Inserts restricted to a trigger

On 6/17/19 4:54 PM, Miles Elam wrote:

Is there are way to restrict direct access to a table for inserts but
allow a trigger on another table to perform an insert for that user?

I'm trying to implement an audit table without allowing user tampering
with the audit information.

Would the below not work?:
CREATE the table as superuser or other privileged user
Have trigger function run as above user(use SECURITY DEFINER)

Thanks in advance,

Miles Elam

--
Adrian Klaver
adrian.klaver@aklaver.com

Adrian Klaver wrote:

On 6/17/19 4:54 PM, Miles Elam wrote:

Is there are way to restrict direct access to a table for inserts but
allow a trigger on another table to perform an insert for that user?

I'm trying to implement an audit table without allowing user tampering
with the audit information.

Would the below not work?:
CREATE the table as superuser or other privileged user
Have trigger function run as above user(use SECURITY DEFINER)

and make sure not to give any other users insert/update/delete
permissions on the audit table.

That seems straightforward. Unfortunately I also want to know the user/role
that performed the operation. If I use SECURITY DEFINER, I get the
superuser account back from CURRENT_USER, not the actual user.

Sorry, should have included that in the original email. How do I restrict
access while still retaining info about the current user/role?

On Mon, Jun 17, 2019 at 5:47 PM <raf@raf.org> wrote:

Have you tried session_user?

create function xx() returns table (cur text, sess text)
security definer language sql as $$
select current_user::text, session_user::text;
$$;

Then log in as different user and:

=> select (xx()).*;
cur | sess
----------+-------
postgres | write

On Tue, Jun 18, 2019 at 6:30 PM Miles Elam <miles.elam@productops.com>
wrote:

Thanks for the suggestion. Unfortunately we only have a single login role
(it's a web app) and then we SET ROLE according to the contents of a JSON
Web Token. So we end up with SESSION_USER as the logged in user and the
active role as CURRENT_USER.

It may be that we're just stuck with a gap and need to just try and keep
track of our mutation points, such as limit what is accessible through REST
or GraphQL, and there is no way to fundamentally lock this down in
Postgres. I was checking the mailing list to see if I'd missed anything.

On Tue, Jun 18, 2019 at 9:47 AM Torsten Förtsch <tfoertsch123@gmail.com>
wrote:

On 6/18/19 10:14 AM, Miles Elam wrote:

Thanks for the suggestion. Unfortunately we only have a single login
role (it's a web app) and then we SET ROLE according to the contents of
a JSON Web Token. So we end up with SESSION_USER as the logged in user
and the active role as CURRENT_USER.

Have not tried it but nested function?:

1) Outer function runs as normal user and grabs the CURRENT_USER. This
is passed into 2)

2) Audit function that runs with SECURITY DEFINER.

Other option is to record the CURRENT_USER in the table the trigger is
on and just pass that to the audit function.

It may be that we're just stuck with a gap and need to just try and keep
track of our mutation points, such as limit what is accessible through
REST or GraphQL, and there is no way to fundamentally lock this down in
Postgres. I was checking the mailing list to see if I'd missed anything.

On Tue, Jun 18, 2019 at 9:47 AM Torsten Förtsch <tfoertsch123@gmail.com
<mailto:tfoertsch123@gmail.com>> wrote:

Have you tried session_user?

create function xx() returns table (cur text, sess text)
security definer language sql as $$
    select current_user::text, session_user::text;
$$;

Then log in as different user and:

=> select (xx()).*;
   cur    | sess
----------+-------
 postgres | write

On Tue, Jun 18, 2019 at 6:30 PM Miles Elam
<miles.elam@productops.com <mailto:miles.elam@productops.com>> wrote:

That seems straightforward. Unfortunately I also want to know
the user/role that performed the operation. If I use SECURITY
DEFINER, I get the superuser account back from CURRENT_USER, not
the actual user.

Sorry, should have included that in the original email. How do I
restrict access while still retaining info about the current
user/role?

On Mon, Jun 17, 2019 at 5:47 PM <raf@raf.org
<mailto:raf@raf.org>> wrote:

Adrian Klaver wrote:

On 6/17/19 4:54 PM, Miles Elam wrote:

Is there are way to restrict direct access to a table

for inserts but

allow a trigger on another table to perform an insert

for that user?

I'm trying to implement an audit table without allowing

user tampering

with the audit information.

Would the below not work?:
CREATE the table as superuser or other privileged user
Have trigger function run as above user(use SECURITY DEFINER)

and make sure not to give any other users insert/update/delete
permissions on the audit table.

Thanks in advance,

Miles Elam

--
Adrian Klaver
adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>

--
Adrian Klaver
adrian.klaver@aklaver.com

Hi Adrian, thanks for responding.

How would I restrict access to the SECURITY DEFINER function? If it can be
called by the trigger, it can be called by the user as well I would think.
Same issue as access to the table itself only now with a superuser
intermediary, right?

