Re: [PATCH v2] slub: Fixes freepointer encoding for single free

[Vlastimil Babka]

On 4/30/24 10:52 AM, 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().
> 
> 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().

To make this explanation complete, we should also say that with
slab_free_freelist_hook() this doesn't happen as it always sets the
freepointer to a valid value after the zeroing.

> Object's freepointer thus needs to be avoided when stored outside the
> object if init_on_free is set.

It would be good to add more reasoning why we're not just doing the same
freepointer re-init as slab_free_freelist_hook(), but we decided instead to
allow check_object() to actually catch any overwrite by the user of the
allocated object, which means it did a buffer overflow. And for that we need
to stop wiping or re-initing the outside-object freepointer ourselves...

> 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
> 
> Co-authored-by: Chengming Zhou <chengming.zhou@xxxxxxxxx>

So per Documentation/process/submitting-patches.rst the canonical name is
Co-developed-by: and Chengming Zhou should respond with his Signed-off-by:

> Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx>

Otherwise seems correct, thanks! So if you could just resend with updated
changelog, would be great.

> ---
> Changes since v1:
> https://lore.kernel.org/all/Zij_fGjRS_rK-65r@archlinux/
> 
> * Jump above out of object freepointer if init_on_free is set
> instead of initializing it with set_freepointer() as suggested
> by Vlastimil Babka.
> 
> * Adapt maybe_wipe_obj_freeptr() to avoid wiping out of object
> on alloc freepointer as suggested by Chengming Zhou.
> 
> * Reword commit message.
> ---
>  mm/slub.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 3aa12b9b323d..173c340ec1d3 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2102,15 +2102,20 @@ bool slab_free_hook(struct kmem_cache *s, void *x, bool init)
>  	 *
>  	 * The initialization memset's clear the object and the metadata,
>  	 * but don't touch the SLAB redzone.
> +	 *
> +	 * The object's freepointer is also avoided if stored outside the
> +	 * object.
>  	 */
>  	if (unlikely(init)) {
>  		int rsize;
> +		unsigned int inuse;
>  
> +		inuse = get_info_end(s);
>  		if (!kasan_has_integrated_init())
>  			memset(kasan_reset_tag(x), 0, s->object_size);
>  		rsize = (s->flags & SLAB_RED_ZONE) ? s->red_left_pad : 0;
> -		memset((char *)kasan_reset_tag(x) + s->inuse, 0,
> -		       s->size - s->inuse - rsize);
> +		memset((char *)kasan_reset_tag(x) + inuse, 0,
> +			s->size - inuse - rsize);
>  	}
>  	/* KASAN might put x into memory quarantine, delaying its reuse. */
>  	return !kasan_slab_free(s, x, init);
> @@ -3789,7 +3794,7 @@ static void *__slab_alloc_node(struct kmem_cache *s,
>  static __always_inline void maybe_wipe_obj_freeptr(struct kmem_cache *s,
>  						   void *obj)
>  {
> -	if (unlikely(slab_want_init_on_free(s)) && obj)
> +	if (unlikely(slab_want_init_on_free(s)) && obj && !freeptr_outside_object(s))
>  		memset((void *)((char *)kasan_reset_tag(obj) + s->offset),
>  			0, sizeof(void *));
>  }