Re: [PATCH] ASoC: Intel: Skylake: replace deprecated strncpy with strscpy

["Amadeusz Sławiński" <amadeuszx.slawinski@linux.intel.com>]

On 7/27/2023 12:34 AM, Kees Cook wrote:
> On Wed, Jul 26, 2023 at 09:12:18PM +0000, justinstitt@google.com wrote:
>> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
>>
>> A suitable replacement is `strscpy` [2] due to the fact that it
>> guarantees NUL-termination on its destination buffer argument which is
>> _not_ the case for `strncpy`!
>>
>> It was pretty difficult, in this case, to try and figure out whether or
>> not the destination buffer was zero-initialized. If it is and this
>> behavior is relied on then perhaps `strscpy_pad` is the preferred
>> option here.
>>
>> Kees was able to help me out and identify the following code snippet
>> which seems to show that the destination buffer is zero-initialized.
>>
>> |       skl = devm_kzalloc(&pci->dev, sizeof(*skl), GFP_KERNEL);
>>
>> With this information, I opted for `strscpy` since padding is seemingly
>> not required.
> 
> We did notice that str_elem->string is 44 bytes, but
> skl->lib_info[ref_count].name is 128 bytes. If str_elem->string isn't
> NUL-terminated, this can still hit an over-read condition (though
> CONFIG_FORTIFY_SOURCE would have caught it both before with strncpy()
> and now with strscpy()). So I assume it is expected to be
> NUL-terminated?
> 

Yes it is a filename of additional library which can be loaded, topology 
UAPI only allows for passing 44 bytes long strings per string token (see 
snd_soc_tplg_vendor_array -> union -> string flex array -> 
snd_soc_tplg_vendor_string_elem -> SNDRV_CTL_ELEM_ID_NAME_MAXLEN), so we 
could also change length of
skl->lib_info[ref_count].name and potentially save few bytes. And 
looking at it again I also think that we should not copy destination 
size number of bytes, by which I mean 
ARRAY_SIZE(skl->lib_info[ref_count].name), which is 128 in this case... 
so either need to change destination buffer size to be same as topology 
field or calculate it differently.