netdevsim: psp: rcu protect psp_dev reference

There are two issues with the way psp_dev is used in nsim_do_psp():

1. There is no check for IS_ERR() on the peers psp_dev, before
   dereferencing.
2. The refcount on this psp_dev can be dropped by
   nsim_psp_rereg_write()

To fix this, we can make netdevsim's reference to its psp_dev an rcu
reference, and then nsim_do_psp() can read the fields it needs from an
rcu critical section.

Fixes: f857478 ("netdevsim: a basic test PSP implementation")
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260505-psd-rcu-v1-3-a8f69ec1ab96@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Daniel Zahka

drivers/net/netdevsim/netdevsim.h

@@ -120,7 +120,7 @@ struct netdevsim {
 		u64_stats_t tx_packets;
 		u64_stats_t tx_bytes;
 		struct u64_stats_sync syncp;
-		struct psp_dev *dev;
+		struct psp_dev __rcu *dev;
 		struct dentry *rereg;
 		struct mutex rereg_lock;
 		u32 spi;

drivers/net/netdevsim/psp.c

@@ -19,6 +19,7 @@ nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
 	    struct netdevsim *peer_ns, struct skb_ext **psp_ext)
 {
 	enum skb_drop_reason rc = 0;
+	struct psp_dev *peer_psd;
 	struct psp_assoc *pas;
 	struct net *net;
 	void **ptr;
@@ -48,7 +49,8 @@ nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
 	}
 
 	/* Now pretend we just received this frame */
-	if (peer_ns->psp.dev->config.versions & (1 << pas->version)) {
+	peer_psd = rcu_dereference(peer_ns->psp.dev);
+	if (peer_psd && peer_psd->config.versions & (1 << pas->version)) {
 		bool strip_icv = false;
 		u8 generation;
 
@@ -61,8 +63,7 @@ nsim_do_psp(struct sk_buff *skb, struct netdevsim *ns,
 
 		skb_ext_reset(skb);
 		skb->mac_len = ETH_HLEN;
-		if (psp_dev_rcv(skb, peer_ns->psp.dev->id, generation,
-				strip_icv)) {
+		if (psp_dev_rcv(skb, peer_psd->id, generation, strip_icv)) {
 			rc = SKB_DROP_REASON_PSP_OUTPUT;
 			goto out_unlock;
 		}
@@ -209,35 +210,50 @@ static struct psp_dev_caps nsim_psp_caps = {
 	.assoc_drv_spc = sizeof(void *),
 };
 
-static void __nsim_psp_uninit(struct netdevsim *ns)
+static void __nsim_psp_uninit(struct netdevsim *ns, bool teardown)
 {
-	if (!IS_ERR(ns->psp.dev))
-		psp_dev_unregister(ns->psp.dev);
+	struct psp_dev *psd;
+
+	psd = rcu_dereference_protected(ns->psp.dev,
+					teardown ||
+					lockdep_is_held(&ns->psp.rereg_lock));
+	if (psd) {
+		rcu_assign_pointer(ns->psp.dev, NULL);
+		synchronize_rcu();
+		psp_dev_unregister(psd);
+	}
 	WARN_ON(ns->psp.assoc_cnt);
 }
 
 void nsim_psp_uninit(struct netdevsim *ns)
 {
 	debugfs_remove(ns->psp.rereg);
 	mutex_destroy(&ns->psp.rereg_lock);
-	__nsim_psp_uninit(ns);
+	__nsim_psp_uninit(ns, true);
 }
 
 static ssize_t
 nsim_psp_rereg_write(struct file *file, const char __user *data, size_t count,
 		     loff_t *ppos)
 {
 	struct netdevsim *ns = file->private_data;
-	int err;
+	struct psp_dev *psd;
+	ssize_t ret;
 
 	mutex_lock(&ns->psp.rereg_lock);
-	__nsim_psp_uninit(ns);
+	__nsim_psp_uninit(ns, false);
+
+	psd = psp_dev_create(ns->netdev, &nsim_psp_ops, &nsim_psp_caps, ns);
+	if (IS_ERR(psd)) {
+		ret = PTR_ERR(psd);
+		goto out;
+	}
 
-	ns->psp.dev = psp_dev_create(ns->netdev, &nsim_psp_ops,
-				     &nsim_psp_caps, ns);
-	err = PTR_ERR_OR_ZERO(ns->psp.dev);
+	rcu_assign_pointer(ns->psp.dev, psd);
+	ret = count;
+out:
 	mutex_unlock(&ns->psp.rereg_lock);
-	return err ?: count;
+	return ret;
 }
 
 static const struct file_operations nsim_psp_rereg_fops = {
@@ -250,13 +266,13 @@ static const struct file_operations nsim_psp_rereg_fops = {
 int nsim_psp_init(struct netdevsim *ns)
 {
 	struct dentry *ddir = ns->nsim_dev_port->ddir;
-	int err;
+	struct psp_dev *psd;
+
+	psd = psp_dev_create(ns->netdev, &nsim_psp_ops, &nsim_psp_caps, ns);
+	if (IS_ERR(psd))
+		return PTR_ERR(psd);
 
-	ns->psp.dev = psp_dev_create(ns->netdev, &nsim_psp_ops,
-				     &nsim_psp_caps, ns);
-	err = PTR_ERR_OR_ZERO(ns->psp.dev);
-	if (err)
-		return err;
+	rcu_assign_pointer(ns->psp.dev, psd);
 
 	mutex_init(&ns->psp.rereg_lock);
 	ns->psp.rereg = debugfs_create_file("psp_rereg", 0200, ddir, ns,