[PATCH v2 net 1/2] net: Fix dev_net(dev) race in unregister_netdevice_notifier_dev_net().

[Kuniyuki Iwashima <kuniyu@amazon.com>]

After the cited commit, dev_net(dev) is fetched before holding RTNL
and passed to __unregister_netdevice_notifier_net().

However, dev_net(dev) might be different after holding RTNL.

In the reported case [0], while removing a VF device, its netns was
being dismantled and the VF was moved to init_net.

So the following sequence is basically illegal when dev was fetched
without lookup:

  net = dev_net(dev);
  rtnl_net_lock(net);

Let's use a new helper rtnl_net_dev_lock() to fix the race.

It calls maybe_get_net() for dev_net_rcu(dev) and checks dev_net_rcu(dev)
before/after rtnl_net_lock().

The dev_net_rcu(dev) pointer itself is valid, thanks to RCU API, but the
netns might be being dismantled.  maybe_get_net() is to avoid the race.
This can be done by holding pernet_ops_rwsem, but it will be overkill.

[0]:
BUG: KASAN: slab-use-after-free in notifier_call_chain (kernel/notifier.c:75 (discriminator 2))
Read of size 8 at addr ffff88810cefb4c8 by task test-bridge-lag/21127
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl (lib/dump_stack.c:123)
 print_report (mm/kasan/report.c:379 mm/kasan/report.c:489)
 kasan_report (mm/kasan/report.c:604)
 notifier_call_chain (kernel/notifier.c:75 (discriminator 2))
 call_netdevice_notifiers_info (net/core/dev.c:2011)
 unregister_netdevice_many_notify (net/core/dev.c:11551)
 unregister_netdevice_queue (net/core/dev.c:11487)
 unregister_netdev (net/core/dev.c:11635)
 mlx5e_remove (drivers/net/ethernet/mellanox/mlx5/core/en_main.c:6552 drivers/net/ethernet/mellanox/mlx5/core/en_main.c:6579) mlx5_core
 auxiliary_bus_remove (drivers/base/auxiliary.c:230)
 device_release_driver_internal (drivers/base/dd.c:1275 drivers/base/dd.c:1296)
 bus_remove_device (./include/linux/kobject.h:193 drivers/base/base.h:73 drivers/base/bus.c:583)
 device_del (drivers/base/power/power.h:142 drivers/base/core.c:3855)
 mlx5_rescan_drivers_locked (./include/linux/auxiliary_bus.h:241 drivers/net/ethernet/mellanox/mlx5/core/dev.c:333 drivers/net/ethernet/mellanox/mlx5/core/dev.c:535 drivers/net/ethernet/mellanox/mlx5/core/dev.c:549) mlx5_core
 mlx5_unregister_device (drivers/net/ethernet/mellanox/mlx5/core/dev.c:468) mlx5_core
 mlx5_uninit_one (./include/linux/instrumented.h:68 ./include/asm-generic/bitops/instrumented-non-atomic.h:141 drivers/net/ethernet/mellanox/mlx5/core/main.c:1563) mlx5_core
 remove_one (drivers/net/ethernet/mellanox/mlx5/core/main.c:965 drivers/net/ethernet/mellanox/mlx5/core/main.c:2019) mlx5_core
 pci_device_remove (./include/linux/pm_runtime.h:129 drivers/pci/pci-driver.c:475)
 device_release_driver_internal (drivers/base/dd.c:1275 drivers/base/dd.c:1296)
 unbind_store (drivers/base/bus.c:245)
 kernfs_fop_write_iter (fs/kernfs/file.c:338)
 vfs_write (fs/read_write.c:587 (discriminator 1) fs/read_write.c:679 (discriminator 1))
 ksys_write (fs/read_write.c:732)
 do_syscall_64 (arch/x86/entry/common.c:52 (discriminator 1) arch/x86/entry/common.c:83 (discriminator 1))
 entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
RIP: 0033:0x7f6a4d5018b7

