SUNRPC: Close a race with transport setup and module put

mainline inclusion
from mainline-v5.11-rc1
commit 9bccd264
category: bugfix
bugzilla: NA
CVE: NA

--------------------------------

After we've looked up the transport module, we need to ensure it can't
go away until we've finished running the transport setup code.

conflicts:
  net/sunrpc/xprt.c
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: NLu Wei <luwei32@huawei.com>
Reviewed-by: NYue Haibing <yuehaibing@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

net/sunrpc/xprt.c

@@ -149,6 +149,32 @@ xprt_class_release(const struct xprt_class *t)
 	module_put(t->owner);
 }
 
+static const struct xprt_class *
+xprt_class_find_by_ident_locked(int ident)
+{
+	const struct xprt_class *t;
+
+	list_for_each_entry(t, &xprt_list, list) {
+		if (t->ident != ident)
+			continue;
+		if (!try_module_get(t->owner))
+			continue;
+		return t;
+	}
+	return NULL;
+}
+
+static const struct xprt_class *
+xprt_class_find_by_ident(int ident)
+{
+	const struct xprt_class *t;
+
+	spin_lock(&xprt_list_lock);
+	t = xprt_class_find_by_ident_locked(ident);
+	spin_unlock(&xprt_list_lock);
+	return t;
+}
+
 static const struct xprt_class *
 xprt_class_find_by_netid_locked(const char *netid)
 {
@@ -1468,21 +1494,17 @@ static void xprt_init(struct rpc_xprt *xprt, struct net *net)
 struct rpc_xprt *xprt_create_transport(struct xprt_create *args)
 {
 	struct rpc_xprt	*xprt;
-	struct xprt_class *t;
+	const struct xprt_class *t;
 
-	spin_lock(&xprt_list_lock);
-	list_for_each_entry(t, &xprt_list, list) {
-		if (t->ident == args->ident) {
-			spin_unlock(&xprt_list_lock);
-			goto found;
-		}
+	t = xprt_class_find_by_ident(args->ident);
+	if (!t) {
+		dprintk("RPC: transport (%d) not supported\n", args->ident);
+		return ERR_PTR(-EIO);
 	}
-	spin_unlock(&xprt_list_lock);
-	dprintk("RPC: transport (%d) not supported\n", args->ident);
-	return ERR_PTR(-EIO);
 
-found:
 	xprt = t->setup(args);
+	xprt_class_release(t);
+
 	if (IS_ERR(xprt)) {
 		dprintk("RPC:       xprt_create_transport: failed, %ld\n",
 				-PTR_ERR(xprt));