[PATCH RFC] netdevsim: remove debugfs files before freeing net_device

A KASAN slab-use-after-free was detected in debugfs_u32_get() when reading
a debugfs file associated with a netdevsim port.

BUG: KASAN: slab-use-after-free in debugfs_u32_get+0x6c/0x70
Read of size 4 at addr ffff88810cd6db00

Call Trace:
 debugfs_u32_get+0x6c/0x70
 simple_attr_read+0x227/0x490
 debugfs_attr_read+0x76/0x130
 vfs_read+0x213/0xa80

The root cause is that the net_device (and its embedded netdevsim private
data) is freed before its associated debugfs files are removed. When a user
reads a debugfs file like rx_max_pending, debugfs_u32_get() dereferences a
pointer to the freed memory. This happens because nsim_destroy() calls
free_netdev() but does not remove the debugfs files created by
nsim_ethtool_init() or nsim_bpf_init(). These files are only removed later
when nsim_dev_port_debugfs_exit() is called. A similar issue exists in the
error path of nsim_create().

To fix this, swap the teardown order in __nsim_dev_port_del() and the
__nsim_dev_port_add() error path so that nsim_dev_port_debugfs_exit() is
called before nsim_destroy(). This ensures the debugfs directory is
recursively removed before the net_device is freed. Additionally, remove
explicit debugfs_remove() calls from various subsystem teardown functions
(like nsim_psp_uninit, nsim_ipsec_teardown, etc.) since the parent
directory is now removed earlier, which would otherwise lead to a
use-after-free of the dentry itself. Finally, ensure the debugfs directory
is removed before free_netdev() is called in the nsim_create() error path.

Fixes: e05b2d141fef ("netdevsim: move netdev creation/destruction to dev probe")
Assisted-by: Gemini:gemini-3.1-pro-preview Gemini:gemini-3-flash-preview syzbot
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=6c25f4750230faf70be9
Link: https://syzkaller.appspot.com/ai_job?id=84a7c962-d10e-424d-a37a-bf27d4830a40
To: "Andrew Lunn" <[email protected]>
To: "David S. Miller" <[email protected]>
To: "Eric Dumazet" <[email protected]>
To: "Jakub Kicinski" <[email protected]>
To: <[email protected]>
To: "Paolo Abeni" <[email protected]>
Cc: <[email protected]>

---
diff --git a/drivers/net/netdevsim/dev.c b/drivers/net/netdevsim/dev.c
index f00fc2f9e..beecc0c6e 100644
--- a/drivers/net/netdevsim/dev.c
+++ b/drivers/net/netdevsim/dev.c
@@ -1524,7 +1524,9 @@ static int __nsim_dev_port_add(struct nsim_dev *nsim_dev, enum nsim_dev_port_typ
 	return 0;
 
 err_nsim_destroy:
+	nsim_dev_port_debugfs_exit(nsim_dev_port);
 	nsim_destroy(nsim_dev_port->ns);
+	goto err_port_resource_unregister;
 err_port_debugfs_exit:
 	nsim_dev_port_debugfs_exit(nsim_dev_port);
 err_port_resource_unregister:
@@ -1544,8 +1546,8 @@ static void __nsim_dev_port_del(struct nsim_dev_port *nsim_dev_port)
 	list_del(&nsim_dev_port->list);
 	if (nsim_dev_port_is_vf(nsim_dev_port))
 		devl_rate_leaf_destroy(&nsim_dev_port->devlink_port);
-	nsim_destroy(nsim_dev_port->ns);
 	nsim_dev_port_debugfs_exit(nsim_dev_port);
+	nsim_destroy(nsim_dev_port->ns);
 	if (nsim_dev_port_is_pf(nsim_dev_port))
 		devl_port_resources_unregister(devlink_port);
 	devl_port_unregister(devlink_port);
diff --git a/drivers/net/netdevsim/ipsec.c b/drivers/net/netdevsim/ipsec.c
index 36a1be492..0bc9f42a5 100644
--- a/drivers/net/netdevsim/ipsec.c
+++ b/drivers/net/netdevsim/ipsec.c
@@ -292,5 +292,4 @@ void nsim_ipsec_teardown(struct netdevsim *ns)
 	if (ipsec->count)
 		netdev_err(ns->netdev, "tearing down IPsec offload with %d SAs left\n",
 			   ipsec->count);
-	debugfs_remove_recursive(ipsec->pfile);
 }
diff --git a/drivers/net/netdevsim/netdev.c b/drivers/net/netdevsim/netdev.c
index a75076891..6918679ca 100644
--- a/drivers/net/netdevsim/netdev.c
+++ b/drivers/net/netdevsim/netdev.c
@@ -1164,6 +1164,8 @@ struct netdevsim *nsim_create(struct nsim_dev *nsim_dev,
 	return ns;
 
 err_free_netdev:
+	debugfs_remove_recursive(nsim_dev_port->ddir);
+	nsim_dev_port->ddir = NULL;
 	free_netdev(dev);
 	return ERR_PTR(err);
 }
@@ -1174,10 +1176,6 @@ void nsim_destroy(struct netdevsim *ns)
 	struct netdevsim *peer;
 	u16 vid;
 
-	debugfs_remove(ns->vlan_dfs);
-	debugfs_remove(ns->qr_dfs);
-	debugfs_remove(ns->pp_dfs);
-
 	if (ns->nb.notifier_call)
 		unregister_netdevice_notifier_dev_net(ns->netdev, &ns->nb,
 						      &ns->nn);
diff --git a/drivers/net/netdevsim/psp.c b/drivers/net/netdevsim/psp.c
index 6936ecb81..2f27a2114 100644
--- a/drivers/net/netdevsim/psp.c
+++ b/drivers/net/netdevsim/psp.c
@@ -227,7 +227,6 @@ static void __nsim_psp_uninit(struct netdevsim *ns, bool teardown)
 
 void nsim_psp_uninit(struct netdevsim *ns)
 {
-	debugfs_remove(ns->psp.rereg);
 	mutex_destroy(&ns->psp.rereg_lock);
 	__nsim_psp_uninit(ns, true);
 }
diff --git a/drivers/net/netdevsim/udp_tunnels.c b/drivers/net/netdevsim/udp_tunnels.c
index 89fff76e5..a69b15f12 100644
--- a/drivers/net/netdevsim/udp_tunnels.c
+++ b/drivers/net/netdevsim/udp_tunnels.c
@@ -188,9 +188,6 @@ int nsim_udp_tunnels_info_create(struct nsim_dev *nsim_dev,
 
 void nsim_udp_tunnels_info_destroy(struct net_device *dev)
 {
-	struct netdevsim *ns = netdev_priv(dev);
-
-	debugfs_remove_recursive(ns->udp_ports.ddir);
 	kfree(dev->udp_tunnel_nic_info);
 	dev->udp_tunnel_nic_info = NULL;
 }


base-commit: 8cd9520d35a6c38db6567e97dd93b1f11f185dc6
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