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

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Message-ID: <a5359f24-28e8-ebd6-4451-18027e82814d@canonical.com>
Date:   Tue, 27 Apr 2021 12:50:14 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Bartosz Golaszewski <bgolaszewski@...libre.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 27/04/2021 12:33, Bartosz Golaszewski wrote:
> 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.

OK, good idea, V2 sent.
> 
> Bart
>