linux-kernel - Re: [PATCH] kasan: test: Silence allocation warnings from GCC 12

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Date:   Wed, 16 Feb 2022 08:22:20 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Andrey Konovalov <andreyknvl@...il.com>
Cc:     Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] kasan: test: Silence allocation warnings from GCC 12

On Wed, Feb 16, 2022 at 04:26:46PM +0100, Andrey Konovalov wrote:
> On Sun, Feb 13, 2022 at 7:32 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > GCC 12 is able to see more problems with allocation sizes at compile
> > time, so these must be silenced so the runtime checks will still be
> > available. Use OPTIMIZER_HIDE_VAR() to silence the new warnings:
> >
> > lib/test_kasan.c: In function 'ksize_uaf':
> 
> Hm, the warning mentions ksize_uaf, but none of the changes touch it.

Excellent point -- let me go re-test this.

-Kees

> 
> > lib/test_kasan.c:781:61: warning: array subscript 120 is outside array bounds of 'void[120]' [-Warray-bounds]
> >   781 |         KUNIT_EXPECT_KASAN_FAIL(test, ((volatile char *)ptr)[size]);
> >       |                                       ~~~~~~~~~~~~~~~~~~~~~~^~~~~~
> > lib/test_kasan.c:96:9: note: in definition of macro 'KUNIT_EXPECT_KASAN_FAIL'
> >    96 |         expression;                                                     \
> >       |         ^~~~~~~~~~
> > In function 'kmalloc',
> >     inlined from 'ksize_uaf' at lib/test_kasan.c:775:8:
> > ./include/linux/slab.h:581:24: note: at offset 120 into object of size 120 allocated by 'kmem_cache_alloc_trace'
> >   581 |                 return kmem_cache_alloc_trace(
> >       |                        ^~~~~~~~~~~~~~~~~~~~~~~
> >   582 |                                 kmalloc_caches[kmalloc_type(flags)][index],
> >       |                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >   583 |                                 flags, size);
> >       |                                 ~~~~~~~~~~~~
> >
> > Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>
> > Cc: Alexander Potapenko <glider@...gle.com>
> > Cc: Andrey Konovalov <andreyknvl@...il.com>
> > Cc: Dmitry Vyukov <dvyukov@...gle.com>
> > Cc: kasan-dev@...glegroups.com
> > Signed-off-by: Kees Cook <keescook@...omium.org>
> > ---
> >  lib/test_kasan.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/lib/test_kasan.c b/lib/test_kasan.c
> > index 26a5c9007653..a19b3d608e3e 100644
> > --- a/lib/test_kasan.c
> > +++ b/lib/test_kasan.c
> > @@ -124,6 +124,7 @@ static void kmalloc_oob_right(struct kunit *test)
> >
> >         ptr = kmalloc(size, GFP_KERNEL);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> > +       OPTIMIZER_HIDE_VAR(ptr);
> >
> >         /*
> >          * An unaligned access past the requested kmalloc size.
> > @@ -185,6 +186,7 @@ static void kmalloc_pagealloc_oob_right(struct kunit *test)
> >         ptr = kmalloc(size, GFP_KERNEL);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> >
> > +       OPTIMIZER_HIDE_VAR(ptr);
> >         KUNIT_EXPECT_KASAN_FAIL(test, ptr[size + OOB_TAG_OFF] = 0);
> >
> >         kfree(ptr);
> > @@ -265,6 +267,7 @@ static void kmalloc_large_oob_right(struct kunit *test)
> >         ptr = kmalloc(size, GFP_KERNEL);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> >
> > +       OPTIMIZER_HIDE_VAR(ptr);
> >         KUNIT_EXPECT_KASAN_FAIL(test, ptr[size] = 0);
> >         kfree(ptr);
> >  }
> > @@ -748,6 +751,7 @@ static void ksize_unpoisons_memory(struct kunit *test)
> >
> >         ptr = kmalloc(size, GFP_KERNEL);
> >         KUNIT_ASSERT_NOT_ERR_OR_NULL(test, ptr);
> > +       OPTIMIZER_HIDE_VAR(ptr);
> >         real_size = ksize(ptr);
> >
> >         /* This access shouldn't trigger a KASAN report. */
> > --
> > 2.30.2
> >

-- 
Kees Cook