linux-kernel - Re: [PATCH] tracing: Fix potential double free in create_var

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Date:   Sat, 23 Apr 2022 00:13:11 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Keita Suzuki <keitasuzuki.park@...ab.ics.keio.ac.jp>
Cc:     mhiramat@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Tom Zanussi <zanussi@...nel.org>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH] tracing: Fix potential double free in create_var_ref()

Hi Keita,

On Fri, 22 Apr 2022 06:00:25 +0000
Keita Suzuki <keitasuzuki.park@...ab.ics.keio.ac.jp> wrote:

> In create_var_ref(), init_var_ref() is called to initialize the fields
> of variable ref_field, which is allocated in the previous function call
> to create_hist_field(). Function init_var_ref() allocates the
> corresponding fields such as ref_field->system, but frees these fields
> when the function encounters an error. The caller later calls
> destroy_hist_field() to conduct error handling, which frees the fields
> and the variable itself. This results in double free of the fields which
> are already freed in the previous function.
> 
> Fix this by storing NULL to the corresponding fields when they are freed
> in init_var_ref().
> 

Good catch! this looks good to me.

Reviewed-by: Masami Hiramatsu <mhiramat@...nel.org>


BTW, could you Cc the original code authoer and  if you fix a bug and
add Fixes: tag? That will help the original author can check the bug and
help stable kernel maintainers to pick the patch. :)

To find the original commit, you can use the 'git blame'.

$ git blame kernel/trace/trace_events_hist.c -L 2093,2100
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2093)      return err;
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2094)  free:
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2095)      kfree(ref_field->system);
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2096)      kfree(ref_field->event_name);
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2097)      kfree(ref_field->name);
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2098) 
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2099)      goto out;
067fe038e70f6 (Tom Zanussi 2018-01-15 20:51:56 -0600 2100) }

Then, git show will tell you the original author.
$ git show 067fe038e70f6

And get the format of the commit for Fixes tag.

$ git --no-pager show -s --abbrev-commit --abbrev=12 --pretty=format:"%h (\"%s\")%n" 067fe038e70f6
067fe038e70f ("tracing: Add variable reference handling to hist triggers")

Then you can add lines like:

Fixes: 067fe038e70f ("tracing: Add variable reference handling to hist triggers")
Cc: stable@...r.kernel.org

Thank you,

> Signed-off-by: Keita Suzuki <keitasuzuki.park@...ab.ics.keio.ac.jp>
> ---
>  kernel/trace/trace_events_hist.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 44db5ba9cabb..a0e41906d9ce 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -2093,8 +2093,11 @@ static int init_var_ref(struct hist_field *ref_field,
>  	return err;
>   free:
>  	kfree(ref_field->system);
> +	ref_field->system = NULL;
>  	kfree(ref_field->event_name);
> +	ref_field->event_name = NULL;
>  	kfree(ref_field->name);
> +	ref_field->name = NULL;
>  
>  	goto out;
>  }
> -- 
> 2.25.1
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>