[PATCH] um,ethertap: refactor deprecated strncpy

[Justin Stitt]

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

`gate_buf` should always be NUL-terminated and does not require
NUL-padding. It is used as a string arg inside an argv array given to
`run_helper()`. Due to this, let's use `strscpy` as it guarantees
NUL-terminated on the destination buffer preventing potential buffer
overreads [2].

This exact invocation was changed from `strcpy` to `strncpy` in commit
7879b1d94badb ("um,ethertap: use strncpy") back in 2015. Let's continue
hardening our `str*cpy` apis and use the newer and safer `strscpy`!

Link: 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 at vger.kernel.org
Cc: Kees Cook <keescook at chromium.org>
Signed-off-by: Justin Stitt <justinstitt at google.com>
---
 arch/um/os-Linux/drivers/ethertap_user.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/um/os-Linux/drivers/ethertap_user.c b/arch/um/os-Linux/drivers/ethertap_user.c
index 9483021d86dd..3363851a4ae8 100644
--- a/arch/um/os-Linux/drivers/ethertap_user.c
+++ b/arch/um/os-Linux/drivers/ethertap_user.c
@@ -105,7 +105,7 @@ static int etap_tramp(char *dev, char *gate, int control_me,
 	sprintf(data_fd_buf, "%d", data_remote);
 	sprintf(version_buf, "%d", UML_NET_VERSION);
 	if (gate != NULL) {
-		strncpy(gate_buf, gate, 15);
+		strscpy(gate_buf, gate, sizeof(gate_buf));
 		args = setup_args;
 	}
 	else args = nosetup_args;

---
base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
change-id: 20230911-strncpy-arch-um-os-linux-drivers-ethertap_user-c-859160d13f59

Best regards,
--
Justin Stitt <justinstitt at google.com>