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Improve the CREATE POLICY documentation.

Browse Source 
Provide a correct description of how multiple policies are combined,
clarify when SELECT permissions are required, mention SELECT FOR
UPDATE/SHARE, and do some other more minor tidying up.

Reviewed by Stephen Frost

Discussion: https://postgr.es/m/CAEZATCVrxyYbOFU8XbGHicz%2BmXPYzw%3DhfNL2XTphDt-53TomQQ%40mail.gmail.com

Back-patch to 9.5.
This commit is contained in:
Dean Rasheed 2017-09-27 17:13:37 +01:00
parent bfd5515702
commit 4d5d08c1cd
1 changed files with 134 additions and 72 deletions
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206
doc/src/sgml/ref/create_policy.sgml
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@ -73,20 +73,17 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
<para>
Policies can be applied for specific commands or for specific roles. The
default for newly created policies is that they apply for all commands and
roles, unless otherwise specified. If multiple policies apply to a given
statement, they will be combined using OR (although <literal>ON CONFLICT DO
UPDATE</> and <literal>INSERT</> policies are not combined in this way, but
rather enforced as noted at each stage of <literal>ON CONFLICT</> execution).
roles, unless otherwise specified.
</para>
<para>
For commands that can have both <literal>USING</literal>
and <literal>WITH CHECK</literal> policies (<literal>ALL</literal>
For policies that can have both <literal>USING</literal>
and <literal>WITH CHECK</literal> expressions (<literal>ALL</literal>
and <literal>UPDATE</literal>), if no <literal>WITH CHECK</literal>
policy is defined, then the <literal>USING</literal> policy will be used
both for which rows are visible (normal <literal>USING</literal> case)
and for which rows will be allowed to be added (<literal>WITH
CHECK</literal> case).
expression is defined, then the <literal>USING</literal> expression will be
used both to determine which rows are visible (normal
<literal>USING</literal> case) and which new rows will be allowed to be
added (<literal>WITH CHECK</literal> case).
</para>
<para>
@ -144,6 +141,16 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
which can be accessed as all restrictive policies must be passed for
each record.
</para>
<para>
Note that there needs to be at least one permissive policy to grant
access to records before restrictive policies can be usefully used to
reduce that access. If only restrictive policies exist, then no records
will be accessible. When a mix of permissive and restrictive policies
are present, a record is only accessible if at least one of the
permissive policies passes, in addition to all the restrictive
policies.
</para>
</listitem>
</varlistentry>
@ -210,7 +217,7 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
</variablelist>
<refsect2>
<refsect2>
<title>Per-Command Policies</title>
<variablelist>
@ -223,8 +230,7 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
to all commands, regardless of the type of command. If an
<literal>ALL</literal> policy exists and more specific policies
exist, then both the <literal>ALL</literal> policy and the more
specific policy (or policies) will be combined using
OR, as usual for overlapping policies.
specific policy (or policies) will be applied.
Additionally, <literal>ALL</literal> policies will be applied to
both the selection side of a query and the modification side, using
the <literal>USING</literal> expression for both cases if only
@ -293,11 +299,12 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
<listitem>
<para>
Using <literal>UPDATE</literal> for a policy means that it will apply
to <literal>UPDATE</literal> commands (or auxiliary <literal>ON
CONFLICT DO UPDATE</literal> clauses of <literal>INSERT</literal>
commands). Since <literal>UPDATE</literal> involves pulling an
existing record and then making changes to some portion (but
possibly not all) of the record, <literal>UPDATE</literal>
to <literal>UPDATE</literal>, <literal>SELECT FOR UPDATE</literal>
and <literal>SELECT FOR SHARE</literal> commands, as well as
auxiliary <literal>ON CONFLICT DO UPDATE</literal> clauses of
<literal>INSERT</literal> commands. Since <literal>UPDATE</literal>
involves pulling an existing record and replacing it with a new
modified record, <literal>UPDATE</literal>
policies accept both a <literal>USING</literal> expression and
a <literal>WITH CHECK</literal> expression.
