Re: [mm/slub] 3616799128: BUG_kmalloc-#(Not_tainted):kmalloc_Redzone_overwritten

[Feng Tang <feng.tang@intel.com>]

On Tue, Aug 02, 2022 at 03:06:41PM +0800, Dmitry Vyukov wrote:
> On Tue, 2 Aug 2022 at 08:55, Feng Tang <feng.tang@intel.com> wrote:
> >
> > On Mon, Aug 01, 2022 at 10:23:23PM +0800, Vlastimil Babka wrote:
> > > On 8/1/22 08:21, Feng Tang wrote:
> > [snip]
> > > > Cc kansan  mail list.
> > > >
> > > > This is really related with KASAN debug, that in free path, some
> > > > kmalloc redzone ([orig_size+1, object_size]) area is written by
> > > > kasan to save free meta info.
> > > >
> > > > The callstack is:
> > > >
> > > >   kfree
> > > >     slab_free
> > > >       slab_free_freelist_hook
> > > >           slab_free_hook
> > > >             __kasan_slab_free
> > > >               ____kasan_slab_free
> > > >                 kasan_set_free_info
> > > >                   kasan_set_track
> > > >
> > > > And this issue only happens with "kmalloc-16" slab. Kasan has 2
> > > > tracks: alloc_track and free_track, for x86_64 test platform, most
> > > > of the slabs will reserve space for alloc_track, and reuse the
> > > > 'object' area for free_track.  The kasan free_track is 16 bytes
> > > > large, that it will occupy the whole 'kmalloc-16's object area,
> > > > so when kmalloc-redzone is enabled by this patch, the 'overwritten'
> > > > error is triggered.
> > > >
> > > > But it won't hurt other kmalloc slabs, as kasan's free meta won't
> > > > conflict with kmalloc-redzone which stay in the latter part of
> > > > kmalloc area.
> > > >
> > > > So the solution I can think of is:
> > > > * skip the kmalloc-redzone for kmalloc-16 only, or
> > > > * skip kmalloc-redzone if kasan is enabled, or
> > > > * let kasan reserve the free meta (16 bytes) outside of object
> > > >   just like for alloc meta
> > >
> > > Maybe we could add some hack that if both kasan and SLAB_STORE_USER is
> > > enabled, we bump the stored orig_size from <16 to 16? Similar to what
> > > __ksize() does.
> >
> > How about the following patch:
> >
> > ---
> > diff --git a/mm/slub.c b/mm/slub.c
> > index added2653bb0..33bbac2afaef 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -830,6 +830,16 @@ static inline void set_orig_size(struct kmem_cache *s,
> >         if (!slub_debug_orig_size(s))
> >                 return;
> >
> > +#ifdef CONFIG_KASAN
> > +       /*
> > +        * When kasan is enabled, it could save its free meta data in the
> > +        * start part of object area, so skip the kmalloc redzone check
> > +        * for small kmalloc slabs to avoid the data conflict.
> > +        */
> > +       if (s->object_size <= 32)
> > +               orig_size = s->object_size;
> > +#endif
> > +
> >         p += get_info_end(s);
> >         p += sizeof(struct track) * 2;
> >
> > I extend the size to 32 for potential's kasan meta data size increase.
> > This is tested locally, if people are OK with it, I can ask for 0Day's
> > help to verify this.
> 
> Where is set_orig_size() function defined? Don't see it upstream nor
> in linux-next.
> This looks fine but my only concern is that this should not increase
> memory consumption when slub debug tracking is not enabled, which
> should be the main operation mode when KASAN is enabled. But I can't
> figure this out w/o context.

Yes, the patchset was only posted on LKML, and not in any tree now.
The link to the original patches is:

https://lore.kernel.org/lkml/20220727071042.8796-1-feng.tang@intel.com/t/

Thanks,
Feng