Messages in this thread | | | Date | Mon, 29 Apr 2024 11:09:51 +0200 | Subject | Re: [PATCH] slub: Fixes freepointer encoding for single free | From | Nicolas Bouchinet <> |
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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.
> >>> 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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