Protecting reserved roles with PostgreSQL Hooks

Using Postgres Hooks to protect functionality in your Postgres database.

Supabase Blog · Jul 2, 2021

Protecting reserved roles with PostgreSQL Hooks

PostgreSQL manages permissions through roles. To create these roles, a database user needs the CREATEROLE privilege, which not only allows role creation but also modification of any role (except superusers).

At Supabase, we use dedicated roles for each of our customers' backend services. For instance, our Storage API uses the supabase_storage_admin role for connecting to the database. Giving the CREATEROLE privilege to our customers would allow them to drop this role and take their own Storage API down.

And yet, we want to give our customers the ability to manage roles the same as they do it in on-premises databases, with the usual CREATE ROLE, ALTER ROLE, and DROP ROLE statements.

So, how do we grant them the CREATEROLE privilege and, at the same time, protect our own roles? In this blog post, we explain how we managed to do this by using PostgreSQL Hooks in our SupaUtils extension.

PostgreSQL has a list of predefined roles — all prefixed with "pg_" — that cannot be dropped or altered. Attempting to do so will throw an error mentioning that the role is "reserved".

_10

alter role pg_monitor createdb;

_10

ERROR: role name "pg_monitor" is reserved

_10

DETAIL: Cannot alter reserved roles.

This mechanism is an internal implementation detail. Unfortunately Postgres doesn't allow us to define our own reserved roles.

RDS reserved roles#

Amazon RDS has a similar defense mechanism, all of its predefined roles — prefixed with "rds" — cannot be modified.

_10

alter role rdsadmin rename to another;

_10

ERROR: The "rdsadmin" role cannot be renamed.

_10

DETAIL: The "rdsadmin" role cannot be renamed because either the source

_10

or the target name refer to an Amazon RDS reserved role name.

_10

LOCATION: handle_rename, rdsutils.c:1534

Again, the error mentions that the role is "reserved".

Also note the rdsutils.c mention. That's not a stock Postgres source file. This means that the logic comes from an RDS extension. We can confirm this is the case by showing the preloaded libraries.

_10

show shared_preload_libraries;

_10

shared_preload_libraries

_10

-----------------------------

_10

rdsutils,pg_stat_statements

rdsutils can be seen there. Naturally this lead us into thinking we can achieve the same logic with an extension of our own, and thus the SupaUtils idea was born.

Extending PostgreSQL with Hooks#

PostgreSQL hooks allow us to extend internal functionality. Hooks can modify behavior at different places, including when running SQL statements.

For example, if we wanted to enforce our own password restrictions whenever a user changes passwords, we could use the check_password_hook to verify the password. We would write out our own Custom Logic, and raise an error if the password fails the password requirements.

For SupaUtils, we're particularly interested in the ProcessUtility_hook, which allows us to hook into utility statements: every statement except select, insert, delete or update. They include alter role and drop role, which are the statements we want to hook on.

Hooks are global function pointers#

To use hooks, we can override functions pointers that are global. On the Postgres codebase, the ProcessUtility_hook is basically used1 like this:

_16

// src/backend/tcop/utility.c

_16

// ProcessUtility_hook is NULL by default

_16

ProcessUtility_hook_type ProcessUtility_hook = NULL;

_16

// This function is used for processing all the utility statements

_16

void

_16

ProcessUtility(PARAMS_OMITTED_FOR_BREVITY)

_16

{

_16

// call the ProcessUtility_hook if it's not NULL

_16

if (ProcessUtility_hook)

_16

(*ProcessUtility_hook)(PARAMS_OMITTED_FOR_BREVITY);

_16

// otherwise call the standard function used to process utility statements

_16

else

_16

standard_ProcessUtility(PARAMS_OMITTED_FOR_BREVITY);

_16

}

As you can see, ProcessUtility_hook is NULL by default, so our extension should set it for the hook to run. Also, the standard_ProcessUtility function is the one that actually does the job of creating or modifying the roles (among other things) so our hook should also call it.

Loading and running the hook#

Each extension set in shared_preload_libraries will get its _PG_init function called. This function will allow us to set our hook onto ProcessUtility_hook.

Since hooks are global function pointers, it might be the case that another extension modifies the hook pointer (on its own _PG_init) before us and sets its own hook. So we need to ensure we also run this previously-set hook, before or after our own hook runs.

It's typically2 done like this:

_26

// variable to store the previous hook

_26

static ProcessUtility_hook_type prev_hook = NULL;

_26

// initialize our extension

_26

void

_26

_PG_init(void)

_26

{

_26

// ProcessUtility_hook has the global function pointer.

_26

// Store its value in case another extension already set its own hook.

