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

[Feng Tang]

On Tue, Aug 02, 2022 at 10:38:59PM +0800, Dmitry Vyukov wrote:
> On Tue, 2 Aug 2022 at 15:37, Feng Tang <feng.tang@xxxxxxxxx> wrote:
> >
> > On Tue, Aug 02, 2022 at 06:30:44PM +0800, Dmitry Vyukov wrote:
> > > .On Tue, 2 Aug 2022 at 11:43, Vlastimil Babka <vbabka@xxxxxxx> wrote:
> > > >
> > > > On 8/2/22 09:06, Dmitry Vyukov wrote:
> > > > > On Tue, 2 Aug 2022 at 08:55, Feng Tang <feng.tang@xxxxxxxxx> 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.
> > > >
> > > > Is there maybe some KASAN macro we can use instead of hardcoding 32?
> > >
> > > kasan_free_meta is placed in the object data after freeing, so it can
> > > be sizeof(kasan_free_meta)
> >
> > 'kasan_free_meta' is defined in mm/kasan/kasan.h, to use it we need to
> > include "../kasan/kasan.h" in slub.c, or move its definition to
> > "include/linux/kasan.h"
> >
> > Another idea is to save the info in kasan_info, like:
> >
> > ---
> > diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> > index b092277bf48d..97e899948d0b 100644
> > --- a/include/linux/kasan.h
> > +++ b/include/linux/kasan.h
> > @@ -100,6 +100,7 @@ static inline bool kasan_has_integrated_init(void)
> >  struct kasan_cache {
> >         int alloc_meta_offset;
> >         int free_meta_offset;
> > +       int free_meta_size;
> 
> Storing it here looks fine to me.
> But I would name it based on the meaning for external users (i.e. that
> many bytes are occupied by kasan in freed objects). For some caches
> KASAN does not store anything in freed objects at all.

OK, please review the below patch, thanks!

- Feng

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