On Fri, Jul 28, 2023 at 09:25:24AM +0200, Amadeusz Sławiński wrote:
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
Thanks for the details! And just to confirm, these are (expected to be) NUL-terminated?
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.
If the source is NUL-terminated, it's fine as-is. (And CONFIG_FORTIFY_SOURCE will catch problems if not.)