_26

prev_hook = ProcessUtility_hook;

_26

// Now override the ProcessUtility_hook with our hook

_26

ProcessUtility_hook = our_hook;

_26

}

_26

static void

_26

our_hook(PARAMS_OMITTED_FOR_BREVITY)

_26

{

_26

// our hook logic goes here

_26

// If there was a previous hook, run it after our hook

_26

if (prev_hook)

_26

prev_hook(PARAMS_OMITTED_FOR_BREVITY);

_26

// If there's no previous hook, call the standard function

_26

else

_26

standard_ProcessUtility(PARAMS_OMITTED_FOR_BREVITY);

_26

}

We can use the concepts above to build our extension.

First we'll need a Makefile in order to compile the extension code and include it into our PostgreSQL installation.

_13

# Makefile

_13

# Our shared library

_13

MODULE_big = supautils

_13

# Our object files to build for the library

_13

OBJS = src/supautils.o

_13

# Tell pg_config to pass us the PostgreSQL extensions makefile(PGXS)

_13

# and include it into our own Makefile through the standard "include" directive.

_13

PG_CONFIG = pg_config

_13

PGXS := $(shell $(PG_CONFIG) --pgxs)

_13

include $(PGXS)

For the source file, we'll start with variable definitions and functions declarations.

_30

// src/supautils.c

_30

// include common declarations

_30

#include "postgres.h"

_30

// required macro for extension libraries to work

_30

PG_MODULE_MAGIC;

_30

// variable for the previous hook

_30

static ProcessUtility_hook_type prev_hook = NULL;

_30

// variable for our reserved roles configuration parameter

_30

static char *reserved_roles = NULL;

_30

// function declaration for extension initialization

_30

void _PG_init(void);

_30

// function declaration for our hook

_30

static void supautils_hook(

_30

PlannedStmt *pstmt,

_30

const char *queryString,

_30

ProcessUtilityContext context,

_30

ParamListInfo params,

_30

QueryEnvironment *queryEnv,

_30

DestReceiver *dest,

_30

QueryCompletion *completionTag

_30

);

_30

// function declaration for our pure function that will return a reserved role

_30

static char* look_for_reserved_role(Node *utility_stmt, List *reserved_role_list);

Up next we'll define each one of these function declarations.

Let's now _PG_init our extension. Besides setting the hook here, we want to define our reserved roles as a configuration parameter, that way they can be modified by editing the postgresql.conf file. For this, we can use the DefineCustomStringVariable function, which inserts the parameter into Postgres "Grand Unified Configuration"(GUC) system.

_22

void

_22

_PG_init(void)

_22

{

_22

// Store the previous hook

_22

prev_hook = ProcessUtility_hook;

_22

// Set our hook

_22

ProcessUtility_hook = supautils_hook;

_22

// Define our "supautils.reserved_roles" parameter

_22

// some arguments are unused so they are left as NULL

_22

DefineCustomStringVariable("supautils.reserved_roles",

_22

"Comma-separated list of roles that cannot be altered or dropped",

_22

NULL,

_22

// It will be assigned to the reserved_roles variable

_22

&reserved_roles,

_22

NULL,

_22

// We should be able to reload this parameter without restarting the server,

_22

// e.g. with "select pg_reload_conf()".

_22

PGC_SIGHUP,

_22

0,

_22

NULL, NULL, NULL);

_22

}

Now that our hook is set, we'll define what it will do. As specified in the ProcessUtility_hook_type, the hook's first parameter is a PlannedStmt, this represents the planned statement — the output from the Postgres planner. This is a step before the statement is executed.

We'll look for the presence of a reserved role in the planned statement. If there's one present, we'll report an error and abort the statement execution step.

_55

static void

_55

supautils_hook(

_55

// The planned statement

_55

PlannedStmt *pstmt,

_55

// These parameters are here for completion, we'll not use any of them

_55

const char *queryString,

_55

ProcessUtilityContext context,

_55

ParamListInfo params,

_55

QueryEnvironment *queryEnv,

_55

DestReceiver *dest,

_55

QueryCompletion *completionTag

_55

)

_55

{

_55

// Get the utility statement from the planned statement

_55

Node *utility_stmt = pstmt->utilityStmt;

_55

// Only do the logic if supautils.reserved_roles is not NULL

_55

if(reserved_roles){

_55

// The found reserved role, assume none was found by default

_55

char *reserved_role = NULL;

_55

// Temp var for storing the list of reserved roles

_55

List *reserved_role_list;

_55

// split the comma-separated string into a List by using a

_55

// helper function from varlena.h

_55

if (!SplitIdentifierString(pstrdup(reserved_roles), ',', &reserved_role_list))

_55

// abort and report an error if the splitting fails

_55

ereport(ERROR,

_55

(errcode(ERRCODE_INVALID_PARAMETER_VALUE),

_55

errmsg("parameter \"%s\" must be a comma-separated list of "

