NFS: finish_automount() requires us to hold 2 refs to the mount record

mainline inclusion
from mainline-v5.7-rc1
commit 75da9858
category: bugfix
bugzilla: NA
CVE: NA

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

We must not return from nfs_d_automount() without holding 2 references
to the mount record. Doing so, will trigger the BUG() in finish_automount().
Also ensure that we don't try to reschedule the automount timer with
a negative or zero timeout value.

Fixes: 22a1ae9a ("NFS: If nfs_mountpoint_expiry_timeout < 0, do not expire submounts")
Cc: stable@vger.kernel.org # v5.5+
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

Conflicts:
	fs/nfs/namespace.c
Signed-off-by: NZhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: NZhang Yi <yi.zhang@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/nfs/namespace.c

@@ -146,6 +146,7 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	struct nfs_server *server = NFS_SERVER(d_inode(path->dentry));
 	struct nfs_fh *fh = NULL;
 	struct nfs_fattr *fattr = NULL;
+	int timeout = READ_ONCE(nfs_mountpoint_expiry_timeout);
 
 	if (IS_ROOT(path->dentry))
 		return ERR_PTR(-ESTALE);
@@ -160,12 +161,12 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	if (IS_ERR(mnt))
 		goto out;
 
-	if (nfs_mountpoint_expiry_timeout < 0)
+	mntget(mnt); /* prevent immediate expiration */
+	if (timeout <= 0)
 		goto out;
 
-	mntget(mnt); /* prevent immediate expiration */
 	mnt_set_expiry(mnt, &nfs_automount_list);
-	schedule_delayed_work(&nfs_automount_task, nfs_mountpoint_expiry_timeout);
+	schedule_delayed_work(&nfs_automount_task, timeout);
 
 out:
 	nfs_free_fattr(fattr);
@@ -204,10 +205,11 @@ const struct inode_operations nfs_referral_inode_operations = {
 static void nfs_expire_automounts(struct work_struct *work)
 {
 	struct list_head *list = &nfs_automount_list;
+	int timeout = READ_ONCE(nfs_mountpoint_expiry_timeout);
 
 	mark_mounts_for_expiry(list);
-	if (!list_empty(list))
-		schedule_delayed_work(&nfs_automount_task, nfs_mountpoint_expiry_timeout);
+	if (!list_empty(list) && timeout > 0)
+		schedule_delayed_work(&nfs_automount_task, timeout);
 }
 
 void nfs_release_automount_timer(void)