[PATCH 3/3] driver core: Avoid adding children below a dead parent

[Lukas Wunner <lukas@xxxxxxxxx>]

If CONFIG_OF_DYNAMIC or CONFIG_ACPI is enabled, SPI devices may be added
below a controller at runtime by a DeviceTree overlay or DSDT patch.
But there are no precautions to prevent adding a device below a
controller that's being removed.

This seems like something that should be guarded against in the driver
core because it's not specific to SPI:  Adding a child below a parent
that's going away seems like a bad idea regardless of the bus type.

Take advantage of kill_device() which was added by commit 00289cd87676
("drivers/base: Introduce kill_device()"), call it upon removal of an
SPI controller and teach the driver core to refuse device addition below
a killed parent.  To make this race-free, device_add() needs to take the
parent's dead_sem before checking its "dead" flag and until the child
device has been added to the parent's klist_children.

Signed-off-by: Lukas Wunner <lukas@xxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
Cc: Pantelis Antoniou <pantelis.antoniou@xxxxxxxxxxxx>
Cc: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx>
---
 drivers/base/core.c | 18 ++++++++++++++++--
 drivers/spi/spi.c   |  3 +++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 057da42b1a660..1d4e39696f996 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -2597,6 +2597,14 @@ int device_add(struct device *dev)
 	pr_debug("device: '%s': %s\n", dev_name(dev), __func__);
 
 	parent = get_device(dev->parent);
+	if (parent) {
+		down_read(&parent->p->dead_sem);
+		if (parent->p->dead) {
+			error = -ENODEV;
+			goto parent_error;
+		}
+	}
+
 	kobj = get_device_parent(dev, parent);
 	if (IS_ERR(kobj)) {
 		error = PTR_ERR(kobj);
@@ -2679,9 +2687,11 @@ int device_add(struct device *dev)
 	}
 
 	bus_probe_device(dev);
-	if (parent)
+	if (parent) {
 		klist_add_tail(&dev->p->knode_parent,
 			       &parent->p->klist_children);
+		up_read(&parent->p->dead_sem);
+	}
 
 	if (dev->class) {
 		mutex_lock(&dev->class->p->mutex);
@@ -2722,6 +2732,8 @@ int device_add(struct device *dev)
  Error:
 	cleanup_glue_dir(dev, glue_dir);
 parent_error:
+	if (parent)
+		up_read(&parent->p->dead_sem);
 	put_device(parent);
 name_error:
 	kfree(dev->p);
@@ -2785,7 +2797,9 @@ EXPORT_SYMBOL_GPL(put_device);
  * kill_device - declare device dead
  * @dev: device in question
  *
- * Declare @dev dead to prevent it from binding to a driver.
+ * Declare @dev dead to prevent it from binding to a driver and
+ * to prevent addition of children.
+ *
  * Return true if it was killed or false if it was already dead.
  */
 bool kill_device(struct device *dev)
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 8158e281f3540..005eca4bae089 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -2764,6 +2764,9 @@ void spi_unregister_controller(struct spi_controller *ctlr)
 	struct spi_controller *found;
 	int id = ctlr->bus_num;
 
+	/* Prevent addition of new children, then remove existing ones */
+	if (IS_ENABLED(CONFIG_OF_DYNAMIC) || IS_ENABLED(CONFIG_ACPI))
+		kill_device(&ctlr->dev);
 	device_for_each_child(&ctlr->dev, NULL, __unregister);
 
 	/* First make sure that this controller was ever added */
-- 
2.27.0