linux-hardening - Re: [PATCH] string: Check for "nonstring" attribute on strscpy() arguments

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Message-ID: <1724c53c-5a02-4816-a688-cae18db443d2@intel.com>
Date: Tue, 6 Aug 2024 12:29:30 +0200
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Kees Cook <kees@...nel.org>
CC: Justin Stitt <justinstitt@...gle.com>, Andy Shevchenko <andy@...nel.org>,
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>, Nick Desaulniers
	<ndesaulniers@...gle.com>, Miguel Ojeda <ojeda@...nel.org>, Marco Elver
	<elver@...gle.com>, Nathan Chancellor <nathan@...nel.org>, Hao Luo
	<haoluo@...gle.com>, <linux-sparse@...r.kernel.org>,
	<linux-hardening@...r.kernel.org>, Bill Wendling <morbo@...gle.com>, "Mark
 Rutland" <mark.rutland@....com>, Jakub Kicinski <kuba@...nel.org>, "Alexander
 Lobakin" <aleksander.lobakin@...el.com>, Tony Ambardar
	<tony.ambardar@...il.com>, Petr Pavlu <petr.pavlu@...e.com>,
	<linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>
Subject: Re: [PATCH] string: Check for "nonstring" attribute on strscpy()
 arguments

On 8/5/24 23:43, Kees Cook wrote:
> GCC already checks for arguments that are marked with the "nonstring"[1]
> attribute when used on standard C String API functions (e.g. strcpy). Gain
> this compile-time checking also for the kernel's primary string copying
> function, strscpy().
> 
> Note that Clang has neither "nonstring" nor __builtin_has_attribute().
> 
> Link: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-nonstring-variable-attribute [1]
> Signed-off-by: Kees Cook <kees@...nel.org>



> diff --git a/include/linux/string.h b/include/linux/string.h
> index 9edace076ddb..95b3fc308f4f 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -76,12 +76,16 @@ ssize_t sized_strscpy(char *, const char *, size_t);
>    * known size.
>    */
>   #define __strscpy0(dst, src, ...)	\
> -	sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst))
> -#define __strscpy1(dst, src, size)	sized_strscpy(dst, src, size)
> +	sized_strscpy(dst, src, sizeof(dst) + __must_be_array(dst) +	\
> +				__must_be_cstr(dst) + __must_be_cstr(src))
> +#define __strscpy1(dst, src, size)	\
> +	sized_strscpy(dst, src, size + __must_be_cstr(dst) + __must_be_cstr(src))
>   
>   #define __strscpy_pad0(dst, src, ...)	\
> -	sized_strscpy_pad(dst, src, sizeof(dst) + __must_be_array(dst))
> -#define __strscpy_pad1(dst, src, size)	sized_strscpy_pad(dst, src, size)
> +	sized_strscpy_pad(dst, src, sizeof(dst) + __must_be_array(dst) +	\
> +				    __must_be_cstr(dst) + __must_be_cstr(src))
> +#define __strscpy_pad1(dst, src, size)	\
> +	sized_strscpy_pad(dst, src, size + __must_be_cstr(dst) + __must_be_cstr(src))

any way to avoid the usual caveat of repeating macro argument?

a variant of BUILD_BUG that is checking argument and otherwise pasting
it would nail it, but I didn't pondered how to implement such