ext4: refuse to create ea block when umounted

The ea block expansion need to access s_root while it is
already set as NULL when umount is triggered. Refuse this
request to avoid panic.

Reported-by: [email protected]
Link: https://syzkaller.appspot.com/bug?id=3613786cb88c93aa1c6a279b1df6a7b201347d08
Link: https://lore.kernel.org/r/[email protected]
Cc: [email protected]
Signed-off-by: Jun Nie <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>

Author: niej

fs/ext4/xattr.c

@@ -1471,6 +1471,13 @@ static struct inode *ext4_xattr_inode_create(handle_t *handle,
 	uid_t owner[2] = { i_uid_read(inode), i_gid_read(inode) };
 	int err;
 
+	if (inode->i_sb->s_root == NULL) {
+		ext4_warning(inode->i_sb,
+			     "refuse to create EA inode when umounting");
+		WARN_ON(1);
+		return ERR_PTR(-EINVAL);
+	}
+
 	/*
 	 * Let the next inode be the goal, so we try and allocate the EA inode
 	 * in the same group, or nearby one.