In addition to the SQL-standard privilege system available through GRANT, tables can have row security policies that restrict, on a per-user basis, which rows can be returned by normal queries or inserted, updated, or deleted by data modification commands. This feature is also known as Row-Level Security. By default, tables do not have any policies, so that if a user has access privileges to a table according to the SQL privilege system, all rows within it are equally available for querying or updating.
When row security is enabled on a table (with
ALTER TABLE ... ENABLE ROW LEVEL
SECURITY), all normal access to the table for selecting rows or
modifying rows must be allowed by a row security policy. (However, the
table's owner is typically not subject to row security policies.) If no
policy exists for the table, a default-deny policy is used, meaning that
no rows are visible or can be modified. Operations that apply to the
whole table, such as TRUNCATE
and REFERENCES
,
are not subject to row security.
Row security policies can be specific to commands, or to roles, or to
both. A policy can be specified to apply to ALL
commands, or to SELECT
, INSERT
, UPDATE
,
or DELETE
. Multiple roles can be assigned to a given
policy, and normal role membership and inheritance rules apply.
To specify which rows are visible or modifiable according to a policy,
an expression is required that returns a Boolean result. This
expression will be evaluated for each row prior to any conditions or
functions coming from the user's query. (The only exceptions to this
rule are leakproof
functions, which are guaranteed to
not leak information; the optimizer may choose to apply such functions
ahead of the row-security check.) Rows for which the expression does
not return true
will not be processed. Separate expressions
may be specified to provide independent control over the rows which are
visible and the rows which are allowed to be modified. Policy
expressions are run as part of the query and with the privileges of the
user running the query, although security-definer functions can be used
to access data not available to the calling user.
Superusers and roles with the BYPASSRLS
attribute always
bypass the row security system when accessing a table. Table owners
normally bypass row security as well, though a table owner can choose to
be subject to row security with ALTER
TABLE ... FORCE ROW LEVEL SECURITY.
Enabling and disabling row security, as well as adding policies to a table, is always the privilege of the table owner only.
Policies are created using the CREATE POLICY command, altered using the ALTER POLICY command, and dropped using the DROP POLICY command. To enable and disable row security for a given table, use the ALTER TABLE command.
Each policy has a name and multiple policies can be defined for a table. As policies are table-specific, each policy for a table must have a unique name. Different tables may have policies with the same name.
When multiple policies apply to a given query, they are combined using
either OR
(for permissive policies, which are the
default) or using AND
(for restrictive policies).
This is similar to the rule that a given role has the privileges
of all roles that they are a member of. Permissive vs. restrictive
policies are discussed further below.
As a simple example, here is how to create a policy on
the account
relation to allow only members of
the managers
role to access rows, and only rows of their
accounts:
CREATE TABLE accounts (manager text, company text, contact_email text); ALTER TABLE accounts ENABLE ROW LEVEL SECURITY; CREATE POLICY account_managers ON accounts TO managers USING (manager = current_user);
The policy above implicitly provides a WITH CHECK
clause identical to its USING
clause, so that the
constraint applies both to rows selected by a command (so a manager
cannot SELECT
, UPDATE
,
or DELETE
existing rows belonging to a different
manager) and to rows modified by a command (so rows belonging to a
different manager cannot be created via INSERT
or UPDATE
).
If no role is specified, or the special user name
PUBLIC
is used, then the policy applies to all
users on the system. To allow all users to access only their own row in
a users
table, a simple policy can be used:
CREATE POLICY user_policy ON users USING (user_name = current_user);
This works similarly to the previous example.
To use a different policy for rows that are being added to the table
compared to those rows that are visible, multiple policies can be
combined. This pair of policies would allow all users to view all rows
in the users
table, but only modify their own:
CREATE POLICY user_sel_policy ON users FOR SELECT USING (true); CREATE POLICY user_mod_policy ON users USING (user_name = current_user);
In a SELECT
command, these two policies are combined
using OR
, with the net effect being that all rows
can be selected. In other command types, only the second policy applies,
so that the effects are the same as before.
Row security can also be disabled with the ALTER TABLE
command. Disabling row security does not remove any policies that are
defined on the table; they are simply ignored. Then all rows in the
table are visible and modifiable, subject to the standard SQL privileges
system.
