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

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Date:   Thu, 27 Oct 2022 21:05:45 +0200
From:   Andrey Konovalov <andreyknvl@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Christoph Lameter <cl@...ux.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        linux-mm@...ck.org, kasan-dev@...glegroups.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm: Make ksize() a reporting-only function

On Sat, Oct 22, 2022 at 8:08 PM Kees Cook <keescook@...omium.org> wrote:
>
> With all "silently resizing" callers of ksize() refactored, remove the
> logic in ksize() that would allow it to be used to effectively change
> the size of an allocation (bypassing __alloc_size hints, etc). Users
> wanting this feature need to either use kmalloc_size_roundup() before an
> allocation, or use krealloc() directly.
>
> For kfree_sensitive(), move the unpoisoning logic inline. Replace the
> some of the partially open-coded ksize() in __do_krealloc with ksize()
> now that it doesn't perform unpoisoning.
>
> Adjust the KUnit tests to match the new ksize() behavior.

Hi Kees,

> -/* 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.

>
>         kfree(ptr);
>  }

Thanks!