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

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Message-ID: <CAEVVKH9rwS6vR5_AjHyjS0vknyZvHabooE+c+k_5XNn2Rdac6w@mail.gmail.com>
Date: Fri, 26 Apr 2024 21:14:57 +0800
From: Xiongwei Song <sxwjean@...il.com>
To: Nicolas Bouchinet <nicolas.bouchinet@...p-os.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, cl@...ux.com, 
	penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com, 
	akpm@...ux-foundation.org, vbabka@...e.cz, roman.gushchin@...ux.dev, 
	42.hyeyoo@...il.com
Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free

On Fri, Apr 26, 2024 at 8:18 PM Nicolas Bouchinet
<nicolas.bouchinet@...p-os.org> wrote:
>
> On 4/26/24 11:20, Xiongwei Song wrote:
> > On Wed, Apr 24, 2024 at 8:48 PM Nicolas Bouchinet
> > <nicolas.bouchinet@...p-os.org> 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().
> >>
> >> 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
> > 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=0x7ee4f480ce0ecd7c here is beyond kernel memory space, is the issue from
> > CONFIG_SLAB_FREELIST_HARDENED enabled?
>
> My understanding of the bug is that slab_free_hook() indeed zeroes the
> object and its metadata first, then calls do_slab_free() and directly
> calls __slab_free(), head an tail parameters being set to the object.
>
> If `slub_debug=F` (SLAB_CONSISTENCY_CHECKS) is set, the following call
> path can be executed :
>
> free_to_partial_list() ->
>
> free_debug_processing() ->
>
> free_consistency_checks() ->
>
> check_object() ->
>
> check_valid_pointer(get_freepointer())

I understand the call path. I meant here the freepointer is not ZERO
but an illegal
value( fp=0x7ee4f480ce0ecd7c), then check_valid_pointer return 1 with
it's last line
and then check_object() printed out the error message. I'm not sure if I
misunderstood you.

Thank,
Xiongwei