linux-hardening - Re: [PATCH] mm/slab: remove HAVE_HARDENED_USERCOPY

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Date:   Tue, 23 May 2023 10:28:32 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Hyeonggon Yoo <42.hyeyoo@...il.com>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org, patches@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/slab: remove HAVE_HARDENED_USERCOPY_ALLOCATOR

On 23.05.23 10:19, Lorenzo Stoakes wrote:
> On Tue, May 23, 2023 at 10:14:24AM +0200, David Hildenbrand wrote:
>> On 23.05.23 09:56, Lorenzo Stoakes wrote:
>>> On Tue, May 23, 2023 at 09:46:46AM +0200, Vlastimil Babka wrote:
>>>> On 5/23/23 09:42, Lorenzo Stoakes wrote:
>>>>> On Tue, May 23, 2023 at 09:31:36AM +0200, Vlastimil Babka wrote:
>>>>>> With SLOB removed, both remaining allocators support hardened usercopy,
>>>>>> so remove the config and associated #ifdef.
>>>>>>
>>>>>> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
>>>>>> ---
>>>>>>    mm/Kconfig       | 2 --
>>>>>>    mm/slab.h        | 9 ---------
>>>>>>    security/Kconfig | 8 --------
>>>>>>    3 files changed, 19 deletions(-)
>>>>>>
>>>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>>>> index 7672a22647b4..041f0da42f2b 100644
>>>>>> --- a/mm/Kconfig
>>>>>> +++ b/mm/Kconfig
>>>>>> @@ -221,7 +221,6 @@ choice
>>>>>>    config SLAB
>>>>>>    	bool "SLAB"
>>>>>>    	depends on !PREEMPT_RT
>>>>>> -	select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>>>>    	help
>>>>>>    	  The regular slab allocator that is established and known to work
>>>>>>    	  well in all environments. It organizes cache hot objects in
>>>>>> @@ -229,7 +228,6 @@ config SLAB
>>>>>>
>>>>>>    config SLUB
>>>>>>    	bool "SLUB (Unqueued Allocator)"
>>>>>> -	select HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>>>>    	help
>>>>>>    	   SLUB is a slab allocator that minimizes cache line usage
>>>>>>    	   instead of managing queues of cached objects (SLAB approach).
>>>>>> diff --git a/mm/slab.h b/mm/slab.h
>>>>>> index f01ac256a8f5..695ef96b4b5b 100644
>>>>>> --- a/mm/slab.h
>>>>>> +++ b/mm/slab.h
>>>>>> @@ -832,17 +832,8 @@ struct kmem_obj_info {
>>>>>>    void __kmem_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *slab);
>>>>>>    #endif
>>>>>>
>>>>>> -#ifdef CONFIG_HAVE_HARDENED_USERCOPY_ALLOCATOR
>>>>>>    void __check_heap_object(const void *ptr, unsigned long n,
>>>>>>    			 const struct slab *slab, bool to_user);
>>>>>> -#else
>>>>>> -static inline
>>>>>> -void __check_heap_object(const void *ptr, unsigned long n,
>>>>>> -			 const struct slab *slab, bool to_user)
>>>>>> -{
>>>>>> -}
>>>>>> -#endif
>>>>>
>>>>> Hm, this is still defined in slab.c/slub.c and invoked in usercopy.c, do we
>>>>> not want the prototype?
>>>>
>>>> Well I didn't delete the prototype, just the ifdef/else around, so now it's
>>>> there unconditionally.
>>>>
>>>>> Perhaps replacing with #ifdef
>>>>> CONFIG_HARDENED_USERCOPY instead? I may be missing something here :)
>>>>
>>>> Putting it under that #ifdef would work and match that the implementations
>>>> of that function are under that same ifdef, but maybe it's unnecessary noise
>>>> in the header?
>>>>
>>>
>>> Yeah my brain inserted extra '-'s there, sorry!
>>>
>>> Given we only define __check_heap_object() in sl[au]b.c if
>>> CONFIG_HARDENED_USERCOPY wouldn't we need to keep the empty version around
>>> if !CONFIG_HARDENED_USERCOPY since check_heap_object() appears to be called
>>> unconditionally?
>>>
>>
>> The file is only compiled with CONFIG_HARDENED_USERCOPY:
>>
>> mm/Makefile:obj-$(CONFIG_HARDENED_USERCOPY) += usercopy.o
>>
> 
> Yeah ugh at this sort of implicit thing. Anyway it'd be preferable to stick
> #ifdef CONFIG_HARDENED_USERCOPY around the prototype just so it's
> abundantly clear this function doesn't exist unless that is set.

I recall that it is very common to not use ifdefs unless really 
required. Because less ifefs are obviously preferable ;)

Compilation+linking will fail in any case.

-- 
Thanks,

David / dhildenb