`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@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
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@google.com>

On 11.09.23 20:59, Justin Stitt wrote:
> `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@vger.kernel.org
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Pushed to xen/tip.git for-linus-6.6a


Juergen

On Mon, Sep 11, 2023 at 06:59:31PM +0000, Justin Stitt wrote:
> `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.

Yeah, this will work. Since this is a u32 destination, I do wonder if
strtomem_pad() would be better since we're not really writing a string?
But since this is all hard-coded, it doesn't matter. :)

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

-Kees

> 
> 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@vger.kernel.org
> Cc: Kees Cook <keescook@chromium.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> 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@google.com>
> 

-- 
Kees Cook