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

Kees Cook
2019-07-25T20:03:35Z

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?

diff --git a/lib/string.c b/lib/string.c
index 461fb620f85f..0e0d7628ddc4 100644
--- 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

--
Kees Cook