_55

"identifiers", reserved_roles)));

_55

// look for the reserved role in an internal function

_55

reserved_role = look_for_reserved_role(utility_stmt, reserved_role_list);

_55

// we're done with the list so free it from memory

_55

list_free(reserved_role_list);

_55

// abort and report an error if a reserved role was found

_55

if(reserved_role)

_55

ereport(ERROR,

_55

(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),

_55

errmsg("\"%s\" is a reserved role, it cannot be modified", reserved_role)));

_55

}

_55

// Run the previous hook if defined or call the standard function

_55

if (prev_hook)

_55

prev_hook(pstmt, queryString,

_55

context, params, queryEnv,

_55

dest, completionTag);

_55

else

_55

standard_ProcessUtility(pstmt, queryString,

_55

context, params, queryEnv,

_55

dest, completionTag);

_55

}

Lastly, we'll define how we look for the reserved role.

At this stage, we already have the utility statement and the reserved role list. All that's left to do is to define if the utility statement is an ALTER ROLE or DROP ROLE statement, and whether if the role it affects is a reserved one.

_70

static char*

_70

look_for_reserved_role(Node *utility_stmt, List *reserved_role_list)

_70

{

_70

// Check the utility statement type

_70

switch (utility_stmt->type)

_70

{

_70

// Matches statements like:

_70

// ALTER ROLE role NOLOGIN

_70

case T_AlterRoleStmt:

_70

{

_70

// cast the utility statement to an alter role statement

_70

AlterRoleStmt *stmt = (AlterRoleStmt *) utility_stmt;

_70

//RoleSpec has the role name plus some attributes

_70

RoleSpec *role = stmt->role;

_70

// postgres defines its own list utilities in pg_list.h

_70

// ListCell is an element of the list that we'll use for iteration

_70

ListCell *role_cell;

_70

// pg_list.h includes the foreach macro for iterating over lists

_70

foreach(role_cell, reserved_role_list)

_70

{

_70

// get the list element

_70

char *reserved_role = (char *) lfirst(role_cell);

_70

// compare the statement role with the reserved role

_70

// get_rolespec_name will get the RoleSpec role name,

_70

// even in cases where the role is the special case of

_70

// "current_user" or "session_user"

_70

if (strcmp(get_rolespec_name(role), reserved_role) == 0)

_70

return reserved_role;

_70

}

_70

break;

_70

}

_70

// Matches statements like:

_70

// DROP ROLE role

_70

case T_DropRoleStmt:

_70

{

_70

// cast the utility statement to a drop role statement

_70

DropRoleStmt *stmt = (DropRoleStmt *) utility_stmt;

_70

ListCell *item;

_70

// the logic is the same as before, iterate over the reserved role list

_70

// and find a match

_70

foreach(item, stmt->roles)

_70

{

_70

RoleSpec *role = lfirst(item);

_70

ListCell *role_cell;

_70

foreach(role_cell, reserved_role_list)

_70

{

_70

char *reserved_role = (char *) lfirst(role_cell);

_70

if (strcmp(get_rolespec_name(role), reserved_role) == 0)

_70

return reserved_role;

_70

}

_70

}

_70

break;

_70

}

_70

default:

_70

break;

_70

}

_70

// Didn't find any reserved role on the statement, so return NULL

_70

return NULL;

_70

}

Now that the code is finished, we can test the extension. Since we already have a Makefile, the extension can be installed by doing make && make install. Then, in postgresql.conf:

_10

# set the extension as preloaded, this will require a restart

_10

shared_preload_libraries="supautils"

_10

# the reserved roles

_10

supautils.reserved_roles="supabase_storage_admin, supabase_auth_admin"

We'll now try to alter or drop the reserved roles:

_10

alter role supabase_storage_admin nologin password 'fake';

_10

ERROR: "supabase_storage_admin" is a reserved role, it cannot be modified

_10

drop role supabase_auth_admin;

_10

ERROR: "supabase_auth_admin" is a reserved role, it cannot be modified

_10

-- Success!!

As you can see, PostgreSQL Hooks allow us to intercept SQL statements. There are many types of hooks, you can see unofficial documentation for these at AmatanHead/psql-hooks.

The full SupaUtils code is in our GitHub repository.

By the way, if you like working on PostgreSQL tooling and extensions: we are hiring PostgreSQL experts!

0 Comments

No comments yet. Be the first to share your thoughts.

Protecting reserved roles with PostgreSQL Hooks

RDS reserved roles#

Extending PostgreSQL with Hooks#

Hooks are global function pointers#

Loading and running the hook#

More Postgres resources#

0 Comments