linux-kernel - Re: [PATCH][next] gpio: sim: Fix dereference of free'd pointer config

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Date:   Tue, 27 Apr 2021 13:33:09 +0200
From:   Bartosz Golaszewski <bgolaszewski@...libre.com>
To:     Colin King <colin.king@...onical.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-gpio <linux-gpio@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] gpio: sim: Fix dereference of free'd pointer config

On Tue, Apr 27, 2021 at 12:24 PM Colin King <colin.king@...onical.com> wrote:
>
> From: Colin Ian King <colin.king@...onical.com>
>
> The error return of config->id dereferences the kfree'd object config.
> Fix this by using a temporary variable for the id to avoid this issue.
>
> Addresses-Coverity: ("Read from pointer aftyer free")
> Fixes: a49d14276ac4 ("gpio: sim: allocate IDA numbers earlier")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  drivers/gpio/gpio-sim.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpio/gpio-sim.c b/drivers/gpio/gpio-sim.c
> index 2e2e6399e453..7bba5783a043 100644
> --- a/drivers/gpio/gpio-sim.c
> +++ b/drivers/gpio/gpio-sim.c
> @@ -751,8 +751,10 @@ gpio_sim_config_make_item(struct config_group *group, const char *name)
>
>         config->id = ida_alloc(&gpio_sim_ida, GFP_KERNEL);
>         if (config->id < 0) {
> +               int id = config->id;
> +
>                 kfree(config);
> -               return ERR_PTR(config->id);
> +               return ERR_PTR(id);
>         }
>
>         config_item_init_type_name(&config->item, name,
> --
> 2.30.2
>

Thanks! Can you do something like this:

diff --git a/drivers/gpio/gpio-sim.c b/drivers/gpio/gpio-sim.c
index 2e2e6399e453..b21541c0b700 100644
--- a/drivers/gpio/gpio-sim.c
+++ b/drivers/gpio/gpio-sim.c
@@ -744,20 +744,22 @@ static struct config_item *
 gpio_sim_config_make_item(struct config_group *group, const char *name)
 {
  struct gpio_sim_chip_config *config;
+ int id;

  config = kzalloc(sizeof(*config), GFP_KERNEL);
  if (!config)
  return ERR_PTR(-ENOMEM);

- config->id = ida_alloc(&gpio_sim_ida, GFP_KERNEL);
- if (config->id < 0) {
+ id = ida_alloc(&gpio_sim_ida, GFP_KERNEL);
+ if (id < 0) {
  kfree(config);
- return ERR_PTR(config->id);
+ return ERR_PTR(id);
  }

  config_item_init_type_name(&config->item, name,
     &gpio_sim_chip_config_type);
  config->num_lines = 1;
+ config->id = id;
  mutex_init(&config->lock);

  return &config->item;


I think this looks more elegant without the local variable inside the if.

Bart