Re: [PATCH v2] string: Introduce strtomem() and strtomem_pad()

[Kees Cook <keescook@chromium.org>]

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