linux-hardening - Re: [RFC] string: Allow 2-argument strscpy()

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Message-ID: <20240129215525.4uxchtrywzzsrauc@google.com>
Date: Mon, 29 Jan 2024 21:55:25 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Kees Cook <keescook@...omium.org>
Cc: Andy Shevchenko <andy@...nel.org>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] string: Allow 2-argument strscpy()

Hi,

On Mon, Jan 29, 2024 at 12:29:04PM -0800, Kees Cook wrote:
> Using sizeof(dst) is the overwhelmingly common case for strscpy().
> Instead of requiring this everywhere, allow a 2-argument version to be
> used that will use the sizeof() internally.

Yeah, this is definitely the case. I have a ton of patches replacing
strncpy with strscpy [1] and many of them match the pattern of:
| strscpy(dest, src, sizeof(dest))

BTW, this hack for function overloading is insane. Never really looked into
it before.

>
> Cc: Rasmus Villemoes <linux@...musvillemoes.dk>
> Cc: Andy Shevchenko <andy@...nel.org>
> Cc: linux-hardening@...r.kernel.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> What do people think of this idea? It's usually very redundant to
> include the 3rd argument, so this might improve readability (and
> perhaps make things more robust by avoiding mistakes when the
> destination name changes).

I like this, though, should you include documentation changes/additions?

Reviewed-by: Justin Stitt <justinstitt@...gle.com>

> ---
>  include/linux/fortify-string.h | 4 ++--
>  include/linux/string.h         | 9 ++++++++-
>  lib/string.c                   | 4 ++--
>  3 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 89a6888f2f9e..56be4d4a5dea 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -215,7 +215,7 @@ __kernel_size_t __fortify_strlen(const char * const POS p)
>  }
>
>  /* Defined after fortified strnlen() to reuse it. */
> -extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
> +extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(sized_strscpy);
>  /**
>   * strscpy - Copy a C-string into a sized buffer
>   *
> @@ -234,7 +234,7 @@ extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
>   * Returns the number of characters copied in @p (not including the
>   * trailing %NUL) or -E2BIG if @size is 0 or the copy of @q was truncated.
>   */
> -__FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, size_t size)
> +__FORTIFY_INLINE ssize_t sized_strscpy(char * const POS p, const char * const POS q, size_t size)
>  {
>  	/* Use string size rather than possible enclosing struct size. */
>  	const size_t p_size = __member_size(p);
> diff --git a/include/linux/string.h b/include/linux/string.h
> index ab148d8dbfc1..0bb1c8d05f18 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -67,9 +67,16 @@ extern char * strcpy(char *,const char *);
>  extern char * strncpy(char *,const char *, __kernel_size_t);
>  #endif
>  #ifndef __HAVE_ARCH_STRSCPY
> -ssize_t strscpy(char *, const char *, size_t);
> +ssize_t sized_strscpy(char *, const char *, size_t);
>  #endif
>
> +#define __strscpy0(dst, src, ...)	sized_strscpy(dst, src, sizeof(dst))
> +
> +#define __strscpy1(dst, src, size)	sized_strscpy(dst, src, size)
> +
> +#define strscpy(dst, src, ...)	\
> +	CONCATENATE(__strscpy, COUNT_ARGS(__VA_ARGS__))(dst, src, __VA_ARGS__)
> +
>  /* Wraps calls to strscpy()/memset(), no arch specific code required */
>  ssize_t strscpy_pad(char *dest, const char *src, size_t count);
>
> diff --git a/lib/string.c b/lib/string.c
> index 6891d15ce991..2869895a1180 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -104,7 +104,7 @@ EXPORT_SYMBOL(strncpy);
>  #endif
>
>  #ifndef __HAVE_ARCH_STRSCPY
> -ssize_t strscpy(char *dest, const char *src, size_t count)
> +ssize_t sized_strscpy(char *dest, const char *src, size_t count)
>  {
>  	const struct word_at_a_time constants = WORD_AT_A_TIME_CONSTANTS;
>  	size_t max = count;
> @@ -170,7 +170,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
>
>  	return -E2BIG;
>  }
> -EXPORT_SYMBOL(strscpy);
> +EXPORT_SYMBOL(sized_strscpy);
>  #endif
>
>  /**
> --
> 2.34.1
>

[1]: https://lore.kernel.org/all/?q=f%3A%22justinstitt%40google.com%22+AND+b%3Astrscpy+AND+NOT+s%3A%22Re%22

Thanks
Justin