[PATCH v4] arm64/sysreg: refactor deprecated strncpy

[Justin Stitt <justinstitt@google.com>]

strncpy is deprecated [1] and should not be used if the src string is
not NUL-terminated.

When dealing with `cmdline` we are counting the number of characters
until a space then copying these over into `buf`. Let's not use any of
the str*() functions since the src string is not necessarily NUL-terminated.

Prefer `memcpy()` alongside a forced NUL-termination as it more
accurately describes what is going on within this function, i.e: copying
from non NUL-terminated buffer into a NUL-terminated buffer.

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: linux-hardening@vger.kernel.org
Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v4:
- rebase onto mainline (thanks Will)
- Link to v3: https://lore.kernel.org/r/20230831-strncpy-arch-arm64-v3-1-cdbb1e7ea5e1@google.com

Changes in v3:
- Fix faulty logic and use memcpy over strscpy (thanks Mostafa and Kees)
- Use '\0' instead of 0 to make it abundantly clear that `buf` is a NUL-terminated string
- Link to v2: https://lore.kernel.org/r/20230811-strncpy-arch-arm64-v2-1-ba84eabffadb@google.com

Changes in v2:
- Utilize return value from strscpy and check for truncation (thanks Kees)
- Link to v1: https://lore.kernel.org/r/20230810-strncpy-arch-arm64-v1-1-f67f3685cd64@google.com
---
 arch/arm64/kernel/idreg-override.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/kernel/idreg-override.c b/arch/arm64/kernel/idreg-override.c
index 2fe2491b692c..3addc09f8746 100644
--- a/arch/arm64/kernel/idreg-override.c
+++ b/arch/arm64/kernel/idreg-override.c
@@ -263,8 +263,8 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
 			return;
 
 		len = min(len, ARRAY_SIZE(buf) - 1);
-		strncpy(buf, cmdline, len);
-		buf[len] = 0;
+		memcpy(buf, cmdline, len);
+		buf[len] = '\0';
 
 		if (strcmp(buf, "--") == 0)
 			return;

---
base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
change-id: 20230810-strncpy-arch-arm64-1f3d328bd9b8

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