fuse: fix control dir setup and teardown

syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
clear d_inode(dentry)->i_private field.

Fix by only adding the dentry to the array after being fully set up.

When tearing down the control directory, do d_invalidate() on it to get rid
of any mounts that might have been added.

[1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6Reported-by: syzbot <syzbot+32c236387d66c4516827@syzkaller.appspotmail.com>
Fixes: bafa9654 ("[PATCH] fuse: add control filesystem")
Cc: <stable@vger.kernel.org> # v2.6.18
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/control.c

@@ -211,10 +211,11 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
 	if (!dentry)
 		return NULL;
 
-	fc->ctl_dentry[fc->ctl_ndents++] = dentry;
 	inode = new_inode(fuse_control_sb);
-	if (!inode)
+	if (!inode) {
+		dput(dentry);
 		return NULL;
+	}
 
 	inode->i_ino = get_next_ino();
 	inode->i_mode = mode;
@@ -228,6 +229,9 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
 	set_nlink(inode, nlink);
 	inode->i_private = fc;
 	d_add(dentry, inode);
+
+	fc->ctl_dentry[fc->ctl_ndents++] = dentry;
+
 	return dentry;
 }
 
@@ -284,7 +288,10 @@ void fuse_ctl_remove_conn(struct fuse_conn *fc)
 	for (i = fc->ctl_ndents - 1; i >= 0; i--) {
 		struct dentry *dentry = fc->ctl_dentry[i];
 		d_inode(dentry)->i_private = NULL;
-		d_drop(dentry);
+		if (!i) {
+			/* Get rid of submounts: */
+			d_invalidate(dentry);
+		}
 		dput(dentry);
 	}
 	drop_nlink(d_inode(fuse_control_sb->s_root));