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From: Xiongwei Song <sxwjean@gmail.com>
Date: Fri, 26 Apr 2024 17:20:03 +0800
Message-ID: <CAEVVKH8Oagbih8E8YNPpNhyh75fWnBLdod+eEGQm9i8ciNv7sQ@mail.gmail.com>
Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free
To: Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, cl@linux.com, 
	penberg@kernel.org, rientjes@google.com, iamjoonsoo.kim@lge.com, 
	akpm@linux-foundation.org, vbabka@suse.cz, roman.gushchin@linux.dev, 
	42.hyeyoo@gmail.com
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On Wed, Apr 24, 2024 at 8:48=E2=80=AFPM Nicolas Bouchinet
<nicolas.bouchinet@clip-os.org> wrote:
>
> From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
>
> Commit 284f17ac13fe ("mm/slub: handle bulk and single object freeing
> separately") splits single and bulk object freeing in two functions
> slab_free() and slab_free_bulk() which leads slab_free() to call
> slab_free_hook() directly instead of slab_free_freelist_hook().
>
> If `init_on_free` is set, slab_free_hook() zeroes the object.
> Afterward, if `slub_debug=3DF` and `CONFIG_SLAB_FREELIST_HARDENED` are
> set, the do_slab_free() slowpath executes freelist consistency
> checks and try to decode a zeroed freepointer which leads to a
> "Freepointer corrupt" detection in check_object().
>
> Object's freepointer thus needs to be properly set using
> set_freepointer() after init_on_free.
>
> To reproduce, set `slub_debug=3DFU init_on_free=3D1 log_level=3D7` on the
> command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=3Dy`.
>
> dmesg sample log:
> [   10.708715] =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D
> [   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Free=
pointer corrupt
> [   10.712695] ----------------------------------------------------------=
-------------------
> [   10.712695]
> [   10.712695] Slab 0xffffd8bdc400d580 objects=3D32 used=3D4 fp=3D0xffff9=
d9a80356f80 flags=3D0x200000000000a00(workingset|slab|node=3D0|zone=3D2)
> [   10.716698] Object 0xffff9d9a80356600 @offset=3D1536 fp=3D0x7ee4f480ce=
0ecd7c

If init_on_free is set,  slab_free_hook() zeros the object first, then
do_slab_free() calls
set_freepointer() to set the fp value, so there are 8 bytes non-zero
at the moment?
Hence, the issue is not related to init_on_free?

The fp=3D0x7ee4f480ce0ecd7c here is beyond kernel memory space, is the issu=
e from
CONFIG_SLAB_FREELIST_HARDENED enabled?

Thanks,
Xiongwei

> [   10.716698]
> [   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 0=
0 00 00 00 00 00  ................
> [   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 0=
0 00 00 00 00 00  ................
> [   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 0=
0 00 00 00 00 00  ................
> [   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 0=
0 00 00 00 00 00  ................
> [   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                    =
                  ....
> [   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not fr=
eed
>
> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
> ---
>  mm/slub.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 3aa12b9b323d9..71dbff9ad8f17 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4342,10 +4342,16 @@ static __fastpath_inline
>  void slab_free(struct kmem_cache *s, struct slab *slab, void *object,
>                unsigned long addr)
>  {
> +       bool init =3D false;
> +
>         memcg_slab_free_hook(s, slab, &object, 1);
> +       init =3D slab_want_init_on_free(s);
>
> -       if (likely(slab_free_hook(s, object, slab_want_init_on_free(s))))
> +       if (likely(slab_free_hook(s, object, init))) {
> +               if (init)
> +                       set_freepointer(s, object, NULL);
>                 do_slab_free(s, slab, object, object, 1, addr);
> +       }
>  }
>
>  static __fastpath_inline
> --
> 2.44.0
>
>