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Message-ID: <28095247-b28d-47f9-a28c-775432d2d6d3@clip-os.org>
Date: Tue, 30 Apr 2024 11:19:59 +0200
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Subject: Re: [PATCH] slub: Fixes freepointer encoding for single free
To: Vlastimil Babka <vbabka@suse.cz>,
 Chengming Zhou <chengming.zhou@linux.dev>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org
Cc: cl@linux.com, penberg@kernel.org, rientjes@google.com,
 iamjoonsoo.kim@lge.com, akpm@linux-foundation.org, roman.gushchin@linux.dev,
 42.hyeyoo@gmail.com, Xiongwei Song <xiongwei.song@windriver.com>
References: <Zij_fGjRS_rK-65r@archlinux>
 <f9c133a9-8886-4c86-a24f-4778997547f2@linux.dev>
 <bbf2063f-54d4-43f0-84b3-1ea789470914@clip-os.org>
 <5c34b253-b476-494a-96c9-fe3c95b9b74d@linux.dev>
 <6f977874-2a18-44ef-b207-9eb0baad9d66@suse.cz>
 <d17d7b20-b99e-46ce-b7bf-fb7058a66e79@clip-os.org>
 <7074c0b4-62d4-444d-8e59-e23bbbccf9b8@clip-os.org>
 <e03c8cbb-ddbf-4721-8e71-01e96379b0cc@linux.dev>
 <83fda406-0340-4b7b-9f02-e9eb41c77f0e@clip-os.org>
 <73f80886-e390-4320-84dd-68e7cd7e8c62@linux.dev>
 <10c9a07a-1c6d-4ea7-8c1d-03a7dc5b29d8@clip-os.org>
 <ae15114f-24d3-499b-9c99-ae7e098badd9@suse.cz>
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From: Nicolas Bouchinet <nicolas.bouchinet@clip-os.org>
In-Reply-To: <ae15114f-24d3-499b-9c99-ae7e098badd9@suse.cz>
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On 4/29/24 22:22, Vlastimil Babka wrote:
> On 4/29/24 6:16 PM, Nicolas Bouchinet wrote:
>> On 4/29/24 16:52, Chengming Zhou wrote:
>>> On 2024/4/29 22:32, Nicolas Bouchinet wrote:
>>>> On 4/29/24 15:35, Chengming Zhou wrote:
>>>>> On 2024/4/29 20:59, Nicolas Bouchinet wrote:
>>>>>>> I help maintaining the Linux-Hardened patchset in which we have a slab object canary feature that helps detecting overflows. It is located just after the object freepointer.
>>>>>> I've tried a patch where the freepointer is avoided but it results in the same bug. It seems that the commit 0f181f9fbea8bc7ea ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk allocations") inits the freepointer on allocation if init_on_free is set in order to return a clean initialized object to the caller.
>>>>>>
>>>>> Good catch! You may need to change maybe_wipe_obj_freeptr() too,
>>>>> I haven't tested this, not sure whether it works for you. :)
>>>>>
>>>>> diff --git a/mm/slub.c b/mm/slub.c
>>>>> index 3e33ff900d35..3f250a167cb5 100644
>>>>> --- a/mm/slub.c
>>>>> +++ b/mm/slub.c
>>>>> @@ -3796,7 +3796,8 @@ 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 *));
>>>>>     }
>>>>>
>>>>> Thanks!
>>>> Indeed since check_object() avoids objects for which freepointer is in the object and since val is equal to SLUB_RED_ACTIVE in our specific case it should work. Do you want me to add you as Co-authored ?
>>>>
>>> Ok, it's great. Thanks!
>> Now I think of it, doesn't it seems a bit odd to only properly
>> init_on_free object's freepointer only if it's inside the object ? IMHO
>> it is equally necessary to avoid information leaking about the
>> freepointer whether it is inside or outside the object.
>> I think it break the semantic of the commit 0f181f9fbea8bc7ea
>> ("mm/slub.c: init_on_free=1 should wipe freelist ptr for bulk
>> allocations") ?
> Hm, AFAIU, wiping inside object prevents misuse of some buggy kernel code
> that would allocate and accidentally leak prior content (including the
> in-object freepointer) somewhere the attacker can read. Now for wiping the
> freepointer outside the object to be useful it would have assume said
> leak-prone code to additionally be reading past the allocated object size,
> i.e. a read buffer overflow. That to me seems to be a much more rare
> combination, and also in that case such code could also likely read even
> further past the object, i.e. leak the next object's data? IOW I don't think
> it buys us much additional security protection in practice?
>
Moreover, with CONFIG_SLAB_FREELIST_HARDENED activated, freepointers are 
encoded and harder to exploit.


>> Thanks.
>>
>