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Date:   Thu, 1 Dec 2022 08:51:26 -0800
From:   Kees Cook <keescook@chromium.org>
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        linux-mm@kvack.org, kasan-dev@googlegroups.com,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        linux-hardening@vger.kernel.org
Subject: Re: [PATCH v2] mm: Make ksize() a reporting-only function
Message-ID: <202212010851.33009FA7@keescook>
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On Wed, Nov 30, 2022 at 03:11:35PM +0100, Andrey Konovalov wrote:
> On Sun, Nov 27, 2022 at 1:55 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > > I just realized there's an issue here with the tag-based modes, as
> > > they align the unpoisoned area to 16 bytes.
> > >
> > > One solution would be to change the allocation size to 128 -
> > > KASAN_GRANULE_SIZE - 5, the same way kmalloc_oob_right test does it,
> > > so that the last 16-byte granule won't get unpoisoned for the
> > > tag-based modes. And then check that the ptr[size] access fails only
> > > for the Generic mode.
> >
> > Ah! Good point. Are you able to send a patch? I suspect you know exactly
> > what to change; it might take me a bit longer to double-check all of
> > those details.
> 
> Let's do it like this:
> 
> size_t size = 128 - KASAN_GRANULE_SIZE - 5, real_size.
> 
> ...
> 
> /* These must trigger a KASAN report. */
> if (IS_ENABLED(CONFIG_KASAN_GENERIC))
>     KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
> KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size + 5]);
> KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[real_size - 1]);

Done, and tested! Thanks :)

-- 
Kees Cook