Below is a larger example of how this feature can be used in production
environments. The table passwd
emulates a Unix password
file:
-- Simple passwd-file based example CREATE TABLE passwd ( user_name text UNIQUE NOT NULL, pwhash text, uid int PRIMARY KEY, gid int NOT NULL, real_name text NOT NULL, home_phone text, extra_info text, home_dir text NOT NULL, shell text NOT NULL ); CREATE ROLE admin; -- Administrator CREATE ROLE bob; -- Normal user CREATE ROLE alice; -- Normal user -- Populate the table INSERT INTO passwd VALUES ('admin','xxx',0,0,'Admin','111-222-3333',null,'/root','/bin/dash'); INSERT INTO passwd VALUES ('bob','xxx',1,1,'Bob','123-456-7890',null,'/home/bob','/bin/zsh'); INSERT INTO passwd VALUES ('alice','xxx',2,1,'Alice','098-765-4321',null,'/home/alice','/bin/zsh'); -- Be sure to enable row level security on the table ALTER TABLE passwd ENABLE ROW LEVEL SECURITY; -- Create policies -- Administrator can see all rows and add any rows CREATE POLICY admin_all ON passwd TO admin USING (true) WITH CHECK (true); -- Normal users can view all rows CREATE POLICY all_view ON passwd FOR SELECT USING (true); -- Normal users can update their own records, but -- limit which shells a normal user is allowed to set CREATE POLICY user_mod ON passwd FOR UPDATE USING (current_user = user_name) WITH CHECK ( current_user = user_name AND shell IN ('/bin/bash','/bin/sh','/bin/dash','/bin/zsh','/bin/tcsh') ); -- Allow admin all normal rights GRANT SELECT, INSERT, UPDATE, DELETE ON passwd TO admin; -- Users only get select access on public columns GRANT SELECT (user_name, uid, gid, real_name, home_phone, extra_info, home_dir, shell) ON passwd TO public; -- Allow users to update certain columns GRANT UPDATE (pwhash, real_name, home_phone, extra_info, shell) ON passwd TO public;
As with any security settings, it's important to test and ensure that the system is behaving as expected. Using the example above, this demonstrates that the permission system is working properly.
-- admin can view all rows and fields postgres=> set role admin; SET postgres=> table passwd; user_name | pwhash | uid | gid | real_name | home_phone | extra_info | home_dir | shell -----------+--------+-----+-----+-----------+--------------+------------+-------------+----------- admin | xxx | 0 | 0 | Admin | 111-222-3333 | | /root | /bin/dash bob | xxx | 1 | 1 | Bob | 123-456-7890 | | /home/bob | /bin/zsh alice | xxx | 2 | 1 | Alice | 098-765-4321 | | /home/alice | /bin/zsh (3 rows) -- Test what Alice is able to do postgres=> set role alice; SET postgres=> table passwd; ERROR: permission denied for relation passwd postgres=> select user_name,real_name,home_phone,extra_info,home_dir,shell from passwd; user_name | real_name | home_phone | extra_info | home_dir | shell -----------+-----------+--------------+------------+-------------+----------- admin | Admin | 111-222-3333 | | /root | /bin/dash bob | Bob | 123-456-7890 | | /home/bob | /bin/zsh alice | Alice | 098-765-4321 | | /home/alice | /bin/zsh (3 rows) postgres=> update passwd set user_name = 'joe'; ERROR: permission denied for relation passwd -- Alice is allowed to change her own real_name, but no others postgres=> update passwd set real_name = 'Alice Doe'; UPDATE 1 postgres=> update passwd set real_name = 'John Doe' where user_name = 'admin'; UPDATE 0 postgres=> update passwd set shell = '/bin/xx'; ERROR: new row violates WITH CHECK OPTION for "passwd" postgres=> delete from passwd; ERROR: permission denied for relation passwd postgres=> insert into passwd (user_name) values ('xxx'); ERROR: permission denied for relation passwd -- Alice can change her own password; RLS silently prevents updating other rows postgres=> update passwd set pwhash = 'abc'; UPDATE 1
All of the policies constructed thus far have been permissive policies,
meaning that when multiple policies are applied they are combined using
the “OR” Boolean operator. While permissive policies can be constructed
to only allow access to rows in the intended cases, it can be simpler to
combine permissive policies with restrictive policies (which the records
must pass and which are combined using the “AND” Boolean operator).
Building on the example above, we add a restrictive policy to require
the administrator to be connected over a local Unix socket to access the
records of the passwd
table:
CREATE POLICY admin_local_only ON passwd AS RESTRICTIVE TO admin USING (pg_catalog.inet_client_addr() IS NULL);
We can then see that an administrator connecting over a network will not see any records, due to the restrictive policy:
=> SELECT current_user; current_user -------------- admin (1 row) => select inet_client_addr(); inet_client_addr ------------------ 127.0.0.1 (1 row) => SELECT current_user; current_user -------------- admin (1 row) => TABLE passwd; user_name | pwhash | uid | gid | real_name | home_phone | extra_info | home_dir | shell -----------+--------+-----+-----+-----------+------------+------------+----------+------- (0 rows) => UPDATE passwd set pwhash = NULL; UPDATE 0
Referential integrity checks, such as unique or primary key constraints and foreign key references, always bypass row security to ensure that data integrity is maintained. Care must be taken when developing schemas and row level policies to avoid “covert channel” leaks of information through such referential integrity checks.
