The permanent bar comes from a 1996 law called the Immigration Reform and Immigrant Responsibility Act (“IRAIRA”). This law amended the Immigration and Naturalization Act (“INA”), which is where all U.S. immigration laws are contained. Section 212 (a)(9)(C) of the INA is the provision that talks about the permanent bar. This section states that any alien who has been unlawfully present in the United States for an aggregate period of more than one year, and who enters or attempts to reenter the United States without being admitted, is inadmissible (meaning, cannot obtain lawful admission to the U.S.). Although called “permanent,” this bar lasts for only 10 years but there is no waiver an immigrant can give to U.S. Citizenship and Immigration Services—this is why it is usually referred as permanent.
There may be some exceptions to the permanent bar. To truly find out if you have—or not—the permanent bar, you should consult with an immigration attorney who can help you calculate the time.