linux-kernel - Re: [PATCH v3] slub: Fixes freepointer encoding for single free

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Date: Tue, 30 Apr 2024 13:58:01 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Cc: Chengming Zhou <chengming.zhou@...ux.dev>,
 Christoph Lameter <cl@...ux.com>, Pekka Enberg <penberg@...nel.org>,
 David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Roman Gushchin <roman.gushchin@...ux.dev>,
 Hyeonggon Yoo <42.hyeyoo@...il.com>
Subject: Re: [PATCH v3] slub: Fixes freepointer encoding for single free

On 4/30/24 1:34 PM, Nicolas Bouchinet wrote:
> From: Nicolas Bouchinet <nicolas.bouchinet@....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=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().
> 
> During bulk free, slab_free_freelist_hook() isn't affected as it always
> sets it objects freepointer using set_freepointer() to maintain its
> reconstructed freelist after `init_on_free`.
> 
> For single free, object's freepointer thus needs to be avoided when
> stored outside the object if `init_on_free` is set. The freepointer left
> as is, check_object() may later detect an invalid pointer value due to
> objects overflow.
> 
> 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-developed-by: Chengming Zhou <chengming.zhou@...ux.dev>

Chengming Zhou, could you provide your s-o-b please?

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

Added to slab/for-6.9-rc7/fixes, thanks!