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Date:   Thu, 27 Oct 2022 12:13:20 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        linux-mm@kvack.org, kasan-dev@googlegroups.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH] mm: Make ksize() a reporting-only function
Message-ID: <202210271212.EB69EF1@keescook>
References: <20221022180455.never.023-kees@kernel.org>
 <CA+fCnZcj_Hq1NQv1L2U7+A8quqj+4kA=8A7LwOWz5eYNQFra+A@mail.gmail.com>
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On Thu, Oct 27, 2022 at 09:05:45PM +0200, Andrey Konovalov wrote:
> On Sat, Oct 22, 2022 at 8:08 PM Kees Cook <keescook@chromium.org> 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