spi: Fix per-page mapping of unaligned vmalloc-ed buffer

spi_map_buf() processes mapping of vmalloc-ed buffers in a special way,
making mapping of every page separately. However, if the buffer is not
aligned to page boundary (e.g. sub-array in a vmalloc-ed array), it
fills the scatter table with page-size unaligned pieces, that cross
page boundaries. This is incorrect and can, for example, cause memory
corruption and various crashes when working with ubifs on spi-nor chips
(though those drivers are themselves buggy in that they should be
providing DMAable memory to the SPI framework).

Fix this by using proper scatter table size and intra-page buffer lengths,
so that the whole buffer splits into separate scatter table entries on
page boundaries.
Signed-off-by: NAndrew Gabbasov <andrew_gabbasov@mentor.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -476,21 +476,30 @@ static int spi_map_buf(struct spi_master *master, struct device *dev,
 		       enum dma_data_direction dir)
 {
 	const bool vmalloced_buf = is_vmalloc_addr(buf);
-	const int desc_len = vmalloced_buf ? PAGE_SIZE : master->max_dma_len;
-	const int sgs = DIV_ROUND_UP(len, desc_len);
+	int desc_len;
+	int sgs;
 	struct page *vm_page;
 	void *sg_buf;
 	size_t min;
 	int i, ret;
 
+	if (vmalloced_buf) {
+		desc_len = PAGE_SIZE;
+		sgs = DIV_ROUND_UP(len + offset_in_page(buf), desc_len);
+	} else {
+		desc_len = master->max_dma_len;
+		sgs = DIV_ROUND_UP(len, desc_len);
+	}
+
 	ret = sg_alloc_table(sgt, sgs, GFP_KERNEL);
 	if (ret != 0)
 		return ret;
 
 	for (i = 0; i < sgs; i++) {
-		min = min_t(size_t, len, desc_len);
 
 		if (vmalloced_buf) {
+			min = min_t(size_t,
+				    len, desc_len - offset_in_page(buf));
 			vm_page = vmalloc_to_page(buf);
 			if (!vm_page) {
 				sg_free_table(sgt);
@@ -499,6 +508,7 @@ static int spi_map_buf(struct spi_master *master, struct device *dev,
 			sg_set_page(&sgt->sgl[i], vm_page,
 				    min, offset_in_page(buf));
 		} else {
+			min = min_t(size_t, len, desc_len);
 			sg_buf = buf;
 			sg_set_buf(&sgt->sgl[i], sg_buf, min);
 		}