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

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Date:   Fri, 2 Sep 2022 13:56:41 -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 08:53:34AM +0700, Bagas Sanjaya wrote:
> On 9/2/22 02:09, Kees Cook wrote:
> > One of the "legitimate" uses of strncpy() is copying a NUL-terminated
> > string into a fixed-size non-NUL-terminated character array. To avoid
> > the weaknesses and ambiguity of intent when using strncpy(), provide
> > replacement functions that explicitly distinguish between trailing
> > padding and not, and require the destination buffer size be discoverable
> > by the compiler.
> >> For example:
> > 
> > struct obj {
> > 	int foo;
> > 	char small[4] __nonstring;
> > 	char big[8] __nonstring;
> > 	int bar;
> > };
> > 
> > struct obj p;
> > 
> > /* This will truncate to 4 chars with no trailing NUL */
> > strncpy(p.small, "hello", sizeof(p.small));
> > /* p.small contains 'h', 'e', 'l', 'l' */
> > 
> > /* This will NUL pad to 8 chars. */
> > strncpy(p.big, "hello", sizeof(p.big));
> > /* p.big contains 'h', 'e', 'l', 'l', 'o', '\0', '\0', '\0' */
> > 
> > When the "__nonstring" attributes are missing, the intent of the
> > programmer becomes ambiguous for whether the lack of a trailing NUL
> > in the p.small copy is a bug. Additionally, it's not clear whether
> > the trailing padding in the p.big copy is _needed_. Both cases
> > become unambiguous with:
> > 
> > strtomem(p.small, "hello");
> > strtomem_pad(p.big, "hello", 0);
> > 
> > See also https://github.com/KSPP/linux/issues/90
> > 
> 
> Should'nt strscpy() do the job?

strscpy() will always NUL-terminate. If someone is moving a
NUL-terminated string to a fixed-length buffer (that is _not_
NUL-terminated), using strscpy() will force the final byte to be 0x00,
which will likely be a regression. For example:

struct wifi_driver {
	...
	char essid[8];
	...
};

struct wifi_driver fw;

char *essed = "12345678";

strncpy(fw.essid, essid, sizeof(fw.essid));

	fw.essid will contain: 1 2 3 4 5 6 7 8

strscpy(fw.essid, essid, sizeof(fw.essid)):

	fw.essid will contain: 1 2 3 4 5 6 7 '\0'


-- 
Kees Cook