net: convert inet_peer.refcnt from atomic_t to refcount_t

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.
This conversion requires overall +1 on the whole
refcounting scheme.

Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inetpeer.h

@@ -46,7 +46,7 @@ struct inet_peer {
 		struct rcu_head     gc_rcu;
 	};
 	/*
-	 * Once inet_peer is queued for deletion (refcnt == -1), following field
+	 * Once inet_peer is queued for deletion (refcnt == 0), following field
 	 * is not available: rid
 	 * We can share memory with rcu_head to help keep inet_peer small.
 	 */
@@ -60,7 +60,7 @@ struct inet_peer {
 
 	/* following fields might be frequently dirtied */
 	__u32			dtime;	/* the time of last use of not referenced entries */
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 };
 
 struct inet_peer_base {

net/ipv4/inetpeer.c

@@ -115,7 +115,7 @@ static void inetpeer_gc_worker(struct work_struct *work)
 
 		n = list_entry(p->gc_list.next, struct inet_peer, gc_list);
 
-		if (!atomic_read(&p->refcnt)) {
+		if (refcount_read(&p->refcnt) == 1) {
 			list_del(&p->gc_list);
 			kmem_cache_free(peer_cachep, p);
 		}
@@ -202,10 +202,11 @@ static struct inet_peer *lookup_rcu(const struct inetpeer_addr *daddr,
 		int cmp = inetpeer_addr_cmp(daddr, &u->daddr);
 		if (cmp == 0) {
 			/* Before taking a reference, check if this entry was
-			 * deleted (refcnt=-1)
+			 * deleted (refcnt=0)
 			 */
-			if (!atomic_add_unless(&u->refcnt, 1, -1))
+			if (!refcount_inc_not_zero(&u->refcnt)) {
 				u = NULL;
+			}
 			return u;
 		}
 		if (cmp == -1)
@@ -382,11 +383,10 @@ static int inet_peer_gc(struct inet_peer_base *base,
 	while (stackptr > stack) {
 		stackptr--;
 		p = rcu_deref_locked(**stackptr, base);
-		if (atomic_read(&p->refcnt) == 0) {
+		if (refcount_read(&p->refcnt) == 1) {
 			smp_rmb();
 			delta = (__u32)jiffies - p->dtime;
-			if (delta >= ttl &&
-			    atomic_cmpxchg(&p->refcnt, 0, -1) == 0) {
+			if (delta >= ttl && refcount_dec_if_one(&p->refcnt)) {
 				p->gc_next = gchead;
 				gchead = p;
 			}
@@ -432,7 +432,7 @@ struct inet_peer *inet_getpeer(struct inet_peer_base *base,
 relookup:
 	p = lookup(daddr, stack, base);
 	if (p != peer_avl_empty) {
-		atomic_inc(&p->refcnt);
+		refcount_inc(&p->refcnt);
 		write_sequnlock_bh(&base->lock);
 		return p;
 	}
@@ -444,7 +444,7 @@ relookup:
 	p = create ? kmem_cache_alloc(peer_cachep, GFP_ATOMIC) : NULL;
 	if (p) {
 		p->daddr = *daddr;
-		atomic_set(&p->refcnt, 1);
+		refcount_set(&p->refcnt, 2);
 		atomic_set(&p->rid, 0);
 		p->metrics[RTAX_LOCK-1] = INETPEER_METRICS_NEW;
 		p->rate_tokens = 0;
@@ -468,7 +468,7 @@ void inet_putpeer(struct inet_peer *p)
 {
 	p->dtime = (__u32)jiffies;
 	smp_mb__before_atomic();
-	atomic_dec(&p->refcnt);
+	refcount_dec(&p->refcnt);
 }
 EXPORT_SYMBOL_GPL(inet_putpeer);