Re: [PATCH] HID: uhid: refactor deprecated strncpy

Kees Cook
2023-09-15T05:14:03Z

On Thu, Sep 14, 2023 at 10:47:21PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
>
> We should prefer more robust and less ambiguous string interfaces.
>
> A suitable replacement is `strscpy` [2] due to the fact that it
> guarantees NUL-termination on the destination buffer without
> unnecessarily NUL-padding.
>
> Looking at: Commit 4d26d1d1e806 ("Revert "HID: uhid: use strlcpy() instead of strncpy()"")
> we see referenced the fact that many attempts have been made to change
> these strncpy's into strlcpy to no success. I think strscpy is an
> objectively better interface here as it doesn't unnecessarily NUL-pad
> the destination buffer whilst allowing us to drop the `len = min(...)`
> pattern as strscpy will implicitly limit the number of bytes copied by
> the smaller of its dest and src arguments.

I think the worry here is that ev->u.create2.name may not be %NUL
terminated. But I think that doesn't matter, see below...

Also, note, this should CC David Herrmann <dh.herrmann@gmail.com>
(now added).

> So while the existing code may not have a bug (i.e: overread problems)
> we should still favor strscpy due to readability (plus a very slight
> performance boost).
>
> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
> Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
> Link: https://github.com/KSPP/linux/issues/90
> Cc: linux-hardening@vger.kernel.org
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> drivers/hid/uhid.c | 12 ++++--------
> 1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
> index 4588d2cd4ea4..00e1566ad37b 100644
> --- a/drivers/hid/uhid.c
> +++ b/drivers/hid/uhid.c
> @@ -490,7 +490,7 @@ static int uhid_dev_create2(struct uhid_device *uhid,
> const struct uhid_event *ev)
> {
> struct hid_device *hid;
> - size_t rd_size, len;
> + size_t rd_size;
> void *rd_data;
> int ret;
>
> @@ -514,13 +514,9 @@ static int uhid_dev_create2(struct uhid_device *uhid,
> goto err_free;
> }
>
> - /* @hid is zero-initialized, strncpy() is correct, strlcpy() not */
> - len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
> - strncpy(hid->name, ev->u.create2.name, len);
> - len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
> - strncpy(hid->phys, ev->u.create2.phys, len);
> - len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
> - strncpy(hid->uniq, ev->u.create2.uniq, len);

ev->u.create2 is:
struct uhid_create2_req {
__u8 name[128];
__u8 phys[64];
__u8 uniq[64];
...

hid is:
struct hid_device { /* device report descriptor */
...
char name[128]; /* Device name */
char phys[64]; /* Device physical location */
char uniq[64]; /* Device unique identifier (serial #) */

So these "min" calls are redundant -- it wants to copy at most 1 less so
it can be %NUL terminated. Which is what strscpy() already does. And
source and dest are the same size, so we can't over-read source if it
weren't terminated (since strscpy won't overread like strlcpy).

> + strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
> + strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
> + strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

>
> hid->ll_driver = &uhid_hid_driver;
> hid->bus = ev->u.create2.bus;
>
> ---
> base-commit: 3669558bdf354cd352be955ef2764cde6a9bf5ec
> change-id: 20230914-strncpy-drivers-hid-uhid-c-a465f3e784dd
>
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
>

--
Kees Cook