[PATCH net 3/3] netdevsim: psp: rcu protect psp_dev reference

Daniel Zahka
2026-05-05T10:42:32Z

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: f857478d6206 ("netdevsim: a basic test PSP implementation")
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
---
drivers/net/netdevsim/netdevsim.h | 2 +-
drivers/net/netdevsim/psp.c | 54 +++++++++++++++++++++++++--------------
2 files changed, 36 insertions(+), 20 deletions(-)

diff --git a/drivers/net/netdevsim/netdevsim.h b/drivers/net/netdevsim/netdevsim.h
index e373ffc26b0c..d909c4160ea1 100644
--- a/drivers/net/netdevsim/netdevsim.h
+++ b/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;
diff --git a/drivers/net/netdevsim/psp.c b/drivers/net/netdevsim/psp.c
index 86d84b7e566b..6936ecb8173e 100644
--- a/drivers/net/netdevsim/psp.c
+++ b/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;
}
@@ -209,10 +210,18 @@ 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);
}

@@ -220,7 +229,7 @@ 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
@@ -228,16 +237,23 @@ 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,

--
2.52.0