LOCKD: fix races in nsm_client_get

Commit e9406db20fecbfcab646bad157b4cfdc7cadddfb (lockd: per-net
NSM client creation and destruction helpers introduced) contains
a nasty race on initialisation of the per-net NSM client because
it doesn't check whether or not the client is set after grabbing
the nsm_create_mutex.

Reported-by: Nix <nix@esperi.org.uk>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org

1 files changed, 26 insertions, 17 deletions

diff --git a/fs/lockd/mon.c b/fs/lockd/mon.c
index e4fb3ba5a58a..fe695603e395 100644
--- a/fs/lockd/mon.c
+++ b/fs/lockd/mon.c
@@ -85,29 +85,38 @@ static struct rpc_clnt *nsm_create(struct net *net)
         return rpc_create(&args);
 }
 
+static struct rpc_clnt *nsm_client_set(struct lockd_net *ln,
+                struct rpc_clnt *clnt)
+{
+        spin_lock(&ln->nsm_clnt_lock);
+        if (ln->nsm_users == 0) {
+                if (clnt == NULL)
+                        goto out;
+                ln->nsm_clnt = clnt;
+        }
+        clnt = ln->nsm_clnt;
+        ln->nsm_users++;
+out:
+        spin_unlock(&ln->nsm_clnt_lock);
+        return clnt;
+}
+
 static struct rpc_clnt *nsm_client_get(struct net *net)
 {
-        static DEFINE_MUTEX(nsm_create_mutex);
-        struct rpc_clnt *clnt;
+        struct rpc_clnt *clnt, *new;
         struct lockd_net *ln = net_generic(net, lockd_net_id);
 
-        spin_lock(&ln->nsm_clnt_lock);
-        if (ln->nsm_users) {
-                ln->nsm_users++;
-                clnt = ln->nsm_clnt;
-                spin_unlock(&ln->nsm_clnt_lock);
+        clnt = nsm_client_set(ln, NULL);
+        if (clnt != NULL)
                 goto out;
-        }
-        spin_unlock(&ln->nsm_clnt_lock);
 
-        mutex_lock(&nsm_create_mutex);
-        clnt = nsm_create(net);
-        if (!IS_ERR(clnt)) {
-                ln->nsm_clnt = clnt;
-                smp_wmb();
-                ln->nsm_users = 1;
-        }
-        mutex_unlock(&nsm_create_mutex);
+        clnt = new = nsm_create(net);
+        if (IS_ERR(clnt))
+                goto out;
+
+        clnt = nsm_client_set(ln, new);
+        if (clnt != new)
+                rpc_shutdown_client(new);
 out:
         return clnt;
 }