Where does SQL Server store the SA password and SQL Server password policy? On Linux, where does SQL Server store the "SQL Server password policy" and the SA user's password? I get it, "password" isn't secure.


  ERROR: Unable to set system administrator password: Password validation failed. The password does not meet SQL Server password policy requirements because it is too short. The password must be at least 8 characters..


and then,


  ERROR: Unable to set system administrator password: Password validation failed. The password does not meet SQL Server password policy requirements because it is not complex enough. The password must be at least 8 characters long and contain characters from three of the following four sets: Uppercase letters, Lowercase letters, Base 10 digits, and Symbols..


I'm guessing the policy is actually compiled into the database? And the password gets stored in an obscure location?
The sa user is a SQL Server login and its password is encrypted and stored in the DMV sys.sql_logins (Database Management View) in the master database.
Reference: sys.sql_logins (Transact-SQL)
You might notice that these views can only be found in the following branch:
Server | Databases | master | Views | System Views | .... 

The DMVs reference some system base tables, which can be accessed (but shouldn't):
Reference: System Base Tables
The policy you are looking for is built in to the code of SQL Server and is set per default for each new account. When creating a SQL Server Login you can decide to turn off the defaults:

To enforce password policy options for complexity and enforcement, select Enforce password policy. For more information, see Password Policy. This is a default option when SQL Server authentication is selected.

... via CHECK_POLICY=OFF. See the full syntax:
CREATE LOGIN login_name { WITH &lt;option_list1&gt; | FROM &lt;sources&gt; }  

&lt;option_list1&gt; ::=   
    PASSWORD = { 'password' | hashed_password HASHED } [ MUST_CHANGE ]  
    [ , &lt;option_list2&gt; [ ,... ] ]  

&lt;option_list2&gt; ::=    
    SID = sid  
    | DEFAULT_DATABASE = database      
    | DEFAULT_LANGUAGE = language  
    | CHECK_EXPIRATION = { ON | OFF}  
    | CHECK_POLICY = { ON | OFF}  
    | CREDENTIAL = credential_name   

&lt;sources&gt; ::=  
    WINDOWS [ WITH &lt;windows_options&gt;[ ,... ] ]  
    | CERTIFICATE certname  
    | ASYMMETRIC KEY asym_key_name  

&lt;windows_options&gt; ::=        
    DEFAULT_DATABASE = database  
    | DEFAULT_LANGUAGE = language  

Reference: CREATE LOGIN (Transact-SQL)
If the SQL Server is a member of a Windows Domain, then it will retrieve the password policy from Active Directory. Otherwise the defaults are:

Password complexity policies are designed to deter brute force attacks by increasing the number of possible passwords. When password complexity policy is enforced, new passwords must meet the following guidelines:

The password does not contain the account name of the user.
The password is at least eight characters long.
The password contains characters from three of the following four categories:
- Latin uppercase letters (A through Z)
- Latin lowercase letters (a through z)
- Base 10 digits (0 through 9)
- Non-alphanumeric characters such as: exclamation point (!), dollar sign ($), number sign (#), or percent (%).
Passwords can be up to 128 characters long. You should use passwords that are as long and complex as possible.


Reference: Password Policy