In some contexts it is important to be sure that row security is
not being applied. For example, when taking a backup, it could be
disastrous if row security silently caused some rows to be omitted
from the backup. In such a situation, you can set the
row_security configuration parameter
to off
. This does not in itself bypass row security;
what it does is throw an error if any query's results would get filtered
by a policy. The reason for the error can then be investigated and
fixed.
In the examples above, the policy expressions consider only the current
values in the row to be accessed or updated. This is the simplest and
best-performing case; when possible, it's best to design row security
applications to work this way. If it is necessary to consult other rows
or other tables to make a policy decision, that can be accomplished using
sub-SELECT
s, or functions that contain SELECT
s,
in the policy expressions. Be aware however that such accesses can
create race conditions that could allow information leakage if care is
not taken. As an example, consider the following table design:
-- definition of privilege groups CREATE TABLE groups (group_id int PRIMARY KEY, group_name text NOT NULL); INSERT INTO groups VALUES (1, 'low'), (2, 'medium'), (5, 'high'); GRANT ALL ON groups TO alice; -- alice is the administrator GRANT SELECT ON groups TO public; -- definition of users' privilege levels CREATE TABLE users (user_name text PRIMARY KEY, group_id int NOT NULL REFERENCES groups); INSERT INTO users VALUES ('alice', 5), ('bob', 2), ('mallory', 2); GRANT ALL ON users TO alice; GRANT SELECT ON users TO public; -- table holding the information to be protected CREATE TABLE information (info text, group_id int NOT NULL REFERENCES groups); INSERT INTO information VALUES ('barely secret', 1), ('slightly secret', 2), ('very secret', 5); ALTER TABLE information ENABLE ROW LEVEL SECURITY; -- a row should be visible to/updatable by users whose security group_id is -- greater than or equal to the row's group_id CREATE POLICY fp_s ON information FOR SELECT USING (group_id <= (SELECT group_id FROM users WHERE user_name = current_user)); CREATE POLICY fp_u ON information FOR UPDATE USING (group_id <= (SELECT group_id FROM users WHERE user_name = current_user)); -- we rely only on RLS to protect the information table GRANT ALL ON information TO public;
Now suppose that alice
wishes to change the “slightly
secret” information, but decides that mallory
should not
be trusted with the new content of that row, so she does:
BEGIN; UPDATE users SET group_id = 1 WHERE user_name = 'mallory'; UPDATE information SET info = 'secret from mallory' WHERE group_id = 2; COMMIT;
That looks safe; there is no window wherein mallory
should be
able to see the “secret from mallory” string. However, there is
a race condition here. If mallory
is concurrently doing,
say,
SELECT * FROM information WHERE group_id = 2 FOR UPDATE;
and her transaction is in READ COMMITTED
mode, it is possible
for her to see “secret from mallory”. That happens if her
transaction reaches the information
row just
after alice
's does. It blocks waiting
for alice
's transaction to commit, then fetches the updated
row contents thanks to the FOR UPDATE
clause. However, it
does not fetch an updated row for the
implicit SELECT
from users
, because that
sub-SELECT
did not have FOR UPDATE
; instead
the users
row is read with the snapshot taken at the start
of the query. Therefore, the policy expression tests the old value
of mallory
's privilege level and allows her to see the
updated row.
There are several ways around this problem. One simple answer is to use
SELECT ... FOR SHARE
in sub-SELECT
s in row
security policies. However, that requires granting UPDATE
privilege on the referenced table (here users
) to the
affected users, which might be undesirable. (But another row security
policy could be applied to prevent them from actually exercising that
privilege; or the sub-SELECT
could be embedded into a security
definer function.) Also, heavy concurrent use of row share locks on the
referenced table could pose a performance problem, especially if updates
of it are frequent. Another solution, practical if updates of the
referenced table are infrequent, is to take an
ACCESS EXCLUSIVE
lock on the
referenced table when updating it, so that no concurrent transactions
could be examining old row values. Or one could just wait for all
concurrent transactions to end after committing an update of the
referenced table and before making changes that rely on the new security
situation.
For additional details see CREATE POLICY and ALTER TABLE.