dma-iommu: Check that swiotlb is active before trying to use it

If the IOMMU is in use and an untrusted device is connected to an external
facing port but the address requested isn't page aligned will cause the
kernel to attempt to use bounce buffers.

If for some reason the bounce buffers have not been allocated this is a
problem that should be made apparent to the user.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20220404204723.9767-3-mario.limonciello@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/dma-iommu.c

@@ -971,6 +971,11 @@ static dma_addr_t iommu_dma_map_page(struct device *dev, struct page *page,
 		void *padding_start;
 		size_t padding_size, aligned_size;
 
+		if (!is_swiotlb_active(dev)) {
+			dev_warn_once(dev, "DMA bounce buffers are inactive, unable to map unaligned transaction.\n");
+			return DMA_MAPPING_ERROR;
+		}
+
 		aligned_size = iova_align(iovad, size);
 		phys = swiotlb_tbl_map_single(dev, phys, size, aligned_size,
 					      iova_mask(iovad), dir, attrs);