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

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

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.

Thanks,
Feng

> 
> > I don't have way to test kasan's SW/HW tag configuration, which
> > is only enabled on arm64 now. And I don't know if there will
> > also be some conflict.
> > 
> > Thanks,
> > Feng
> > 
>