Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms

[Joe Perches <joe@perches.com>]

On Thu, 2019-07-25 at 13:03 -0700, Kees Cook wrote:
> On Wed, Jul 24, 2019 at 10:08:57AM -0700, Linus Torvalds wrote:
> > On Wed, Jul 24, 2019 at 6:09 AM Rasmus Villemoes
> > <linux@rasmusvillemoes.dk> wrote:
> > > The kernel's snprintf() does not behave in a non-standard way, at least
> > > not with respect to its return value.
> > 
> > Note that the kernels snprintf() *does* very much protect against the
> > overflow case - not by changing the return value, but simply by having
> > 
> >         /* Reject out-of-range values early.  Large positive sizes are
> >            used for unknown buffer sizes. */
> >         if (WARN_ON_ONCE(size > INT_MAX))
> >                 return 0;
> > 
> > at the very top.
> > 
> > So you can't actually overflow in the kernel by using the repeated
> > 
> >         offset += vsnprintf( .. size - offset ..);
> > 
> > model.
> > 
> > Yes, it's the wrong thing to do, but it is still _safe_.
> 
> Actually, perhaps we should add this test to strscpy() too?

Doesn't seem to have a reason not to be added
but maybe it's better to add another WARN_ON_ONCE.

> diff --git a/lib/string.c b/lib/string.c
[]
> @@ -182,7 +182,7 @@ ssize_t strscpy(char *dest, const char *src, size_t count)
>  	size_t max = count;
>  	long res = 0;
>  
> -	if (count == 0)
> +	if (count == 0 || count > INT_MAX)
>  		return -E2BIG;
>  
>  #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
>