Re: [PATCH] tracing: Fix use-after-free and double-free on last_cmd

[Steven Rostedt <rostedt@xxxxxxxxxxx>]

On Fri, 17 Mar 2023 13:30:44 +0800
Cheng-Jui Wang <cheng-jui.wang@xxxxxxxxxxxx> wrote:

> From: "Tze-nan Wu" <Tze-nan.Wu@xxxxxxxxxxxx>

Hi!

Thanks for the report and the patch. Some nits below.

Also change the subject to:

	tracing/synthetic: Fix races on freeing last_cmd

> ---
>  kernel/trace/trace_events_synth.c | 23 +++++++++++++++++++----
>  1 file changed, 19 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
> index 46d0abb32d0f..ce438eccab2e 100644
> --- a/kernel/trace/trace_events_synth.c
> +++ b/kernel/trace/trace_events_synth.c
> @@ -42,16 +42,25 @@ enum { ERRORS };
>  #undef C
>  #define C(a, b)		b
>  
> +static DEFINE_MUTEX(lastcmd_mutex);
> +
>  static const char *err_text[] = { ERRORS };
>  
>  static char *last_cmd;

Please keep the mutex and the variable it protects next to each other:

static DEFINE_MUTEX(lastcmd_mutex);
static char *last_cmd;

>  
>  static int errpos(const char *str)
>  {
> -	if (!str || !last_cmd)
> -		return 0;
> +	int ret = 0;
> +
> +	mutex_lock(&lastcmd_mutex);
> +	if (!str || !last_cmd) {

Change this to just:

	if (!str || !last_cmd)
		goto out;

> +		mutex_unlock(&lastcmd_mutex);
> +		return ret;

> +	}
>  
> -	return err_pos(last_cmd, str);
> +	ret = err_pos(last_cmd, str);

Add:

 out:

> +	mutex_unlock(&lastcmd_mutex);
> +	return ret;
>  }
>  
>  static void last_cmd_set(const char *str)
> @@ -59,18 +68,24 @@ static void last_cmd_set(const char *str)
>  	if (!str)
>  		return;
>  
> +	mutex_lock(&lastcmd_mutex);
>  	kfree(last_cmd);
>  

In this case, you can remove the space:

	mutex_lock(&lastcmd_mutex);
	kfree(last_cmd);
	last_cmd = kstrdup(str, GFP_KERNEL);
	mutex_unlock(&lastcmd_mutex);

>  	last_cmd = kstrdup(str, GFP_KERNEL);
> +	mutex_unlock(&lastcmd_mutex);
>  }
>  
>  static void synth_err(u8 err_type, u16 err_pos)
>  {
> -	if (!last_cmd)
> +	mutex_lock(&lastcmd_mutex);
> +	if (!last_cmd) {

This should be:

	if (!last_cmd)
		goto out;

> +		mutex_unlock(&lastcmd_mutex);
>  		return;
> +	}
>  
>  	tracing_log_err(NULL, "synthetic_events", last_cmd, err_text,
>  			err_type, err_pos);

 out:

> +	mutex_unlock(&lastcmd_mutex);
>  }
>  
>  static int create_synth_event(const char *raw_command);

Thanks,

-- Steve