Merge branch 'atm-clip-fix-infinite-recursion-potential-null-ptr-dere…

…f-and-memleak'

Kuniyuki Iwashima says:

====================
atm: clip: Fix infinite recursion, potential null-ptr-deref, and memleak.

Patch 1 fixes racy access to atmarpd found while checking RTNL usage
in clip.c.

Patch 2 fixes memory leak by ioctl(ATMARP_MKIP) and ioctl(ATMARPD_CTRL).

Patch 3 fixes infinite recursive call of clip_vcc->old_push(), which
was reported by syzbot.

v1: https://lore.kernel.org/20250702020437.703698-1-kuniyu@google.com
====================

Link: https://patch.msgid.link/20250704062416.1613927-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/atm/clip.c

@@ -45,32 +45,44 @@
 #include <net/atmclip.h>
 
 static struct net_device *clip_devs;
-static struct atm_vcc *atmarpd;
+static struct atm_vcc __rcu *atmarpd;
+static DEFINE_MUTEX(atmarpd_lock);
 static struct timer_list idle_timer;
 static const struct neigh_ops clip_neigh_ops;
 
 static int to_atmarpd(enum atmarp_ctrl_type type, int itf, __be32 ip)
 {
 	struct sock *sk;
 	struct atmarp_ctrl *ctrl;
+	struct atm_vcc *vcc;
 	struct sk_buff *skb;
+	int err = 0;
 
 	pr_debug("(%d)\n", type);
-	if (!atmarpd)
-		return -EUNATCH;
+
+	rcu_read_lock();
+	vcc = rcu_dereference(atmarpd);
+	if (!vcc) {
+		err = -EUNATCH;
+		goto unlock;
+	}
 	skb = alloc_skb(sizeof(struct atmarp_ctrl), GFP_ATOMIC);
-	if (!skb)
-		return -ENOMEM;
+	if (!skb) {
+		err = -ENOMEM;
+		goto unlock;
+	}
 	ctrl = skb_put(skb, sizeof(struct atmarp_ctrl));
 	ctrl->type = type;
 	ctrl->itf_num = itf;
 	ctrl->ip = ip;
-	atm_force_charge(atmarpd, skb->truesize);
+	atm_force_charge(vcc, skb->truesize);
 
-	sk = sk_atm(atmarpd);
+	sk = sk_atm(vcc);
 	skb_queue_tail(&sk->sk_receive_queue, skb);
 	sk->sk_data_ready(sk);
-	return 0;
+unlock:
+	rcu_read_unlock();
+	return err;
 }
 
 static void link_vcc(struct clip_vcc *clip_vcc, struct atmarp_entry *entry)
@@ -417,6 +429,8 @@ static int clip_mkip(struct atm_vcc *vcc, int timeout)
 
 	if (!vcc->push)
 		return -EBADFD;
+	if (vcc->user_back)
+		return -EINVAL;
 	clip_vcc = kmalloc(sizeof(struct clip_vcc), GFP_KERNEL);
 	if (!clip_vcc)
 		return -ENOMEM;
@@ -607,10 +621,12 @@ static void atmarpd_close(struct atm_vcc *vcc)
 {
 	pr_debug("\n");
 
-	rtnl_lock();
-	atmarpd = NULL;
+	mutex_lock(&atmarpd_lock);
+	RCU_INIT_POINTER(atmarpd, NULL);
+	mutex_unlock(&atmarpd_lock);
+
+	synchronize_rcu();
 	skb_queue_purge(&sk_atm(vcc)->sk_receive_queue);
-	rtnl_unlock();
 
 	pr_debug("(done)\n");
 	module_put(THIS_MODULE);
@@ -631,15 +647,18 @@ static struct atm_dev atmarpd_dev = {
 
 static int atm_init_atmarp(struct atm_vcc *vcc)
 {
-	rtnl_lock();
+	if (vcc->push == clip_push)
+		return -EINVAL;
+
+	mutex_lock(&atmarpd_lock);
 	if (atmarpd) {
-		rtnl_unlock();
+		mutex_unlock(&atmarpd_lock);
 		return -EADDRINUSE;
 	}
 
 	mod_timer(&idle_timer, jiffies + CLIP_CHECK_INTERVAL * HZ);
 
-	atmarpd = vcc;
+	rcu_assign_pointer(atmarpd, vcc);
 	set_bit(ATM_VF_META, &vcc->flags);
 	set_bit(ATM_VF_READY, &vcc->flags);
 	    /* allow replies and avoid getting closed if signaling dies */
@@ -648,13 +667,14 @@ static int atm_init_atmarp(struct atm_vcc *vcc)
 	vcc->push = NULL;
 	vcc->pop = NULL; /* crash */
 	vcc->push_oam = NULL; /* crash */
-	rtnl_unlock();
+	mutex_unlock(&atmarpd_lock);
 	return 0;
 }
 
 static int clip_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 {
 	struct atm_vcc *vcc = ATM_SD(sock);
+	struct sock *sk = sock->sk;
 	int err = 0;
 
 	switch (cmd) {
@@ -675,14 +695,18 @@ static int clip_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 		err = clip_create(arg);
 		break;
 	case ATMARPD_CTRL:
+		lock_sock(sk);
 		err = atm_init_atmarp(vcc);
 		if (!err) {
 			sock->state = SS_CONNECTED;
 			__module_get(THIS_MODULE);
 		}
+		release_sock(sk);
 		break;
 	case ATMARP_MKIP:
+		lock_sock(sk);
 		err = clip_mkip(vcc, arg);
+		release_sock(sk);
 		break;
 	case ATMARP_SETENTRY:
 		err = clip_setentry(vcc, (__force __be32)arg);