The <literal>USING</literal> expression determines which records
@ -306,22 +313,6 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
modified rows are allowed to be stored back into the relation.
</para>
<para>
When an <literal>UPDATE</literal> command is used with a
<literal>WHERE</literal> clause or a <literal>RETURNING</literal>
clause, <literal>SELECT</literal> rights are also required on the
relation being updated and the appropriate <literal>SELECT</literal>
and <literal>ALL</literal> policies will be combined (using OR for any
overlapping <literal>SELECT</literal> related policies found) with the
<literal>USING</literal> clause of the <literal>UPDATE</literal> policy
using AND. Therefore, in order for a user to be able to
<literal>UPDATE</literal> specific rows, the user must have access
to the row(s) through a <literal>SELECT</literal>
or <literal>ALL</literal> policy and the row(s) must pass
the <literal>UPDATE</literal> policy's <literal>USING</>
expression.
</para>
<para>
Any rows whose updated values do not pass the
<literal>WITH CHECK</literal> expression will cause an error, and the
@ -331,21 +322,33 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
</para>
<para>
Note, however, that <literal>INSERT</literal> with <literal>ON CONFLICT
DO UPDATE</literal> requires that an <literal>UPDATE</literal> policy
<literal>USING</literal> expression always be enforced as a
<literal>WITH CHECK</literal> expression. This
<literal>UPDATE</literal> policy must always pass when the
<literal>UPDATE</literal> path is taken. Any existing row that
necessitates that the <literal>UPDATE</literal> path be taken must
pass the (<literal>UPDATE</literal> or <literal>ALL</literal>)
<literal>USING</literal> qualifications (combined using OR), which
are always enforced as <literal>WITH CHECK</literal>
options in this context. (The <literal>UPDATE</literal> path will
<emphasis>never</> be silently avoided; an error will be thrown
instead.) Finally, the final row appended to the relation must pass
any <literal>WITH CHECK</literal> options that a conventional
<literal>UPDATE</literal> is required to pass.
Typically an <literal>UPDATE</literal> command also needs to read
data from columns in the relation being updated (e.g., in a
<literal>WHERE</literal> clause or a <literal>RETURNING</literal>
clause, or in an expression on the right hand side of the
<literal>SET</literal> clause). In this case,
<literal>SELECT</literal> rights are also required on the relation
being updated, and the appropriate <literal>SELECT</literal> or
<literal>ALL</literal> policies will be applied in addition to
the <literal>UPDATE</literal> policies. Thus the user must have
access to the row(s) being updated through a <literal>SELECT</literal>
or <literal>ALL</literal> policy in addition to being granted
permission to update the row(s) via an <literal>UPDATE</literal>
or <literal>ALL</literal> policy.
</para>
<para>
When an <literal>INSERT</literal> command has an auxiliary
<literal>ON CONFLICT DO UPDATE</literal> clause, if the
<literal>UPDATE</literal> path is taken, the row to be updated is
first checked against the <literal>USING</literal> expressions of
any <literal>UPDATE</literal> policies, and then the new updated row
is checked against the <literal>WITH CHECK</literal> expressions.
Note, however, that unlike a standalone <literal>UPDATE</literal>
command, if the existing row does not pass the
<literal>USING</literal> expressions, an error will be thrown (the
<literal>UPDATE</literal> path will <emphasis>never</> be silently
avoided).
</para>
</listitem>
</varlistentry>
@ -364,19 +367,18 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
</para>
<para>
When a <literal>DELETE</literal> command is used with a
<literal>WHERE</literal> clause or a <literal>RETURNING</literal>
clause, <literal>SELECT</literal> rights are also required on the
relation being updated and the appropriate <literal>SELECT</literal>
and <literal>ALL</literal> policies will be combined (using OR for any
overlapping <literal>SELECT</literal> related policies found) with the
<literal>USING</literal> clause of the <literal>DELETE</literal> policy
using AND. Therefore, in order for a user to be able to
<literal>DELETE</literal> specific rows, the user must have access
to the row(s) through a <literal>SELECT</literal>
or <literal>ALL</literal> policy and the row(s) must pass
the <literal>DELETE</literal> policy's <literal>USING</>
expression.
