Re: [kernel-hardening] [PATCH 09/38] usercopy: Mark kmalloc caches as usercopy caches

[Christian Borntraeger]

On 07.04.20 10:00, Vlastimil Babka wrote:
> On 1/31/20 1:03 PM, Jann Horn wrote:
> 
>> 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.
>  
> So, let's do that?
> 
> ----8<----
> From d5190e4e871689a530da3c3fd327be45a88f006a Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@xxxxxxx>
> 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@xxxxxxx/
> [3] https://lore.kernel.org/kernel-hardening/CAG48ez1a4waGk9kB0WLaSbs4muSoK0AYAVk8=XYaKj4_+6e6Hg@xxxxxxxxxxxxxx/
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>

> ---
>  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
>