Re: [PATCH] powerpc: Initialize local variable fdt to NULL in elf64_load()

[Lakshmi Ramasubramanian <nramas@linux.microsoft.com>]

On 4/16/21 2:05 AM, Michael Ellerman wrote:

> Daniel Axtens <dja@axtens.net> writes:
>>> On 4/15/21 12:14 PM, Lakshmi Ramasubramanian wrote:
>>>
>>> Sorry - missed copying device-tree and powerpc mailing lists.
>>>
>>>> There are a few "goto out;" statements before the local variable "fdt"
>>>> is initialized through the call to of_kexec_alloc_and_setup_fdt() in
>>>> elf64_load(). This will result in an uninitialized "fdt" being passed
>>>> to kvfree() in this function if there is an error before the call to
>>>> of_kexec_alloc_and_setup_fdt().
>>>>
>>>> Initialize the local variable "fdt" to NULL.
>>>>
>> I'm a huge fan of initialising local variables! But I'm struggling to
>> find the code path that will lead to an uninit fdt being returned...
>>
>> The out label reads in part:
>>
>> 	/* Make kimage_file_post_load_cleanup free the fdt buffer for us. */
>> 	return ret ? ERR_PTR(ret) : fdt;
>>
>> As far as I can tell, any time we get a non-zero ret, we're going to
>> return an error pointer rather than the uninitialised value...

As Dan pointed out, the new code is in linux-next.

I have copied the new one below - the function doesn't return fdt, but 
instead sets it in the arch specific field (please see the link to the 
updated elf_64.c below).

https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git/tree/arch/powerpc/kexec/elf_64.c?h=for-next

>>
>> (btw, it does look like we might leak fdt if we have an error after we
>> successfully kmalloc it.)
>>
>> Am I missing something? Can you link to the report for the kernel test
>> robot or from Dan?

/*
          * Once FDT buffer has been successfully passed to 
kexec_add_buffer(),
          * the FDT buffer address is saved in image->arch.fdt. In that 
case,
          * the memory cannot be freed here in case of any other error.
          */
         if (ret && !image->arch.fdt)
                 kvfree(fdt);

         return ret ? ERR_PTR(ret) : NULL;

In case of an error, the memory allocated for fdt is freed unless it has 
already been passed to kexec_add_buffer().

thanks,
  -lakshmi

>>
>> FWIW, I think it's worth including this patch _anyway_ because initing
>> local variables is good practice, but I'm just not sure on the
>> justification.
> 
> Why is it good practice?
> 
> It defeats -Wuninitialized. So you're guaranteed to be returning
> something initialised, but not necessarily initialised to the right
> value.
> 
> In a case like this NULL seems like a safe choice, but it's still wrong.
> The function is meant to return a pointer to the successfully allocated
> fdt, or an ERR_PTR() value. NULL is neither of those.
> 
> I agree there are security reasons that initialising stack variables is
> desirable, but I think that should be handled by the compiler, not at
> the source level.
> 
> cheers
>