Re: [PATCH] slub: Fixes freepointer encoding for single free

[Chengming Zhou]

On 2024/4/24 20:47, Nicolas Bouchinet wrote:
> From: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
> 
> 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().

Right.

> 
> If `init_on_free` is set, slab_free_hook() zeroes the object.
> Afterward, if `slub_debug=F` 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().

IIUC, the "freepointer" can be checked on the free path only when
it's outside the object memory. Here slab_free_hook() zeroed the
freepointer and caused the problem.

But why we should zero the memory outside the object_size? It seems
more reasonable to only zero the object_size when init_on_free is set?

Thanks.

> 
> Object's freepointer thus needs to be properly set using
> set_freepointer() after init_on_free.
> 
> To reproduce, set `slub_debug=FU init_on_free=1 log_level=7` on the
> command line of a kernel build with `CONFIG_SLAB_FREELIST_HARDENED=y`.
> 
> dmesg sample log:
> [   10.708715] =============================================================================
> [   10.710323] BUG kmalloc-rnd-05-32 (Tainted: G    B           T ): Freepointer corrupt
> [   10.712695] -----------------------------------------------------------------------------
> [   10.712695]
> [   10.712695] Slab 0xffffd8bdc400d580 objects=32 used=4 fp=0xffff9d9a80356f80 flags=0x200000000000a00(workingset|slab|node=0|zone=2)
> [   10.716698] Object 0xffff9d9a80356600 @offset=1536 fp=0x7ee4f480ce0ecd7c
> [   10.716698]
> [   10.716698] Bytes b4 ffff9d9a803565f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> [   10.720703] Object   ffff9d9a80356600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> [   10.720703] Object   ffff9d9a80356610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> [   10.724696] Padding  ffff9d9a8035666c: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> [   10.724696] Padding  ffff9d9a8035667c: 00 00 00 00                                      ....
> [   10.724696] FIX kmalloc-rnd-05-32: Object at 0xffff9d9a80356600 not freed
> 
> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>
> ---
>  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 = false;
> +
>  	memcg_slab_free_hook(s, slab, &object, 1);
> +	init = 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