[PATCH] usb: dwc3: dwc3-qcom: Avoid use-after-free when USB defers or unbinds

[Douglas Anderson]

On sc7180-trogdor class devices with 'fw_devlink=permissive' and KASAN
enabled, you'll see a Use-After-Free reported at bootup.

The root of the problem is that dwc3_qcom_of_register_core() is adding
a devm-allocated "tx-fifo-resize" property to its device tree node
using of_add_property().

The issue is that of_add_property() makes a _permanent_ addition to
the device tree that lasts until reboot. That means allocating memory
for the property using "devm" managed memory is a terrible idea since
that memory will be freed upon probe deferral or device
unbinding. Let's change to just allocate memory once and never free
it. This sorta looks like a leak but isn't truly one, since only one
property will be allocated per device tree node per boot.

NOTE: one would think that perhaps it would be better to use
of_remove_property() and then be able to free the property on device
remove. That sounds good until you read the comments for
of_remove_property(), which says that properties are never really
removed and they're just moved to the side.

Fixes: cefdd52fa045 ("usb: dwc3: dwc3-qcom: Enable tx-fifo-resize property by default")
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
---

 drivers/usb/dwc3/dwc3-qcom.c | 32 +++++++++++++++++++++-----------
 1 file changed, 21 insertions(+), 11 deletions(-)

diff --git a/drivers/usb/dwc3/dwc3-qcom.c b/drivers/usb/dwc3/dwc3-qcom.c
index 9abbd01028c5..34b054033116 100644
--- a/drivers/usb/dwc3/dwc3-qcom.c
+++ b/drivers/usb/dwc3/dwc3-qcom.c
@@ -658,18 +658,28 @@ static int dwc3_qcom_of_register_core(struct platform_device *pdev)
 		return -ENODEV;
 	}
 
-	prop = devm_kzalloc(dev, sizeof(*prop), GFP_KERNEL);
-	if (!prop) {
-		ret = -ENOMEM;
-		dev_err(dev, "unable to allocate memory for property\n");
-		goto node_put;
-	}
+	/*
+	 * Permanently add the "tx-fifo-resize" to the device tree. Even if
+	 * our device is unregistered this property will still be part
+	 * of the device tree until reboot. Because this is a "permanent"
+	 * change, we allocate memory _without_ devm. For some context, see
+	 * the fact that of_remove_property() doesn't actually remove things.
+	 */
+	if (!of_find_property(dwc3_np, "tx-fifo-resize", NULL)) {
+		prop = kzalloc(sizeof(*prop), GFP_KERNEL);
+		if (!prop) {
+			ret = -ENOMEM;
+			dev_err(dev, "unable to allocate memory for property\n");
+			goto node_put;
+		}
 
-	prop->name = "tx-fifo-resize";
-	ret = of_add_property(dwc3_np, prop);
-	if (ret) {
-		dev_err(dev, "unable to add property\n");
-		goto node_put;
+		prop->name = "tx-fifo-resize";
+		ret = of_add_property(dwc3_np, prop);
+		if (ret) {
+			dev_err(dev, "unable to add property\n");
+			kfree(prop);
+			goto node_put;
+		}
 	}
 
 	ret = of_platform_populate(np, NULL, NULL, dev);
-- 
2.34.1.400.ga245620fadb-goog