linux-kernel - Re: [syzbot] linux-next boot error: KASAN: slab-out-of-bounds Read in _find_next

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Message-Id: <20220906225312.263c4493a744cbcb66288283@linux-foundation.org>
Date:   Tue, 6 Sep 2022 22:53:12 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     syzbot <syzbot+08ca1fa706a22cc17efe@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-next@...r.kernel.org, sfr@...b.auug.org.au,
        syzkaller-bugs@...glegroups.com,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>
Subject: Re: [syzbot] linux-next boot error: KASAN: slab-out-of-bounds Read
 in _find_next_bit

On Tue, 6 Sep 2022 17:45:07 -0700 Yury Norov <yury.norov@...il.com> wrote:

> > > The buggy address belongs to the object at ffff888017576600
> > >  which belongs to the cache kmalloc-192 of size 192
> > > The buggy address is located 184 bytes inside of
> > >  192-byte region [ffff888017576600, ffff8880175766c0)
> > 
> > At offset 184 of a 192-byte region.
> > 
> > So what's wrong with doing that?  Does KASAN have an off-by-one?
> 
> Hi Andrew, all,
> 
> This is a bug in FIND_NEXT_BIT(). It should be 
>   if (idx >= sz / BITS_PER_LONG)                                   \
>           goto out;                                                \
> 
> instead of 
>   if (idx > sz / BITS_PER_LONG)                                    \
>           goto out;                                                \
> 
> The fix is in bitmap-for-next, expected to be in -next by tomorrow.
> Sorry for the noise.

OK... but why is KASAN reporting a bad access from an area
which appears to be OK?