linux-kernel - Re: [PATCH v2 2/7] nvmem: fix another memory leak in error path

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Date:   Tue, 18 Feb 2020 10:11:17 +0000
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Bartosz Golaszewski <brgl@...ev.pl>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        Khouloud Touil <ktouil@...libre.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2 2/7] nvmem: fix another memory leak in error path



On 18/02/2020 10:05, Bartosz Golaszewski wrote:
> wt., 18 lut 2020 o 10:56 Srinivas Kandagatla
> <srinivas.kandagatla@...aro.org> napisał(a):
>>
>>
>>
>> On 18/02/2020 09:42, Bartosz Golaszewski wrote:
>>> From: Bartosz Golaszewski <bgolaszewski@...libre.com>
>>>
>>> The nvmem struct is only freed on the first error check after its
>>> allocation and leaked after that. Fix it with a new label.
>>>
>>> Cc: <stable@...r.kernel.org>
>>> Signed-off-by: Bartosz Golaszewski <bgolaszewski@...libre.com>
>>> ---
>>>    drivers/nvmem/core.c | 8 ++++----
>>>    1 file changed, 4 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
>>> index b0be03d5f240..c9b3f4047154 100644
>>> --- a/drivers/nvmem/core.c
>>> +++ b/drivers/nvmem/core.c
>>> @@ -343,10 +343,8 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>>>                return ERR_PTR(-ENOMEM);
>>>
>>>        rval  = ida_simple_get(&nvmem_ida, 0, 0, GFP_KERNEL);
>>> -     if (rval < 0) {
>>> -             kfree(nvmem);
>>> -             return ERR_PTR(rval);
>>> -     }
>>> +     if (rval < 0)
>>> +             goto err_free_nvmem;
>>>        if (config->wp_gpio)
>>>                nvmem->wp_gpio = config->wp_gpio;
>>>        else
>>> @@ -432,6 +430,8 @@ struct nvmem_device *nvmem_register(const struct nvmem_config *config)
>>>        put_device(&nvmem->dev);
>>>    err_ida_remove:
>>>        ida_simple_remove(&nvmem_ida, nvmem->id);
>>> +err_free_nvmem:
>>> +     kfree(nvmem);
>>
>> This is not correct fix to start with, if the device has already been
>> intialized before jumping here then nvmem would be freed as part of
>> nvmem_release().
>>
>> So the bug was actually introduced by adding err_ida_remove label.
>>
>> You can free nvmem at that point but not at any point after that as
>> device core would be holding a reference to it.
>>
> 
> OK I see - I missed the release() callback assignment. Frankly: I find
> this split of resource management responsibility confusing. Since the
> users are expected to call nvmem_unregister() anyway - wouldn't it be
> more clear to just free all resources there? What is the advantage of
> defining the release() callback for device type here?

Because we are using dev pointer from nvmem structure, and this dev 
pointer should be valid until release callback is invoked.

Freeing nvmem at any early stage would make dev pointer invalid and 
device core would dereference it!

--srini
> 
> Bartosz
>