fs: fix dropping of rcu-walk from force_reval_path

As J. R. Okajima noted, force_reval_path passes in the same dentry to
d_revalidate as the one in the nameidata structure (other callers pass in a
child), so the locking breaks. This can oops with a chrooted nfs mount, for
example. Similarly there can be other problems with revalidating a dentry
which is already in nameidata of the path walk.
Signed-off-by: NNick Piggin <npiggin@kernel.dk>

fs/namei.c

@@ -479,6 +479,14 @@ static int nameidata_dentry_drop_rcu(struct nameidata *nd, struct dentry *dentry
 	struct fs_struct *fs = current->fs;
 	struct dentry *parent = nd->path.dentry;
 
+	/*
+	 * It can be possible to revalidate the dentry that we started
+	 * the path walk with. force_reval_path may also revalidate the
+	 * dentry already committed to the nameidata.
+	 */
+	if (unlikely(parent == dentry))
+		return nameidata_drop_rcu(nd);
+
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
 	if (nd->root.mnt) {
 		spin_lock(&fs->lock);