[PATCH] xen/efi: refactor deprecated strncpy

[Justin Stitt <justinstitt@xxxxxxxxxx>]

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

`efi_loader_signature` has space for 4 bytes. We are copying "Xen" (3 bytes)
plus a NUL-byte which makes 4 total bytes. With that being said, there is
currently not a bug with the current `strncpy()` implementation in terms of
buffer overreads but we should favor a more robust string interface
either way.

A suitable replacement is `strscpy` [2] due to the fact that it guarantees
NUL-termination on the destination buffer while being functionally the
same in this case.

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@xxxxxxxxxxxxxxx
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
---
Note: build-tested
---
 arch/x86/xen/efi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/xen/efi.c b/arch/x86/xen/efi.c
index 863d0d6b3edc..7250d0e0e1a9 100644
--- a/arch/x86/xen/efi.c
+++ b/arch/x86/xen/efi.c
@@ -138,7 +138,7 @@ void __init xen_efi_init(struct boot_params *boot_params)
        if (efi_systab_xen == NULL)
                return;
 
-       strncpy((char *)&boot_params->efi_info.efi_loader_signature, "Xen",
+       strscpy((char *)&boot_params->efi_info.efi_loader_signature, "Xen",
                        sizeof(boot_params->efi_info.efi_loader_signature));
        boot_params->efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
        boot_params->efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 
32);

---
base-commit: 2dde18cd1d8fac735875f2e4987f11817cc0bc2c
change-id: 20230911-strncpy-arch-x86-xen-efi-c-14292f5a79ee

Best regards,
--
Justin Stitt <justinstitt@xxxxxxxxxx>