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

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Message-ID: <4674f97f-5f97-45b7-a4b9-a19ca46b7ce1@lucifer.local>
Date:   Tue, 23 May 2023 09:19:30 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     David Hildenbrand <david@...hat.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 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.

>
>
> Reviewed-by: David Hildenbrand <david@...hat.com>
>
> --
> Thanks,
>
> David / dhildenb
>