linux-kernel - Re: [PATCH] s390/ipl: refactor deprecated strncpy

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Message-ID: <202308141626.3F82919BFD@keescook>
Date:   Mon, 14 Aug 2023 16:32:11 -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/ipl: refactor deprecated strncpy

On Fri, Aug 11, 2023 at 09:56:15PM +0000, Justin Stitt wrote:
> `strncpy` is deprecated for use on NUL-terminated destination strings [1].
> 
> Use `strscpy_pad` which has the same behavior as `strncpy` here with the
> extra safeguard of guaranteeing NUL-termination of destination strings.
> In it's current form, this may result in silent truncation if the src
> string has the same size as the destination string.
> 
> Link: 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>
> ---
>  arch/s390/kernel/ipl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kernel/ipl.c b/arch/s390/kernel/ipl.c
> index 85a00d97a314..a6dcf6f28197 100644
> --- a/arch/s390/kernel/ipl.c
> +++ b/arch/s390/kernel/ipl.c
> @@ -266,7 +266,7 @@ static ssize_t sys_##_prefix##_##_name##_store(struct kobject *kobj,	\
>  		struct kobj_attribute *attr,				\
>  		const char *buf, size_t len)				\
>  {									\
> -	strncpy(_value, buf, sizeof(_value) - 1);			\
> +	strscpy_pad(_value, buf, sizeof(_value));			\

Padding isn't needed here -- the string are consumed by __cpcmd(), which
explicitly uses strlen() and a memcpy to pass them off.

>  	strim(_value);							\

This existing code line is buggy, though -- it will not trim leading
whitespace in the buffer. (It _returns_ a string that has been
forward-adjusted.)

I think this is an API mistake -- strim() should either do in-place
changes for both ends or be defined with __much_check so the return
value doesn't get lost. (But this is a separate issue.)

-Kees

-- 
Kees Cook