbc2441763b 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -2736,14 +2736,14 @@ static const struct file_operations ftrace_show_event_filters_fops = {
 	.open = ftrace_event_show_filters_open,
 	.read = seq_read,
 	.llseek = seq_lseek,
-	.release = seq_release,
+	.release = ftrace_event_release,
 };
 
 static const struct file_operations ftrace_show_event_triggers_fops = {
 	.open = ftrace_event_show_triggers_open,
 	.read = seq_read,
 	.llseek = seq_lseek,
-	.release = seq_release,
+	.release = ftrace_event_release,
 };
 
 static const struct file_operations ftrace_set_event_pid_fops = {
@@ -2908,7 +2908,17 @@ ftrace_event_set_open(struct inode *inode, struct file *file)
 static int
 ftrace_event_show_filters_open(struct inode *inode, struct file *file)
 {
-	return ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
+	struct trace_array *tr = inode->i_private;
+	int ret;
+
+	ret = tracing_check_open_get_tr(tr);
+	if (ret)
+		return ret;
+
+	ret = ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
+	if (ret < 0)
+		trace_array_put(tr);
+	return ret;
 }
 
 /**
@@ -2922,7 +2932,17 @@ ftrace_event_show_filters_open(struct inode *inode, struct file *file)
 static int
 ftrace_event_show_triggers_open(struct inode *inode, struct file *file)
 {
-	return ftrace_event_open(inode, file, &show_show_event_triggers_seq_ops);
+	struct trace_array *tr = inode->i_private;
+	int ret;
+
+	ret = tracing_check_open_get_tr(tr);
+	if (ret)
+		return ret;
+
+	ret = ftrace_event_open(inode, file, &show_show_event_triggers_seq_ops);
+	if (ret < 0)
+		trace_array_put(tr);
+	return ret;
 }
 
 static int
-- 
2.53.0

Re: [PATCH] tracing: Have show_event_filters/triggers files take trace array ref

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [Critical] Use-after-free in trace_options_core_read and trace_options_core_write due to missing trace_array reference.
--

commit ea5806b528a93a9450975f8b2320a019a3fbc9c7
Author: Steven Rostedt <rostedt@goodmis.org>

tracing: Have show_event_filters/triggers files take trace array ref

This commit fixes a use-after-free issue when reading show_event_filters or
show_event_triggers files by ensuring their open operations take a reference
on the trace array. This prevents the trace array from being freed via rmdir
while a task is still reading the files.

Closes: https://lore.kernel.org/all/CA+0ovCjerKZJLwXScM9bF2ga2rLi4_XOpUfK41NDbENpeu98jA@mail.gmail.com/

> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 9f8f2d02276ca..e676601943362 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c

[ ... ]

> @@ -2908,7 +2908,17 @@ ftrace_event_set_open(struct inode *inode, struct file *file)
>  static int
>  ftrace_event_show_filters_open(struct inode *inode, struct file *file)
>  {
> -	return ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
> +	struct trace_array *tr = inode->i_private;
> +	int ret;
> +
> +	ret = tracing_check_open_get_tr(tr);

[Severity: Critical]
This isn't a bug introduced by this patch, but since we are fixing missing
trace array references, does a similar use-after-free exist for the trace
options files?

Looking at trace_options_core_fops in kernel/trace/trace.c, it registers
tracing_open_generic() as the open handler. That function passes NULL
to tracing_check_open_get_tr(), bypassing the trace_array_get() call:

kernel/trace/trace.c:tracing_open_generic() {
	int ret;

	ret = tracing_check_open_get_tr(NULL);
	if (ret)
		return ret;

	filp->private_data = inode->i_private;
	return 0;
}

Because no reference is taken on the trace array, could a user remove the
trace instance via rmdir while holding the options file open, and then
trigger a use-after-free when reading or writing?

