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Date:   Mon, 20 Apr 2020 10:43:37 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Jiri Slaby <jslaby@suse.cz>
Cc:     Vlastimil Babka <vbabka@suse.cz>, Jann Horn <jannh@google.com>,
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Subject: Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches
 as usercopy caches
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On Mon, Apr 20, 2020 at 09:53:20AM +0200, Jiri Slaby wrote:
> On 07. 04. 20, 10:00, Vlastimil Babka wrote:
> > From d5190e4e871689a530da3c3fd327be45a88f006a Mon Sep 17 00:00:00 2001
> > From: Vlastimil Babka <vbabka@suse.cz>
> > Date: Tue, 7 Apr 2020 09:58:00 +0200
> > Subject: [PATCH] usercopy: Mark dma-kmalloc caches as usercopy caches
> > 
> > We have seen a "usercopy: Kernel memory overwrite attempt detected to SLUB
> > object 'dma-kmalloc-1 k' (offset 0, size 11)!" error on s390x, as IUCV uses
> > kmalloc() with __GFP_DMA because of memory address restrictions.
> > The issue has been discussed [2] and it has been noted that if all the kmalloc
> > caches are marked as usercopy, there's little reason not to mark dma-kmalloc
> > caches too. The 'dma' part merely means that __GFP_DMA is used to restrict
> > memory address range.
> > 
> > As Jann Horn put it [3]:
> > 
> > "I think dma-kmalloc slabs should be handled the same way as normal
> > kmalloc slabs. When a dma-kmalloc allocation is freshly created, it is
> > just normal kernel memory - even if it might later be used for DMA -,
> > and it should be perfectly fine to copy_from_user() into such
> > allocations at that point, and to copy_to_user() out of them at the
> > end. If you look at the places where such allocations are created, you
> > can see things like kmemdup(), memcpy() and so on - all normal
> > operations that shouldn't conceptually be different from usercopy in
> > any relevant way."
> > 
> > Thus this patch marks the dma-kmalloc-* caches as usercopy.
> > 
> > [1] https://bugzilla.suse.com/show_bug.cgi?id=1156053
> > [2] https://lore.kernel.org/kernel-hardening/bfca96db-bbd0-d958-7732-76e36c667c68@suse.cz/
> > [3] https://lore.kernel.org/kernel-hardening/CAG48ez1a4waGk9kB0WLaSbs4muSoK0AYAVk8=XYaKj4_+6e6Hg@mail.gmail.com/
> > 
> > Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> 
> Friendly ping.
> 
> Acked-by: Jiri Slaby <jslaby@suse.cz>

Should this go via -mm?

-Kees

> 
> > ---
> >  mm/slab_common.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index 5282f881d2f5..ae9486160594 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -1303,7 +1303,8 @@ void __init create_kmalloc_caches(slab_flags_t flags)
> >  			kmalloc_caches[KMALLOC_DMA][i] = create_kmalloc_cache(
> >  				kmalloc_info[i].name[KMALLOC_DMA],
> >  				kmalloc_info[i].size,
> > -				SLAB_CACHE_DMA | flags, 0, 0);
> > +				SLAB_CACHE_DMA | flags, 0,
> > +				kmalloc_info[i].size);
> >  		}
> >  	}
> >  #endif
> > 
> 
> thanks,
> -- 
> js
> suse labs

-- 
Kees Cook