Merge branch 'net-fix-issues-around-register_netdevice-failures'

Jakub Kicinski says:

====================
net: fix issues around register_netdevice() failures

This series attempts to clean up the life cycle of struct
net_device. Dave has added dev->needs_free_netdev in the
past to fix double frees, we can lean on that mechanism
a little more to fix remaining issues with register_netdevice().

This is the next chapter of the saga which already includes:
commit 0e0eee2465df ("net: correct error path in rtnl_newlink()")
commit e51fb152318e ("rtnetlink: fix a memory leak when ->newlink fails")
commit cf124db566e6 ("net: Fix inconsistent teardown and release of private netdev state.")
commit 93ee31f14f6f ("[NET]: Fix free_netdev on register_netdev failure.")
commit 814152a89ed5 ("net: fix memleak in register_netdevice()")
commit 10cc514f451a ("net: Fix null de-reference of device refcount")

The immediate problem which gets fixed here is that calling
free_netdev() right after unregister_netdevice() is illegal
because we need to release rtnl_lock first, to let the
unregistration finish. Note that unregister_netdevice() is
just a wrapper of unregister_netdevice_queue(), it only
does half of the job.

Where this limitation becomes most problematic is in failure
modes of register_netdevice(). There is a notifier call right
at the end of it, which lets other subsystems veto the entire
thing. At which point we should really go through a full
unregister_netdevice(), but we can't because callers may
go straight to free_netdev() after the failure, and that's
no bueno (see the previous paragraph).

This set makes free_netdev() more lenient, when device
is still being unregistered free_netdev() will simply set
dev->needs_free_netdev and let the unregister process do
the freeing.

With the free_netdev() problem out of the way failures in
register_netdevice() can make use of net_todo, again.
Users are still expected to call free_netdev() right after
failure but that will only set dev->needs_free_netdev.

To prevent the pathological case of:

 dev->needs_free_netdev = true;
 if (register_netdevice(dev)) {
   rtnl_unlock();
   free_netdev(dev);
 }

make register_netdevice()'s failure clear dev->needs_free_netdev.

Problems described above are only present with register_netdevice() /
unregister_netdevice(). We have two parallel APIs for registration
of devices:
 - those called outside rtnl_lock (register_netdev(), and
   unregister_netdev());
 - and those to be used under rtnl_lock - register_netdevice()
   and unregister_netdevice().
The former is trivial and has no problems. The alternative
approach to fix the latter would be to also separate the
freeing functions - i.e. add free_netdevice(). This has been
implemented (incl. converting all relevant calls in the tree)
but it feels a little unnecessary to put the burden of choosing
the right free_netdev{,ice}() call on the programmer when we
can "just do the right thing" by default.
====================

Link: https://lore.kernel.org/r/20210106184007.1821480-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

3 files changed, 22 insertions, 30 deletions

diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index 15bbfaf943fd..8b644113715e 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -284,9 +284,7 @@ static int register_vlan_device(struct net_device *real_dev, u16 vlan_id)
 	return 0;
 
 out_free_newdev:
-	if (new_dev->reg_state == NETREG_UNINITIALIZED ||
-	    new_dev->reg_state == NETREG_UNREGISTERED)
-		free_netdev(new_dev);
+	free_netdev(new_dev);
 	return err;
 }
 
diff --git a/net/core/dev.c b/net/core/dev.c
index 8fa739259041..0071a11a6dc3 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -10077,17 +10077,11 @@ int register_netdevice(struct net_device *dev)
 	ret = call_netdevice_notifiers(NETDEV_REGISTER, dev);
 	ret = notifier_to_errno(ret);
 	if (ret) {
+		/* Expect explicit free_netdev() on failure */
+		dev->needs_free_netdev = false;
 		rollback_registered(dev);
-		rcu_barrier();
-
-		dev->reg_state = NETREG_UNREGISTERED;
-		/* We should put the kobject that hold in
-		 * netdev_unregister_kobject(), otherwise
-		 * the net device cannot be freed when
-		 * driver calls free_netdev(), because the
-		 * kobject is being hold.
-		 */
-		kobject_put(&dev->dev.kobj);
+		net_set_todo(dev);
+		goto out;
 	}
 	/*
 	 *	Prevent userspace races by waiting until the network
@@ -10631,6 +10625,17 @@ void free_netdev(struct net_device *dev)
 	struct napi_struct *p, *n;
 
 	might_sleep();
+
+	/* When called immediately after register_netdevice() failed the unwind
+	 * handling may still be dismantling the device. Handle that case by
+	 * deferring the free.
+	 */
+	if (dev->reg_state == NETREG_UNREGISTERING) {
+		ASSERT_RTNL();
+		dev->needs_free_netdev = true;
+		return;
+	}
+
 	netif_free_tx_queues(dev);
 	netif_free_rx_queues(dev);
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index bb0596c41b3e..3d6ab194d0f5 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -3439,26 +3439,15 @@ replay:
 
 	dev->ifindex = ifm->ifi_index;
 
-	if (ops->newlink) {
+	if (ops->newlink)
 		err = ops->newlink(link_net ? : net, dev, tb, data, extack);
-		/* Drivers should call free_netdev() in ->destructor
-		 * and unregister it on failure after registration
-		 * so that device could be finally freed in rtnl_unlock.
-		 */
-		if (err < 0) {
-			/* If device is not registered at all, free it now */
-			if (dev->reg_state == NETREG_UNINITIALIZED ||
-			    dev->reg_state == NETREG_UNREGISTERED)
-				free_netdev(dev);
-			goto out;
-		}
-	} else {
+	else
 		err = register_netdevice(dev);
-		if (err < 0) {
-			free_netdev(dev);
-			goto out;
-		}
+	if (err < 0) {
+		free_netdev(dev);
+		goto out;
 	}
+
 	err = rtnl_configure_link(dev, ifm);
 	if (err < 0)
 		goto out_unregister;