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

Dmitry Vyukov
2022-08-04T10:48:20Z

On Thu, 4 Aug 2022 at 08:29, Feng Tang <feng.tang@intel.com> 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;
> >
> > 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
>
> ---8<---
> From c4fc739ea4d5222f0aba4b42b59668d64a010082 Mon Sep 17 00:00:00 2001
> From: Feng Tang <feng.tang@intel.com>
> Date: Thu, 4 Aug 2022 13:25:35 +0800
> Subject: [PATCH] mm: kasan: Add free_meta size info in struct kasan_cache
>
> When kasan is enabled for slab/slub, it may save kasan' free_meta
> data in the former part of slab object data area in slab object
> free path, which works fine.
>
> There is ongoing effort to extend slub's debug function which will
> redzone the latter part of kmalloc object area, and when both of
> the debug are enabled, there is possible conflict, especially when
> the kmalloc object has small size, as caught by 0Day bot [1]
>
> For better information for slab/slub, add free_meta's data size
> info 'kasan_cache', so that its users can take right action to
> avoid data conflict.
>
> [1]. https://lore.kernel.org/lkml/YuYm3dWwpZwH58Hu@xsang-OptiPlex-9020/
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Signed-off-by: Feng Tang <feng.tang@intel.com>

Acked-by: Dmitry Vyukov <dvyukov@google.com>

I assume there will be a second patch that uses
free_meta_size_in_object in slub debug code.

> ---
> include/linux/kasan.h | 2 ++
> mm/kasan/common.c | 2 ++
> 2 files changed, 4 insertions(+)
>
> diff --git a/include/linux/kasan.h b/include/linux/kasan.h
> index b092277bf48d..293bdaa0ba09 100644
> --- a/include/linux/kasan.h
> +++ b/include/linux/kasan.h
> @@ -100,6 +100,8 @@ static inline bool kasan_has_integrated_init(void)
> struct kasan_cache {
> int alloc_meta_offset;
> int free_meta_offset;
> + /* size of free_meta data saved in object's data area */
> + int free_meta_size_in_object;
> bool is_kmalloc;
> };
>
> diff --git a/mm/kasan/common.c b/mm/kasan/common.c
> index 78be2beb7453..a627efa267d1 100644
> --- a/mm/kasan/common.c
> +++ b/mm/kasan/common.c
> @@ -201,6 +201,8 @@ void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
> cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
> *size = ok_size;
> }
> + } else {
> + cache->kasan_info.free_meta_size_in_object = sizeof(struct kasan_free_meta);
> }
>
> /* Calculate size with optimal redzone. */
> --
> 2.27.0