Fix handling of username_as_alias during LDAP authentication

There is a bug that was introduced in the LDAP authentication method by hashicorp#11000.
It was thought to be backward compatible but has broken a number of users. Later
a new parameter `username_as_alias` was introduced in hashicorp#14324
to make it possible for operators to restore the previous behavior.
The way it is currently working is not completely backward compatible thought
because when username_as_alias is set, a call to GetUserAliasAttributeValue() will
first be made, then this value is completely discarded in pathLogin() and replaced
by the username as expected.

This is an issue because it makes useless calls to the LDAP server and will break
backward compatibility if one of the constraints in GetUserAliasAttributeValue()
is not respected, even though the resulting value will be discarded anyway.

In order to maintain backward compatibility here we have to only call
GetUserAliasAttributeValue() if necessary.

Since this change of behavior was introduced in 1.9, this fix will need to be
backported to the 1.9, 1.10 and 1.11 branches.

builtin/credential/ldap/backend.go

@@ -56,7 +56,7 @@ type backend struct {
 	*framework.Backend
 }
 
-func (b *backend) Login(ctx context.Context, req *logical.Request, username string, password string) (string, []string, *logical.Response, []string, error) {
+func (b *backend) Login(ctx context.Context, req *logical.Request, username string, password string, usernameAsAlias bool) (string, []string, *logical.Response, []string, error) {
 	cfg, err := b.Config(ctx, req)
 	if err != nil {
 		return "", nil, nil, nil, err
@@ -195,12 +195,15 @@ func (b *backend) Login(ctx context.Context, req *logical.Request, username stri
 	// Policies from each group may overlap
 	policies = strutil.RemoveDuplicates(policies, true)
 
-	entityAliasAttribute, err := ldapClient.GetUserAliasAttributeValue(cfg.ConfigEntry, c, username)
-	if err != nil {
-		return "", nil, logical.ErrorResponse(err.Error()), nil, nil
-	}
-	if entityAliasAttribute == "" {
-		return "", nil, logical.ErrorResponse("missing entity alias attribute value"), nil, nil
+	entityAliasAttribute := username
+	if !usernameAsAlias {
+		entityAliasAttribute, err = ldapClient.GetUserAliasAttributeValue(cfg.ConfigEntry, c, username)
+		if err != nil {
+			return "", nil, logical.ErrorResponse(err.Error()), nil, nil
+		}
+		if entityAliasAttribute == "" {
+			return "", nil, logical.ErrorResponse("missing entity alias attribute value"), nil, nil
+		}
 	}
 
 	return entityAliasAttribute, policies, ldapResponse, allGroups, nil

builtin/credential/ldap/path_login.go

@@ -73,7 +73,7 @@ func (b *backend) pathLogin(ctx context.Context, req *logical.Request, d *framew
 	username := d.Get("username").(string)
 	password := d.Get("password").(string)
 
-	effectiveUsername, policies, resp, groupNames, err := b.Login(ctx, req, username, password)
+	effectiveUsername, policies, resp, groupNames, err := b.Login(ctx, req, username, password, cfg.UsernameAsAlias)
 	// Handle an internal error
 	if err != nil {
 		return nil, err
@@ -103,10 +103,6 @@ func (b *backend) pathLogin(ctx context.Context, req *logical.Request, d *framew
 		},
 	}
 
-	if cfg.UsernameAsAlias {
-		auth.Alias.Name = username
-	}
-
 	cfg.PopulateTokenAuth(auth)
 
 	// Add in configured policies from mappings
@@ -139,7 +135,7 @@ func (b *backend) pathLoginRenew(ctx context.Context, req *logical.Request, d *f
 	username := req.Auth.Metadata["username"]
 	password := req.Auth.InternalData["password"].(string)
 
-	_, loginPolicies, resp, groupNames, err := b.Login(ctx, req, username, password)
+	_, loginPolicies, resp, groupNames, err := b.Login(ctx, req, username, password, cfg.UsernameAsAlias)
 	if err != nil || (resp != nil && resp.IsError()) {
 		return resp, err
 	}