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

From: Kees Cook <[email-masked]>
Date: Thu, 30 Jan 2020 11:23:38 -0800

On Wed, Jan 29, 2020 at 06:19:56PM +0100, Christian Borntraeger wrote:
> On 29.01.20 18:09, Christoph Hellwig wrote:
> > On Wed, Jan 29, 2020 at 06:07:14PM +0100, Christian Borntraeger wrote:
> >>> DMA can be done to NORMAL memory as well.
> >>
> >> Exactly. 
> >> I think iucv uses GFP_DMA because z/VM needs those buffers to reside below 2GB (which is ZONA_DMA for s390).
> > 
> > The normal way to allocate memory with addressing limits would be to
> > use dma_alloc_coherent and friends.  Any chance to switch iucv over to
> > that?  Or is there no device associated with it?
> 
> There is not necessarily a device for that. It is a hypervisor interface (an
> instruction that is interpreted by z/VM). We do have the netiucv driver that
> creates a virtual nic, but there is also AF_IUCV which works without a device.
> 
> But back to the original question: If we mark kmalloc caches as usercopy caches,
> we should do the same for DMA kmalloc caches. As outlined by Christoph, this has
> nothing to do with device DMA.

Hm, looks like it's allocated from the low 16MB. Seems like poor naming!
:) There seems to be a LOT of stuff using GFP_DMA, and it seems unlikely
those are all expecting low addresses?

Since this has only been a problem on s390, should just s390 gain the
weakening of the usercopy restriction?  Something like:

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1907cb2903c7..c5bbc141f20b 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1303,7 +1303,9 @@ 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,
+				IS_ENABLED(CONFIG_S390) ?
+					kmalloc_info[i].size : 0);
 		}
 	}
 #endif

-- 
Kees Cook