Messages in this thread | | | Date | Mon, 29 Apr 2024 18:16:40 +0200 | Subject | Re: [PATCH] slub: Fixes freepointer encoding for single free | From | Nicolas Bouchinet <> |
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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: >>>> On 4/29/24 11:09, Nicolas Bouchinet wrote: >>>>> Hi Vlastimil, >>>>> >>>>> thanks for your review and your proposal. >>>>> >>>>> On 4/29/24 10:52, Vlastimil Babka wrote: >>>>>> On 4/25/24 5:14 PM, Chengming Zhou wrote: >>>>>>> On 2024/4/25 23:02, Nicolas Bouchinet wrote: >>>>>> Thanks for finding the bug and the fix! >>>>>> >>>>>>>> Hy, >>>>>>>> >>>>>>>> First of all, thanks a lot for your time. >>>>>>>> >>>>>>>> On 4/25/24 10:36, Chengming Zhou wrote: >>>>>>>>> On 2024/4/24 20:47, Nicolas Bouchinet wrote: >>>>>>>>>> From: Nicolas Bouchinet<nicolas.bouchinet@ssi.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(). >>>>>>>>> Right. >>>>>>>>> y not suitable for a stable-candidate fix we need >>>>>>>>>> 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(). >>>>>>>>> IIUC, the "freepointer" can be checked on the free path only when >>>>>>>>> it's outside the object memory. Here slab_free_hook() zeroed the >>>>>>>>> freepointer and caused the problem. >>>>>>>>> >>>>>>>>> But why we should zero the memory outside the object_size? It seems >>>>>>>>> more reasonable to only zero the object_size when init_on_free is set? >>>>>>>> The original purpose was to avoid leaking information through the object and its metadata / tracking information as described in init_on_free initial Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and init_on_free=1 boot options"). >>>>>>>> >>>>>>>> I have to admit I didn't read the entire lore about the original patchset yet, though it could be interesting to know a bit more the threat models, specifically regarding the object metadata init. >>>>>>> Thank you for the reference! I also don't get why it needs to zero >>>>>>> the metadata and tracking information. >>>>>> Hmm taking a step back, it seems really suboptimal to initialize the >>>>>> outside-object freepointer as part of init_on_free: >>>>>> >>>>>> - the freeing itself will always set it one way or another, in this case >>>>>> free_to_partial_list() will do set_freepointer() after free_debug_processing() >>>>>> >>>>>> - we lose the ability to detect if the allocated slab object's user wrote to >>>>>> it, which is a buffer overflow >>> Ah, right, this ability seems important for debugging overflow problem. >>> >>>>>> So the best option to me would be to adjust the init in slab_free_hook() to >>>>>> avoid the outside-object freepointer similarly to how it avoids the red zone. >>> Agree. >>> >>>>>> We'll still not have the buffer overflow detection ability for bulk free >>>>>> where slab_free_freelist_hook() will set the free pointer before we reach >>>>>> the checks, but changing that is most likely not worth the trouble, and >>>>>> especially not suitable for a stable-candidate fix we need here. >>>>> It seems like a good alternative to me, I'll push a V2 patch with those changes. >>>>> >>>>> 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") ?
Thanks.
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