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>

1 files changed, 27 insertions, 15 deletions

diff --git a/net/ipv4/inetpeer.c b/net/ipv4/inetpeer.c
index 9df4e635fb5f..ce616d92cc54 100644
--- a/net/ipv4/inetpeer.c
+++ b/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) {