[PATCH v2] ARM: dma-mapping: always clear allocated buffers

[YueHaibing]

Sean Wang reported dma_zalloc_coherent doesn't work as expect on his
armv7,the allocated mem is not zeroed.The reason is __alloc_from_pool
doesn't honor __GFP_ZERO.

As commit 6829e274a623 ("arm64: dma-mapping: always clear allocated buffers")
has pointed out,buffers allocated by dma_alloc_coherent() are always zeroed
on most architectures. some drivers rely on this 'feature'. Allocated buffer
might be also exposed to userspace with dma_mmap() call,so clearing it is
desired from security point of view to avoid exposing random memory to userspace.

This patch unifies dma_alloc_coherent() behavior on ARM architecture with other
implementations by unconditionally zeroing allocated buffer.Also to fix
dma_zalloc_coherent behavior.

Reported-by: Sean Wang <sean.wang@xxxxxxxxxxxx>
Signed-off-by: YueHaibing <yuehaibing@xxxxxxxxxx>
Reviewed-by: zhong jiang <zhongjiang@xxxxxxxxxx>
---
v2: reference more argument from arm64 commit as Christoph suggested
---
 arch/arm/mm/dma-mapping.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 6656647..cf5882f 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -564,6 +564,7 @@ static void *__alloc_from_pool(size_t size, struct page **ret_page)
 
 		*ret_page = phys_to_page(phys);
 		ptr = (void *)val;
+		memset(ptr, 0, size);
 	}
 
 	return ptr;
-- 
2.7.0