linux-kernel - Re: [PATCH 1/1] tracing: fix possible null pointer dereference

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Date:   Wed, 18 May 2022 15:28:39 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Yuanjun Gong <ruc_gongyuanjun@....com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/1] tracing: fix possible null pointer dereference

On Tue, 17 May 2022 17:57:23 +0800
Yuanjun Gong <ruc_gongyuanjun@....com> wrote:

> From: Gong Yuanjun <ruc_gongyuanjun@....com>
> 
> In hist_trigger_elt_data_alloc(), elt_data is freed by
> hist_elt_data_free() if kcalloc fails.
> 
> static int hist_trigger_elt_data_alloc(struct tracing_map_elt *elt)
> {
> ...
> elt_data->field_var_str = kcalloc(n_str, sizeof(char *), GFP_KERNEL);
>         if (!elt_data->field_var_str) {

But at this position elt_data->n_field_var_str is zero.

>                 hist_elt_data_free(elt_data);
>                 return -EINVAL;
>         }
> ...}
> 
> In hist_elt_data_free() the elt_data->field_var_str field should be
> checked before dereference.
> 
> Signed-off-by: Gong Yuanjun <ruc_gongyuanjun@....com>
> ---
>  kernel/trace/trace_events_hist.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 44db5ba9cabb..73177c9f94b2 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -1576,11 +1576,11 @@ static void hist_elt_data_free(struct hist_elt_data *elt_data)
>  {
>  	unsigned int i;
>  
> -	for (i = 0; i < elt_data->n_field_var_str; i++)

This loop will not execute because n_field_var_str is zero, and 0 < 0 is
false.

> -		kfree(elt_data->field_var_str[i]);
> -
> -	kfree(elt_data->field_var_str);

freeing a NULL pointer is OK to do.

There is nothing wrong with the current code. NACK.

-- Steve

> -
> +	if (elt_data->field_var_str) {
> +		for (i = 0; i < elt_data->n_field_var_str; i++)
> +			kfree(elt_data->field_var_str[i]);
> +		kfree(elt_data->field_var_str);
> +	}
>  	kfree(elt_data->comm);
>  	kfree(elt_data);
>  }