inetpeer: fix race in unused_list manipulations

Several crashes in cleanup_once() were reported in recent kernels.

Commit d6cc1d642de9 (inetpeer: various changes) added a race in
unlink_from_unused().

One way to avoid taking unused_peers.lock before doing the list_empty()
test is to catch 0->1 refcnt transitions, using full barrier atomic
operations variants (atomic_cmpxchg() and atomic_inc_return()) instead
of previous atomic_inc() and atomic_add_unless() variants.

We then call unlink_from_unused() only for the owner of the 0->1
transition.

Add a new atomic_add_unless_return() static helper

With help from Arun Sharma.

Refs: https://bugzilla.kernel.org/show_bug.cgi?id=32772

Reported-by: Arun Sharma <asharma@fb.com>
Reported-by: Maximilian Engelhardt <maxi@daemonizer.de>
Reported-by: Yann Dupont <Yann.Dupont@univ-nantes.fr>
Reported-by: Denys Fedoryshchenko <denys@visp.net.lb>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inetpeer.c

@@ -154,11 +154,9 @@ void __init inet_initpeers(void)
 /* Called with or without local BH being disabled. */
 static void unlink_from_unused(struct inet_peer *p)
 {
-	if (!list_empty(&p->unused)) {
-		spin_lock_bh(&unused_peers.lock);
-		list_del_init(&p->unused);
-		spin_unlock_bh(&unused_peers.lock);
-	}
+	spin_lock_bh(&unused_peers.lock);
+	list_del_init(&p->unused);
+	spin_unlock_bh(&unused_peers.lock);
 }
 
 static int addr_compare(const struct inetpeer_addr *a,
@@ -205,6 +203,20 @@ static int addr_compare(const struct inetpeer_addr *a,
 	u;							\
 })
 
+static bool atomic_add_unless_return(atomic_t *ptr, int a, int u, int *newv)
+{
+	int cur, old = atomic_read(ptr);
+
+	while (old != u) {
+		*newv = old + a;
+		cur = atomic_cmpxchg(ptr, old, *newv);
+		if (cur == old)
+			return true;
+		old = cur;
+	}
+	return false;
+}
+
 /*
  * Called with rcu_read_lock()
  * Because we hold no lock against a writer, its quite possible we fall
@@ -213,7 +225,8 @@ static int addr_compare(const struct inetpeer_addr *a,
  * We exit from this function if number of links exceeds PEER_MAXDEPTH
  */
 static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
-				    struct inet_peer_base *base)
+				    struct inet_peer_base *base,
+				    int *newrefcnt)
 {
 	struct inet_peer *u = rcu_dereference(base->root);
 	int count = 0;
@@ -226,7 +239,7 @@ static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
 			 * distinction between an unused entry (refcnt=0) and
 			 * a freed one.
 			 */
-			if (unlikely(!atomic_add_unless(&u->refcnt, 1, -1)))
+			if (!atomic_add_unless_return(&u->refcnt, 1, -1, newrefcnt))
 				u = NULL;
 			return u;
 		}
@@ -465,22 +478,23 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
 	struct inet_peer_base *base = family_to_base(daddr->family);
 	struct inet_peer *p;
 	unsigned int sequence;
-	int invalidated;
+	int invalidated, newrefcnt = 0;
 
 	/* Look up for the address quickly, lockless.
 	 * Because of a concurrent writer, we might not find an existing entry.
 	 */
 	rcu_read_lock();
 	sequence = read_seqbegin(&base->lock);
-	p = lookup_rcu(daddr, base);
+	p = lookup_rcu(daddr, base, &newrefcnt);
 	invalidated = read_seqretry(&base->lock, sequence);
 	rcu_read_unlock();
 
 	if (p) {
-		/* The existing node has been found.
+found:		/* The existing node has been found.
 		 * Remove the entry from unused list if it was there.
 		 */
-		unlink_from_unused(p);
+		if (newrefcnt == 1)
+			unlink_from_unused(p);
 		return p;
 	}
 
@@ -494,11 +508,9 @@ struct inet_peer *inet_getpeer(struct inetpeer_addr *daddr, int create)
 	write_seqlock_bh(&base->lock);
 	p = lookup(daddr, stack, base);
 	if (p != peer_avl_empty) {
-		atomic_inc(&p->refcnt);
+		newrefcnt = atomic_inc_return(&p->refcnt);
 		write_sequnlock_bh(&base->lock);
-		/* Remove the entry from unused list if it was there. */
-		unlink_from_unused(p);
-		return p;
+		goto found;
 	}
 	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
 	if (p) {