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

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Message-ID: <ae3ff438-5e5a-4a75-b4e9-575324a584f7@lucifer.local>
Date:   Tue, 23 May 2023 08:56:38 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     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 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?