dev_addr: add a modification check

netdev->dev_addr should only be modified via helpers,
but someone may be casting off the const. Add a runtime
check to catch abuses.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -1942,6 +1942,8 @@ enum netdev_ml_priv_type {
  *	@unlink_list:	As netif_addr_lock() can be called recursively,
  *			keep a list of interfaces to be deleted.
  *
+ *	@dev_addr_shadow:	Copy of @dev_addr to catch direct writes.
+ *
  *	FIXME: cleanup struct net_device such that network protocol info
  *	moves out.
  */
@@ -2268,6 +2270,8 @@ struct net_device {
 
 	/* protected by rtnl_lock */
 	struct bpf_xdp_entity	xdp_state[__MAX_XDP_MODE];
+
+	u8 dev_addr_shadow[MAX_ADDR_LEN];
 };
 #define to_net_dev(d) container_of(d, struct net_device, dev)
 
@@ -4288,6 +4292,7 @@ int dev_addr_del(struct net_device *dev, const unsigned char *addr,
 		 unsigned char addr_type);
 void dev_addr_flush(struct net_device *dev);
 int dev_addr_init(struct net_device *dev);
+void dev_addr_check(struct net_device *dev);
 
 /* Functions used for unicast addresses handling */
 int dev_uc_add(struct net_device *dev, const unsigned char *addr);

net/core/dev.c

@@ -1377,6 +1377,7 @@ static int __dev_open(struct net_device *dev, struct netlink_ext_ack *extack)
 	int ret;
 
 	ASSERT_RTNL();
+	dev_addr_check(dev);
 
 	if (!netif_device_present(dev)) {
 		/* may be detached because parent is runtime-suspended */

net/core/dev_addr_lists.c

@@ -498,6 +498,21 @@ EXPORT_SYMBOL(__hw_addr_init);
  * Device addresses handling functions
  */
 
+/* Check that netdev->dev_addr is not written to directly as this would
+ * break the rbtree layout. All changes should go thru dev_addr_set() and co.
+ * Remove this check in mid-2024.
+ */
+void dev_addr_check(struct net_device *dev)
+{
+	if (!memcmp(dev->dev_addr, dev->dev_addr_shadow, MAX_ADDR_LEN))
+		return;
+
+	netdev_warn(dev, "Current addr:  %*ph\n", MAX_ADDR_LEN, dev->dev_addr);
+	netdev_warn(dev, "Expected addr: %*ph\n",
+		    MAX_ADDR_LEN, dev->dev_addr_shadow);
+	netdev_WARN(dev, "Incorrect netdev->dev_addr\n");
+}
+
 /**
  *	dev_addr_flush - Flush device address list
  *	@dev: device
@@ -509,6 +524,7 @@ EXPORT_SYMBOL(__hw_addr_init);
 void dev_addr_flush(struct net_device *dev)
 {
 	/* rtnl_mutex must be held here */
+	dev_addr_check(dev);
 
 	__hw_addr_flush(&dev->dev_addrs);
 	dev->dev_addr = NULL;
@@ -552,8 +568,11 @@ void dev_addr_mod(struct net_device *dev, unsigned int offset,
 {
 	struct netdev_hw_addr *ha;
 
+	dev_addr_check(dev);
+
 	ha = container_of(dev->dev_addr, struct netdev_hw_addr, addr[0]);
 	memcpy(&ha->addr[offset], addr, len);
+	memcpy(&dev->dev_addr_shadow[offset], addr, len);
 }
 EXPORT_SYMBOL(dev_addr_mod);