On Fri, Oct 20, 2023 at 05:52:43PM +0000, Justin Stitt wrote:
> strncpy() is deprecated for use on NUL-terminated destination strings
> [1] and as such we should prefer more robust and less ambiguous
> interfaces.
>
> We expect ec_fw_string to be NUL-terminated based on its use with format
> strings in thinkpad_acpi.c:
> 11241 | pr_notice("ThinkPad firmware release %s doesn't match the known patterns\n",
> 11242 | ec_fw_string);
>
> Moreover, NUL-padding is not required since ec_fw_string is explicitly
> zero-initialized:
> 11185 | char ec_fw_string[18] = {0};
>
> When carefully copying bytes from one buffer to another in
> pre-determined blocks (like what's happening here with dmi_data):
>
> | static void find_new_ec_fwstr(const struct dmi_header *dm, void *private)
> | {
> | char *ec_fw_string = (char *) private;
> | const char *dmi_data = (const char *)dm;
> | /*
> | * ThinkPad Embedded Controller Program Table on newer models
> | *
> | * Offset | Name | Width | Description
> | * ----------------------------------------------------
> | * 0x00 | Type | BYTE | 0x8C
> | * 0x01 | Length | BYTE |
> | * 0x02 | Handle | WORD | Varies
> | * 0x04 | Signature | BYTEx6 | ASCII for "LENOVO"
> | * 0x0A | OEM struct offset | BYTE | 0x0B
> | * 0x0B | OEM struct number | BYTE | 0x07, for this structure
> | * 0x0C | OEM struct revision | BYTE | 0x01, for this format
> | * 0x0D | ECP version ID | STR ID |
> | * 0x0E | ECP release date | STR ID |
> | */
> |
> | /* Return if data structure not match */
> | if (dm->type != 140 || dm->length < 0x0F ||
> | memcmp(dmi_data + 4, "LENOVO", 6) != 0 ||
> | dmi_data[0x0A] != 0x0B || dmi_data[0x0B] != 0x07 ||
> | dmi_data[0x0C] != 0x01)
> | return;
> |
> | /* fwstr is the first 8byte string */
> | strncpy(ec_fw_string, dmi_data + 0x0F, 8);
>
> ... we shouldn't be using a C string api. Let's instead use memcpy() as
> this more properly relays the intended behavior.
>
> Do note that ec_fw_string will still end up being NUL-terminated since
> we are memcpy'ing only 8 bytes into a buffer full of 18 zeroes. There's
> still some trailing NUL-bytes there. To ensure this behavior, let's add
> a BUILD_BUG_ON checking the length leaves space for at least one
> trailing NUL-byte.
>
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
This looks like the best choice given how this code is designed. The
"char *private" prototype is weird since there's only one user, but
okay. :P
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
--
Kees Cook
