Messages in this thread | | | Date | Tue, 30 Apr 2024 11:19:59 +0200 | Subject | Re: [PATCH] slub: Fixes freepointer encoding for single free | From | Nicolas Bouchinet <> |
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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. >> >
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