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

From: Andrew Morton <[email-masked]>
Date: Tue, 6 Sep 2022 22:53:12 -0700

On Tue, 6 Sep 2022 17:45:07 -0700 Yury Norov <[email-masked]> 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?