dmaengine: imx-sdma: fix use-after-free on probe error path

If probe() fails anywhere beyond the point where
sdma_get_firmware() is called, then a kernel oops may occur.

Problematic sequence of events:
1. probe() calls sdma_get_firmware(), which schedules the
   firmware callback to run when firmware becomes available,
   using the sdma instance structure as the context
2. probe() encounters an error, which deallocates the
   sdma instance structure
3. firmware becomes available, firmware callback is
   called with deallocated sdma instance structure
4. use after free - kernel oops !

Solution: only attempt to load firmware when we're certain
that probe() will succeed. This guarantees that the firmware
callback's context will remain valid.

Note that the remove() path is unaffected by this issue: the
firmware loader will increment the driver module's use count,
ensuring that the module cannot be unloaded while the
firmware callback is pending or running.
Signed-off-by: Sven Van Asbroeck <TheSven73@gmail.com>
Reviewed-by: Robin Gong <yibin.gong@nxp.com>
[vkoul: fixed braces for if condition]
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/imx-sdma.c

@@ -2096,27 +2096,6 @@ static int sdma_probe(struct platform_device *pdev)
 	if (pdata && pdata->script_addrs)
 		sdma_add_scripts(sdma, pdata->script_addrs);
 
-	if (pdata) {
-		ret = sdma_get_firmware(sdma, pdata->fw_name);
-		if (ret)
-			dev_warn(&pdev->dev, "failed to get firmware from platform data\n");
-	} else {
-		/*
-		 * Because that device tree does not encode ROM script address,
-		 * the RAM script in firmware is mandatory for device tree
-		 * probe, otherwise it fails.
-		 */
-		ret = of_property_read_string(np, "fsl,sdma-ram-script-name",
-					      &fw_name);
-		if (ret)
-			dev_warn(&pdev->dev, "failed to get firmware name\n");
-		else {
-			ret = sdma_get_firmware(sdma, fw_name);
-			if (ret)
-				dev_warn(&pdev->dev, "failed to get firmware from device tree\n");
-		}
-	}
-
 	sdma->dma_device.dev = &pdev->dev;
 
 	sdma->dma_device.device_alloc_chan_resources = sdma_alloc_chan_resources;
@@ -2161,6 +2140,33 @@ static int sdma_probe(struct platform_device *pdev)
 		of_node_put(spba_bus);
 	}
 
+	/*
+	 * Kick off firmware loading as the very last step:
+	 * attempt to load firmware only if we're not on the error path, because
+	 * the firmware callback requires a fully functional and allocated sdma
+	 * instance.
+	 */
+	if (pdata) {
+		ret = sdma_get_firmware(sdma, pdata->fw_name);
+		if (ret)
+			dev_warn(&pdev->dev, "failed to get firmware from platform data\n");
+	} else {
+		/*
+		 * Because that device tree does not encode ROM script address,
+		 * the RAM script in firmware is mandatory for device tree
+		 * probe, otherwise it fails.
+		 */
+		ret = of_property_read_string(np, "fsl,sdma-ram-script-name",
+					      &fw_name);
+		if (ret) {
+			dev_warn(&pdev->dev, "failed to get firmware name\n");
+		} else {
+			ret = sdma_get_firmware(sdma, fw_name);
+			if (ret)
+				dev_warn(&pdev->dev, "failed to get firmware from device tree\n");
+		}
+	}
+
 	return 0;
 
 err_register: