Re: [PATCH v2 06/10] fortify: strcat: Move definition to use fortified strlcat()

[Nick Desaulniers]

On Fri, Apr 7, 2023 at 12:27 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> Move the definition of fortified strcat() to after strlcat() to use it
> for bounds checking.
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  include/linux/fortify-string.h | 53 +++++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 27 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index 8cf17ef81905..ab058d092817 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -151,33 +151,6 @@ char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
>         return __underlying_strncpy(p, q, size);
>  }
>
> -/**
> - * strcat - Append a string to an existing string
> - *
> - * @p: pointer to NUL-terminated string to append to
> - * @q: pointer to NUL-terminated source string to append from
> - *
> - * Do not use this function. While FORTIFY_SOURCE tries to avoid
> - * read and write overflows, this is only possible when the
> - * destination buffer size is known to the compiler. Prefer
> - * building the string with formatting, via scnprintf() or similar.
> - * At the very least, use strncat().
> - *
> - * Returns @p.
> - *
> - */
> -__FORTIFY_INLINE __diagnose_as(__builtin_strcat, 1, 2)
> -char *strcat(char * const POS p, const char *q)
> -{
> -       const size_t p_size = __member_size(p);
> -
> -       if (p_size == SIZE_MAX)
> -               return __underlying_strcat(p, q);
> -       if (strlcat(p, q, p_size) >= p_size)
> -               fortify_panic(__func__);
> -       return p;
> -}
> -
>  extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(strnlen);
>  /**
>   * strnlen - Return bounded count of characters in a NUL-terminated string
> @@ -435,6 +408,32 @@ size_t strlcat(char * const POS p, const char * const POS q, size_t avail)
>         return wanted;
>  }
>
> +/* Defined after fortified strlcat() to reuse it. */

I don't follow; the previous location was already defined in terms of
calls to strlcat.  Why is this patch necessary?

Could this be fixed in 5/10
https://lore.kernel.org/linux-hardening/20230407192717.636137-5-keescook@xxxxxxxxxxxx/
by just putting strlcat in the expected place in the first place?

> +/**
> + * strcat - Append a string to an existing string
> + *
> + * @p: pointer to NUL-terminated string to append to
> + * @q: pointer to NUL-terminated source string to append from
> + *
> + * Do not use this function. While FORTIFY_SOURCE tries to avoid
> + * read and write overflows, this is only possible when the
> + * destination buffer size is known to the compiler. Prefer
> + * building the string with formatting, via scnprintf() or similar.
> + * At the very least, use strncat().
> + *
> + * Returns @p.
> + *
> + */
> +__FORTIFY_INLINE __diagnose_as(__builtin_strcat, 1, 2)
> +char *strcat(char * const POS p, const char *q)
> +{
> +       const size_t p_size = __member_size(p);
> +

This drops the `p_size == SIZE_MAX` guard.  Might it be faster at
runtime to dispatch to __underlying_strcat rather than __real_strlcat
in such cases?

What's the convention for __underlying_ vs __real_ prefixes in
include/linux/fortify-string.h?

> +       if (strlcat(p, q, p_size) >= p_size)
> +               fortify_panic(__func__);
> +       return p;
> +}
> +
>  /**
>   * strncat - Append a string to an existing string
>   *
> --
> 2.34.1
>


-- 
Thanks,
~Nick Desaulniers