spi: Assign dummy scatterlist to unidirectional transfers

Commit 8cc3bad ("spi: Remove unneded check for orig_nents")
introduced a regression: unmapped data could now be passed to the DMA
APIs, resulting in null pointer dereferences. Commit 9f788ba ("spi:
Don't mark message DMA mapped when no transfer in it is") and commit
da56009 ("spi: Check if transfer is mapped before calling DMA sync
APIs") addressed the problem, but only partially. Unidirectional
transactions will still result in null pointer dereference. To prevent
that from happening, assign a dummy scatterlist when no data is mapped,
so that the DMA API can be called and not result in a null pointer
dereference.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reported-by: Neil Armstrong <neil.armstrong@linaro.org>
Closes: https://lore.kernel.org/r/8ae675b5-fcf9-4c9b-b06a-4462f70e1322@linaro.org
Reported-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Closes: https://lore.kernel.org/all/d3679496-2e4e-4a7c-97ed-f193bd53af1d@notapiano
Closes: https://lore.kernel.org/all/4748499f-789c-45a8-b50a-2dd09f4bac8c@notapiano
Fixes: 8cc3bad ("spi: Remove unneded check for orig_nents")
Tested-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
[nfraprado: wrote the commit message]
Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
Link: https://msgid.link/r/20240529-dma-oops-dummy-v1-1-bb43aacfb11b@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: andy-shev

drivers/spi/spi.c

@@ -1220,6 +1220,11 @@ void spi_unmap_buf(struct spi_controller *ctlr, struct device *dev,
 	spi_unmap_buf_attrs(ctlr, dev, sgt, dir, 0);
 }
 
+/* Dummy SG for unidirect transfers */
+static struct scatterlist dummy_sg = {
+	.page_link = SG_END,
+};
+
 static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
 {
 	struct device *tx_dev, *rx_dev;
@@ -1258,6 +1263,8 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
 						attrs);
 			if (ret != 0)
 				return ret;
+		} else {
+			xfer->tx_sg.sgl = &dummy_sg;
 		}
 
 		if (xfer->rx_buf != NULL) {
@@ -1271,6 +1278,8 @@ static int __spi_map_msg(struct spi_controller *ctlr, struct spi_message *msg)
 
 				return ret;
 			}
+		} else {
+			xfer->rx_sg.sgl = &dummy_sg;
 		}
 	}
 	/* No transfer has been mapped, bail out with success */