Messages in this thread | | | Date | Mon, 29 Apr 2024 14:59:32 +0200 | Subject | Re: [PATCH] slub: Fixes freepointer encoding for single free | From | Nicolas Bouchinet <> |
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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 >> >> 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. >> >> 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.
> >> >>>> The patch could also be optimized a bit by restricting >>>> set_freepointer() call to the `CONFIG_SLAB_FREELIST_HARDENED` >>>> option value. >>>> >>> Yeah. Maybe memcg_alloc_abort_single() needs this too. >>> >>> Thanks. >>> >>>> Thanks again, Nicolas >>>> >>>>> Thanks. >>>>> >>>>>> 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 >>>>>> [ 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 >>>>>> >>>>>> Signed-off-by: Nicolas Bouchinet<nicolas.bouchinet@ssi.gouv.fr> >>>>>> --- >>>>>> mm/slub.c | 8 +++++++- >>>>>> 1 file changed, 7 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/mm/slub.c b/mm/slub.c >>>>>> index 3aa12b9b323d9..71dbff9ad8f17 100644 >>>>>> --- a/mm/slub.c >>>>>> +++ b/mm/slub.c >>>>>> @@ -4342,10 +4342,16 @@ static __fastpath_inline >>>>>> void slab_free(struct kmem_cache *s, struct slab *slab, void >>>>>> *object, >>>>>> unsigned long addr) >>>>>> { >>>>>> + bool init = false; >>>>>> + >>>>>> memcg_slab_free_hook(s, slab, &object, 1); >>>>>> + init = slab_want_init_on_free(s); >>>>>> - if (likely(slab_free_hook(s, object, >>>>>> slab_want_init_on_free(s)))) >>>>>> + if (likely(slab_free_hook(s, object, init))) { >>>>>> + if (init) >>>>>> + set_freepointer(s, object, NULL); >>>>>> do_slab_free(s, slab, object, object, 1, addr); >>>>>> + } >>>>>> } >>>>>> static __fastpath_inline > Thanks again for your review, > > Nicolas >
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