Fixes: 7fb1073300a2 ("net: Hold rtnl_net_lock() in (un)?register_netdevice_notifier_dev_net().")
Reported-by: Yael Chemla <ychemla@nvidia.com>
Closes: https://lore.kernel.org/netdev/146eabfe-123c-4970-901e-e961b4c09bc3@nvidia.com/
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Tested-by: Yael Chemla <ychemla@nvidia.com>
---
v2:
  * Use dev_net_rcu().
  * Use msleep(1) instead of cond_resched() after maybe_get_net()
  * Remove cond_resched() after net_eq() check

v1: https://lore.kernel.org/netdev/20250130232435.43622-2-kuniyu@amazon.com/
---
 net/core/dev.c | 63 +++++++++++++++++++++++++++++++++++++++-----------
 1 file changed, 50 insertions(+), 13 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index b91658e8aedb..f7430c9d9bc3 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2070,6 +2070,51 @@ static void __move_netdevice_notifier_net(struct net *src_net,
 	__register_netdevice_notifier_net(dst_net, nb, true);
 }
 
+static bool from_cleanup_net(void)
+{
+#ifdef CONFIG_NET_NS
+	return current == cleanup_net_task;
+#else
+	return false;
+#endif
+}
+
+static void rtnl_net_dev_lock(struct net_device *dev)
+{
+	struct net *net;
+
+	DEBUG_NET_WARN_ON_ONCE(from_cleanup_net());
+again:
+	/* netns might be being dismantled. */
+	rcu_read_lock();
+	net = maybe_get_net(dev_net_rcu(dev));
+	rcu_read_unlock();
+	if (!net) {
+		msleep(1);
+		goto again;
+	}
+
+	rtnl_net_lock(net);
+
+	/* dev might have been moved to another netns. */
+	rcu_read_lock();
+	if (!net_eq(net, dev_net_rcu(dev))) {
+		rcu_read_unlock();
+		rtnl_net_unlock(net);
+		put_net(net);
+		goto again;
+	}
+	rcu_read_unlock();
+}
+
+static void rtnl_net_dev_unlock(struct net_device *dev)
+{
+	struct net *net = dev_net(dev);
+
+	rtnl_net_unlock(net);
+	put_net(net);
+}
+
 int register_netdevice_notifier_dev_net(struct net_device *dev,
 					struct notifier_block *nb,
 					struct netdev_net_notifier *nn)
@@ -2077,6 +2122,8 @@ int register_netdevice_notifier_dev_net(struct net_device *dev,
 	struct net *net = dev_net(dev);
 	int err;
 
+	DEBUG_NET_WARN_ON_ONCE(!list_empty(&dev->dev_list));
+
 	rtnl_net_lock(net);
 	err = __register_netdevice_notifier_net(net, nb, false);
 	if (!err) {
@@ -2093,13 +2140,12 @@ int unregister_netdevice_notifier_dev_net(struct net_device *dev,
 					  struct notifier_block *nb,
 					  struct netdev_net_notifier *nn)
 {
-	struct net *net = dev_net(dev);
 	int err;
 
-	rtnl_net_lock(net);
+	rtnl_net_dev_lock(dev);
 	list_del(&nn->list);
-	err = __unregister_netdevice_notifier_net(net, nb);
-	rtnl_net_unlock(net);
+	err = __unregister_netdevice_notifier_net(dev_net(dev), nb);
+	rtnl_net_dev_unlock(dev);
 
 	return err;
 }
@@ -10255,15 +10301,6 @@ static void dev_index_release(struct net *net, int ifindex)
 	WARN_ON(xa_erase(&net->dev_by_index, ifindex));
 }
 
-static bool from_cleanup_net(void)
-{
-#ifdef CONFIG_NET_NS
-	return current == cleanup_net_task;
-#else
-	return false;
-#endif
-}
-
 /* Delayed registration/unregisteration */
 LIST_HEAD(net_todo_list);
 DECLARE_WAIT_QUEUE_HEAD(netdev_unregistering_wq);
-- 
2.39.5 (Apple Git-154)