Re: [PATCH 1/5] string.h: Introduce memtostr() and memtostr_pad()

Kees Cook
2024-04-10T18:33:52Z

On Wed, Apr 10, 2024 at 07:08:10AM +0300, Andy Shevchenko wrote:
> On Wed, Apr 10, 2024 at 5:31 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > Another ambiguous use of strncpy() is to copy from strings that may not
> > be NUL-terminated. These cases depend on having the destination buffer
> > be explicitly larger than the source buffer's maximum size, having
> > the size of the copy exactly match the source buffer's maximum size,
> > and for the destination buffer to get explicitly NUL terminated.
> >
> > This usually happens when parsing protocols or hardware character arrays
> > that are not guaranteed to be NUL-terminated. The code pattern is
> > effectively this:
> >
> > char dest[sizeof(src) + 1];
> >
> > strncpy(dest, src, sizeof(src));
> > dest[sizeof(dest) - 1] = '\0';
> >
> > In practice it usually looks like:
> >
> > struct from_hardware {
> > ...
> > char name[HW_NAME_SIZE] __nonstring;
> > ...
> > };
> >
> > struct from_hardware *p = ...;
> > char name[HW_NAME_SIZE + 1];
> >
> > strncpy(name, p->name, HW_NAME_SIZE);
> > name[NW_NAME_SIZE] = '\0';
> >
> > This cannot be replaced with:
> >
> > strscpy(name, p->name, sizeof(name));
> >
> > because p->name is smaller and not NUL-terminated, so FORTIFY will
> > trigger when strnlen(p->name, sizeof(name)) is used. And it cannot be
> > replaced with:
> >
> > strscpy(name, p->name, sizeof(p->name));
> >
> > because then "name" may contain a 1 character early truncation of
> > p->name.
> >
> > Provide an unambiguous interface for converting a maybe not-NUL-terminated
> > string to a NUL-terminated string, with compile-time buffer size checking
> > so that it can never fail at runtime: memtostr() and memtostr_pad(). Also
> > add KUnit tests for both.
>
> Obvious question, why can't strscpy() be fixed for this corner case?

We would lose the ability to detect normal out-of-bounds reads, or at
least make them ambiguous. I really want these APIs to have distinct and
dependable semantics/behaviors.

--
Kees Cook