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

Feng Tang
2022-08-02T13:37:26Z

On Tue, Aug 02, 2022 at 06:30:44PM +0800, Dmitry Vyukov wrote:
> .On Tue, 2 Aug 2022 at 11:43, Vlastimil Babka <vbabka@suse.cz> wrote:
> >
> > On 8/2/22 09:06, 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.
> >
> > 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;
bool is_kmalloc;
};

diff --git a/mm/kasan/common.c b/mm/kasan/common.c
index c40c0e7b3b5f..7bd82c5ec264 100644
--- a/mm/kasan/common.c
+++ b/mm/kasan/common.c
@@ -178,6 +178,8 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
return;
}

+ cache->kasan_info.free_meta_size = sizeof(struct free_meta_offset);
+
/*
* Add free meta into redzone when it's not possible to store
* it in the object. This is the case when:

Thanks,
Feng