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

Feng Tang
2022-08-02T08:45:36Z

On Tue, Aug 02, 2022 at 03:59:00PM +0800, Dmitry Vyukov wrote:
> On Tue, 2 Aug 2022 at 09:47, 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
>
> I think this can be done only when CONFIG_KASAN_GENERIC.
> Only CONFIG_KASAN_GENERIC stores free meta info in objects:
> https://elixir.bootlin.com/linux/latest/source/mm/kasan/common.c#L176

Thanks for the catch! will change.

> And KASAN_HW_TAGS has chances of being enabled with DEBUG_SLUB in
> real-world uses (with Arm MTE).

I only have device to test the kasan-generic mode, and not SW/HW tag.
But if there is conflict, we may have to apply the similar solution :)

>
> > > > +
> > > > 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/
>
> Lots of code...
>
> This SLAB_STORE_USER seems to be set on all kmalloc slabs by default
> when CONFIG_SLUB_DEBUG is enabled, right?

Christoph has explained in one earlier mail that CONFIG_SLUB_DEBUG only
compile in the debug support but not activate it. Option CONFIG_SLUB_DEBUG_ON
will enable it, and each slub debug flag bits can also be enabled
by changing kernel cmdline for some or all slabs.

> And KASAN enables CONFIG_SLUB_DEBUG, this means that this is stored
> always when KASAN is enabled? Looks wrong.

Thanks,
Feng