linux-kernel - Re: [PATCH v2] string: Introduce strtomem() and strtomem

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Date:   Fri, 2 Sep 2022 14:01:55 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Bagas Sanjaya <bagasdotme@...il.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Len Baker <len.baker@....com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Francis Laniel <laniel_francis@...vacyrequired.com>,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] string: Introduce strtomem() and strtomem_pad()

On Fri, Sep 02, 2022 at 11:21:21AM +0700, Bagas Sanjaya wrote:
> On Thu, Sep 01, 2022 at 12:09:52PM -0700, Kees Cook wrote:
> > + * Do not use this function. While FORTIFY_SOURCE tries to avoid
> > + * over-reads of @q, it cannot defend against writing unterminated
> > + * results to @p. Using strncpy() remains ambiguous and fragile.
> > + * Instead, please choose an alternative, so that the expectation
> > + * of @p's contents is unambiguous:
> > + *
> > + * @p needs to be:     | padded to @size | not padded
> > + * --------------------+-----------------+------------+
> > + *      NUL-terminated | strscpy_pad()   | strscpy()  |
> > + * --------------------+-----------------+------------+
> > + *  not NUL-terminated | strtomem_pad()  | strtomem() |
> > + * --------------------+-----------------+------------+
> > + *
> 
> My htmldocs build doesn't catch any new warnings, but I think the table
> above can be fixed up:
> 
> ---- >8 ----
> 
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index eed2119b23c523..3413a8e561fc62 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -95,12 +95,13 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
>   * Instead, please choose an alternative, so that the expectation
>   * of @p's contents is unambiguous:
>   *
> - * @p needs to be:     | padded to @size | not padded
> - * --------------------+-----------------+------------+
> - *      NUL-terminated | strscpy_pad()   | strscpy()  |
> - * --------------------+-----------------+------------+
> - *  not NUL-terminated | strtomem_pad()  | strtomem() |
> - * --------------------+-----------------+------------+
> + * +--------------------+-----------------+------------+
> + * |@p needs to be:     | padded to @size | not padded |
> + * +====================+=================+============+
> + * |     NUL-terminated | strscpy_pad()   | strscpy()  |
> + * +--------------------+-----------------+------------+
> + * | not NUL-terminated | strtomem_pad()  | strtomem() |
> + * +--------------------+-----------------+------------+

Ah thank you!

> Regardless, I don't see these new table above in the output (am I missing
> something?).

Oh, yes, you're right. I will add an kern-doc directive to pull in the
file.

-- 
Kees Cook