Subject: Re: [PATCH] mm: Make ksize() a reporting-only function

From: Kees Cook <[email-masked]>
Date: Thu, 27 Oct 2022 12:13:20 -0700

On Thu, Oct 27, 2022 at 09:05:45PM +0200, Andrey Konovalov wrote:
> On Sat, Oct 22, 2022 at 8:08 PM Kees Cook <[email-masked]> wrote:
> [...]
> > -/* Check that ksize() makes the whole object accessible. */
> > +/* Check that ksize() does NOT unpoison whole object. */
> >  static void ksize_unpoisons_memory(struct kunit *test)
> >  {
> >         char *ptr;
> > @@ -791,15 +791,17 @@ static void ksize_unpoisons_memory(struct kunit *test)
> >
> >         ptr = kmalloc(size, GFP_KERNEL);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> > +
> >         real_size = ksize(ptr);
> > +       KUNIT_EXPECT_GT(test, real_size, size);
> >
> >         OPTIMIZER_HIDE_VAR(ptr);
> >
> >         /* This access shouldn't trigger a KASAN report. */
> > -       ptr[size] = 'x';
> > +       ptr[size - 1] = 'x';
> >
> >         /* This one must. */
> > -       KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[real_size]);
> > +       KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[real_size - 1]);
> 
> How about also accessing ptr[size] here? It would allow for a more
> precise checking of the in-object redzone.

Sure! Probably both ptr[size] and ptr[real_size -1], yes?

-- 
Kees Cook