linux-kernel - Re: [PATCH] xen/efi: refactor deprecated strncpy

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Message-ID: <202309142009.DD5AE193@keescook>
Date:   Thu, 14 Sep 2023 20:10:19 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Justin Stitt <justinstitt@...gle.com>
Cc:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] xen/efi: refactor deprecated strncpy

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@...omium.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@...r.kernel.org
> Cc: Kees Cook <keescook@...omium.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.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@...gle.com>
> 

-- 
Kees Cook