For example, trace_options_core_read() retrieves the freed trace_array
via get_tr_index() and immediately dereferences it:

kernel/trace/trace.c:trace_options_core_read() {
	...
	get_tr_index(tr_index, &tr, &index);

	if (tr->trace_flags & (1ULL << index))
	...
}

> +	if (ret)
> +		return ret;
> +
> +	ret = ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
> +	if (ret < 0)
> +		trace_array_put(tr);
> +	return ret;
>  }

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260828094153.17b95037@gandalf.local.home?part=1

Re: [PATCH] tracing: Have show_event_filters/triggers files take trace array ref

[Aaron Tomlin]

On Fri, Aug 28, 2026 at 02:41:53PM +0100, Steven Rostedt wrote:
> From: Steven Rostedt <rostedt@goodmis.org>
> 
> The newly added files show_event_filters and show_event_triggers that show
> all filters or triggers that are set within the trace array do not take a
> reference for the trace array it is showing. Without taking a reference,
> the trace_array may be freed via "rmdir" while a task is reading one of
> theses files. Those files iterate all the events within an instance
> (trace_array) and nothing prevents that instance from being freed while
> its data is being read. This causes a use-after-free crash.
> 
> Have the open of both those files take the trace_array reference via the
> trace_array_get() that prevents the trace_array from being freed while the
> files are opened.
> 
> Cc: stable@vger.kernel.org
> Fixes: 729757b96a662 ("tracing: Add show_event_filters to expose active event filters")
> Fixes: 6a80838814eea ("tracing: Add show_event_triggers to expose active event triggers")
> Reported-by: Farhad Alemi <farhad.alemi@berkeley.edu>
> Closes: https://lore.kernel.org/all/CA+0ovCjerKZJLwXScM9bF2ga2rLi4_XOpUfK41NDbENpeu98jA@mail.gmail.com/
> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
> ---
>  kernel/trace/trace_events.c | 28 ++++++++++++++++++++++++----
>  1 file changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index 1d39eaf6a0f7..9dbc2441763b 100644
> --- a/kernel/trace/trace_events.c
> +++ b/kernel/trace/trace_events.c
> @@ -2736,14 +2736,14 @@ static const struct file_operations ftrace_show_event_filters_fops = {
>  	.open = ftrace_event_show_filters_open,
>  	.read = seq_read,
>  	.llseek = seq_lseek,
> -	.release = seq_release,
> +	.release = ftrace_event_release,
>  };
>  
>  static const struct file_operations ftrace_show_event_triggers_fops = {
>  	.open = ftrace_event_show_triggers_open,
>  	.read = seq_read,
>  	.llseek = seq_lseek,
> -	.release = seq_release,
> +	.release = ftrace_event_release,
>  };
>  
>  static const struct file_operations ftrace_set_event_pid_fops = {
> @@ -2908,7 +2908,17 @@ ftrace_event_set_open(struct inode *inode, struct file *file)
>  static int
>  ftrace_event_show_filters_open(struct inode *inode, struct file *file)
>  {
> -	return ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
> +	struct trace_array *tr = inode->i_private;
> +	int ret;
> +
> +	ret = tracing_check_open_get_tr(tr);
> +	if (ret)
> +		return ret;
> +
> +	ret = ftrace_event_open(inode, file, &show_show_event_filters_seq_ops);
> +	if (ret < 0)
> +		trace_array_put(tr);
> +	return ret;
>  }
>  
>  /**
> @@ -2922,7 +2932,17 @@ ftrace_event_show_filters_open(struct inode *inode, struct file *file)
>  static int
>  ftrace_event_show_triggers_open(struct inode *inode, struct file *file)
>  {
> -	return ftrace_event_open(inode, file, &show_show_event_triggers_seq_ops);
> +	struct trace_array *tr = inode->i_private;
> +	int ret;
> +
> +	ret = tracing_check_open_get_tr(tr);
> +	if (ret)
> +		return ret;
> +
> +	ret = ftrace_event_open(inode, file, &show_show_event_triggers_seq_ops);
> +	if (ret < 0)
> +		trace_array_put(tr);
> +	return ret;
>  }
>  
>  static int
> -- 
> 2.53.0
> 

Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>

-- 
Aaron Tomlin