linux-hardening - Re: [PATCH] s390/sclp: replace deprecated strncpy with strtomem

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Date: Tue, 24 Oct 2023 16:46:04 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Heiko Carstens <hca@...ux.ibm.com>, Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>, linux-s390@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] s390/sclp: replace deprecated strncpy with strtomem

On Mon, Oct 23, 2023 at 07:14:49PM +0000, Justin Stitt wrote:
> Let's move away from using strncpy() as it is deprecated [1].
> 
> Instead use strtomem() as `e.id` is already marked as nonstring:
> |       char id[4] __nonstring;

I'm surprised the compiler didn't complain about this -- I thought that
was the point of the __nonstring attribute: to diagnose when used with a
string function. Hmpf.

> 
> We don't need strtomem_pad() because `e` is already memset to 0 --
> rendering any additional NUL-padding useless.
> 
> 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@...r.kernel.org
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>
> ---
> Note: build-tested only.
> 
> Found with: $ rg "strncpy\("
> ---
>  drivers/s390/char/sclp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c
> index 8f74db689a0c..486cf57a1605 100644
> --- a/drivers/s390/char/sclp.c
> +++ b/drivers/s390/char/sclp.c
> @@ -81,7 +81,7 @@ static inline void sclp_trace(int prio, char *id, u32 a, u64 b, bool err)
>  	struct sclp_trace_entry e;
>  
>  	memset(&e, 0, sizeof(e));
> -	strncpy(e.id, id, sizeof(e.id));
> +	strtomem(e.id, id);

Looks right to me; thanks!

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

-Kees

>  	e.a = a;
>  	e.b = b;
>  	debug_event(&sclp_debug, prio, &e, sizeof(e));
> 
> ---
> base-commit: 9c5d00cb7b6bbc5a7965d9ab7d223b5402d1f02c
> change-id: 20231023-strncpy-drivers-s390-char-sclp-c-bb66226a7eaa
> 
> Best regards,
> --
> Justin Stitt <justinstitt@...gle.com>
> 
> 

-- 
Kees Cook