atm: clip: Fix potential null-ptr-deref in to_atmarpd().

atmarpd is protected by RTNL since commit f3a0592 ("[ATM]: clip
causes unregister hang").

However, it is not enough because to_atmarpd() is called without RTNL,
especially clip_neigh_solicit() / neigh_ops->solicit() is unsleepable.

Also, there is no RTNL dependency around atmarpd.

Let's use a private mutex and RCU to protect access to atmarpd in
to_atmarpd().

Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Kuniyuki Iwashima <[email protected]>
Reviewed-by: Simon Horman <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: q2ven

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)
@@ -607,10 +619,12 @@ 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 +645,15 @@ static struct atm_dev atmarpd_dev = {
 
 static int atm_init_atmarp(struct atm_vcc *vcc)
 {
-	rtnl_lock();
+	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 */
@@ -648,7 +662,7 @@ 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;
 }