[PATCH v2] HID: uhid: replace deprecated strncpy with strscpy

[Justin Stitt]

`strncpy` is deprecated for use on NUL-terminated destination strings
[1] and as such 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.

Furthermore, let's make sure `hid->xyz` and `ev->u.create2.xyz` are the
same size at compile time to prevent silent truncation.

With these changes, it is abundantly clear what the intent and behavior
of the code is -- We are getting a string to string copy with
NUL-termination and no truncation.

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@xxxxxxxxxxxxxxx
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Changes in v2:
- update subject + commit message (thanks Kees)
- use BUILD_BUG_ON size mismatch (thanks Kees and David)
- Link to v1: https://lore.kernel.org/r/20230914-strncpy-drivers-hid-uhid-c-v1-1-18a190060d8d@xxxxxxxxxx
---
 drivers/hid/uhid.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
index 4588d2cd4ea4..a54c7995b9be 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 +514,12 @@ 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);
+	BUILD_BUG_ON(sizeof(hid->name) != sizeof(ev->u.create2.name));
+	strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
+	BUILD_BUG_ON(sizeof(hid->phys) != sizeof(ev->u.create2.phys));
+	strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
+	BUILD_BUG_ON(sizeof(hid->uniq) != sizeof(ev->u.create2.uniq));
+	strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));
 
 	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@xxxxxxxxxx>