In most cases a <literal>DELETE</literal> command also needs to read
data from columns in the relation that it is deleting from (e.g.,
in a <literal>WHERE</literal> clause or a
<literal>RETURNING</literal> clause). In this case,
<literal>SELECT</literal> rights are also required on the relation,
and the appropriate <literal>SELECT</literal> or
<literal>ALL</literal> policies will be applied in addition to
the <literal>DELETE</literal> policies. Thus the user must have
access to the row(s) being deleted through a <literal>SELECT</literal>
or <literal>ALL</literal> policy in addition to being granted
permission to delete the row(s) via a <literal>DELETE</literal> or
<literal>ALL</literal> policy.
</para>
<para>
@ -390,6 +392,76 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
</variablelist>
</refsect2>
<refsect2>
<title>Application of Multiple Policies</title>
<para>
When multiple policies of different command types apply to the same command
(for example, <literal>SELECT</literal> and <literal>UPDATE</literal>
policies applied to an <literal>UPDATE</literal> command), then the user
must have both types of permissions (for example, permission to select rows
from the relation as well as permission to update them). Thus the
expressions for one type of policy are combined with the expressions for
the other type of policy using the <literal>AND</literal> operator.
</para>
<para>
When multiple policies of the same command type apply to the same command,
then there must be at least one <literal>PERMISSIVE</literal> policy
granting access to the relation, and all of the
<literal>RESTRICTIVE</literal> policies must pass. Thus all the
<literal>PERMISSIVE</literal> policy expressions are combined using
<literal>OR</literal>, all the <literal>RESTRICTIVE</literal> policy
expressions are combined using <literal>AND</literal>, and the results are
combined using <literal>AND</literal>. If there are no
<literal>PERMISSIVE</literal> policies, then access is denied.
</para>
<para>
Note that, for the purposes of combining multiple policies,
<literal>ALL</literal> policies are treated as having the same type as
whichever other type of policy is being applied.
</para>
<para>
For example, in an <literal>UPDATE</literal> command requiring both
<literal>SELECT</literal> and <literal>UPDATE</literal> permissions, if
there are multiple applicable policies of each type, they will be combined
as follows:
<programlisting>
<replaceable>expression</replaceable> from RESTRICTIVE SELECT/ALL policy 1
AND
<replaceable>expression</replaceable> from RESTRICTIVE SELECT/ALL policy 2
AND
...
AND
(
<replaceable>expression</replaceable> from PERMISSIVE SELECT/ALL policy 1
OR
<replaceable>expression</replaceable> from PERMISSIVE SELECT/ALL policy 2
OR
...
)
AND
<replaceable>expression</replaceable> from RESTRICTIVE UPDATE/ALL policy 1
AND
<replaceable>expression</replaceable> from RESTRICTIVE UPDATE/ALL policy 2
AND
...
AND
(
<replaceable>expression</replaceable> from PERMISSIVE UPDATE/ALL policy 1
OR
<replaceable>expression</replaceable> from PERMISSIVE UPDATE/ALL policy 2
OR
...
)
</programlisting>
</para>
</refsect2>
</refsect1>
<refsect1>
@ -418,16 +490,6 @@ CREATE POLICY <replaceable class="parameter">name</replaceable> ON <replaceable
keys) instead of keys with external meanings.
</para>
<para>
Note that there needs to be at least one permissive policy to grant
access to records before restrictive policies can be usefully used to
reduce that access. If only restrictive policies exist, then no records
will be accessible. When a mix of permissive and restrictive policies
are present, a record is only accessible if at least one of the
permissive policies passes, in addition to all the restrictive
policies.
</para>
<para>
Generally, the system will enforce filter conditions imposed using
security policies prior to qualifications that appear in user queries,