[PATCH] ARM: dma-mapping: Fix dma_get_sgtable() for regions without struct page

[linux@armlinux.org.uk (Russell King - ARM Linux)]

Okay, I'm about to merge the following patch for -rc, since refusing
to create a scattertable for non-page backed memory is the only valid
solution for that case.  I'm intending to queue this for -rc this
evening.

8<====
ARM: dma-mapping: disallow dma_get_sgtable() for non-kernel managed memory

dma_get_sgtable() tries to create a scatterlist table containing valid
struct page pointers for the coherent memory allocation passed in to it.

However, memory can be declared via dma_declare_coherent_memory(), or
via other reservation schemes which means that coherent memory is not
guaranteed to be backed by struct pages.  This means it is not possible
to create a valid scatterlist via this mechanism.

This patch adds detection of such memory, and refuses to create a
scatterlist table for such memory.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
---
 arch/arm/mm/dma-mapping.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
index 63eabb06f9f1..475811f5383a 100644
--- a/arch/arm/mm/dma-mapping.c
+++ b/arch/arm/mm/dma-mapping.c
@@ -935,13 +935,31 @@ static void arm_coherent_dma_free(struct device *dev, size_t size, void *cpu_add
 	__arm_dma_free(dev, size, cpu_addr, handle, attrs, true);
 }
 
+/*
+ * The whole dma_get_sgtable() idea is fundamentally unsafe - it seems
+ * that the intention is to allow exporting memory allocated via the
+ * coherent DMA APIs through the dma_buf API, which only accepts a
+ * scattertable.  This presents a couple of problems:
+ * 1. Not all memory allocated via the coherent DMA APIs is backed by
+ *    a struct page
+ * 2. Passing coherent DMA memory into the streaming APIs is not allowed
+ *    as we will try to flush the memory through a different alias to that
+ *    actually being used (and the flushes are redundant.)
+ */
 int arm_dma_get_sgtable(struct device *dev, struct sg_table *sgt,
 		 void *cpu_addr, dma_addr_t handle, size_t size,
 		 unsigned long attrs)
 {
-	struct page *page = pfn_to_page(dma_to_pfn(dev, handle));
+	unsigned long pfn = dma_to_pfn(dev, handle);
+	struct page *page;
 	int ret;
 
+	/* If the PFN is not valid, we do not have a struct page */
+	if (!pfn_valid(pfn))
+		return -ENXIO;
+
+	page = pfn_to_page(pfn);
+
 	ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
 	if (unlikely(ret))
 		return ret;
-- 
2.7.4

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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