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 <redacted>
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/ (local)
[3] https://lore.kernel.org/kernel-hardening/CAG48ez1a4waGk9kB0WLaSbs4muSoK0AYAVk8=XYaKj4_+6e6Hg@mail.gmail.com/ (local)

Signed-off-by: Vlastimil Babka <redacted>
---
 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

-- 
2.26.0