On Tue, Jun 18, 2019 at 6:20 PM Adrian Klaver <adrian.klaver@aklaver.com>
wrote:

On 6/19/19 3:07 PM, Miles Elam wrote:

Hi Adrian, thanks for responding.

How would I restrict access to the SECURITY DEFINER function? If it can
be called by the trigger, it can be called by the user as well I would
think. Same issue as access to the table itself only now with a
superuser intermediary, right?

We may need to back this up:

1) What is the audit table recording?

2) How much access do your users have to the database itself, outside
the Web app?

3) What access do users have in the Web app?

On Tue, Jun 18, 2019 at 6:20 PM Adrian Klaver <adrian.klaver@aklaver.com
<mailto:adrian.klaver@aklaver.com>> wrote:

On 6/18/19 10:14 AM, Miles Elam wrote:

Thanks for the suggestion. Unfortunately we only have a single login
role (it's a web app) and then we SET ROLE according to the

contents of

a JSON Web Token. So we end up with SESSION_USER as the logged in

user

and the active role as CURRENT_USER.

Have not tried it but nested function?:

1) Outer function runs as normal user and grabs the CURRENT_USER. This
is passed into 2)

2) Audit function that runs with SECURITY DEFINER.

Other option is to record the CURRENT_USER in the table the trigger is
on and just pass that to the audit function.

It may be that we're just stuck with a gap and need to just try

and keep

track of our mutation points, such as limit what is accessible

through

REST or GraphQL, and there is no way to fundamentally lock this

down in

Postgres. I was checking the mailing list to see if I'd missed

anything.

On Tue, Jun 18, 2019 at 9:47 AM Torsten Förtsch

<tfoertsch123@gmail.com <mailto:tfoertsch123@gmail.com>

<mailto:tfoertsch123@gmail.com <mailto:tfoertsch123@gmail.com>>>

wrote:

     Have you tried session_user?

     create function xx() returns table (cur text, sess text)
     security definer language sql as $$
          select current_user::text, session_user::text;
     $$;

     Then log in as different user and:

     => select (xx()).*;
         cur    | sess
     ----------+-------
       postgres | write

     On Tue, Jun 18, 2019 at 6:30 PM Miles Elam
     <miles.elam@productops.com <mailto:miles.elam@productops.com>

<mailto:miles.elam@productops.com
<mailto:miles.elam@productops.com>>> wrote:

         That seems straightforward. Unfortunately I also want to know
         the user/role that performed the operation. If I use SECURITY
         DEFINER, I get the superuser account back from

CURRENT_USER, not

         the actual user.

         Sorry, should have included that in the original email.

How do I

         restrict access while still retaining info about the current
         user/role?

         On Mon, Jun 17, 2019 at 5:47 PM <raf@raf.org

<mailto:raf@raf.org>

         <mailto:raf@raf.org <mailto:raf@raf.org>>> wrote:

             Adrian Klaver wrote:

              > On 6/17/19 4:54 PM, Miles Elam wrote:
              > > Is there are way to restrict direct access to a

table

             for inserts but
              > > allow a trigger on another table to perform an

insert

             for that user?
              > >
              > > I'm trying to implement an audit table without

allowing

             user tampering
              > > with the audit information.
              >
              > Would the below not work?:
              > CREATE the table as superuser or other privileged user
              > Have trigger function run as above user(use

SECURITY DEFINER)

             and make sure not to give any other users

insert/update/delete

             permissions on the audit table.

              > > Thanks in advance,
              > >
              > > Miles Elam
              >
              > --
              > Adrian Klaver
              > adrian.klaver@aklaver.com

<mailto:adrian.klaver@aklaver.com> <mailto:adrian.klaver@aklaver.com
<mailto:adrian.klaver@aklaver.com>>

--
Adrian Klaver
adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>

--
Adrian Klaver
adrian.klaver@aklaver.com

On 6/19/19 3:07 PM, Miles Elam wrote:

Hi Adrian, thanks for responding.

How would I restrict access to the SECURITY DEFINER function? If it can
be called by the trigger, it can be called by the user as well I would
think. Same issue as access to the table itself only now with a
superuser intermediary, right?

Should have also mentioned, if you are not adverse to a third party
solution there is PGAudit:

https://www.pgaudit.org/

--
Adrian Klaver
adrian.klaver@aklaver.com

Thanks for the reply, Adrian.

1. The audit tables (plural) are recording the historical data for a table,
ie., updates and deletes. All the same data as found in a given table along
with the role that performed the operation, the transaction id, and the
time range where this data was in active use.

2. Only thorough a web UI via an API service.

3. Should be limited to web app, but the data scientists may need direct
access in the near future.

PGAudit does not cover our use case. We are making a temporal table system
since PostgreSQL does not support one natively. For example: "What would
this query have returned yesterday at 4:27pm PT?" Access is as expected for
inserts but updates and deletes are logged to history tables. We cannot use
3rd party extensions because we are on AWS managed databases. We are
following the model detailed here (
https://wiki.postgresql.org/wiki/SQL2011Temporal) with some modifications.

Given the model listed in the link, it's not clear how we can prevent user
tampering with history inserts. (History updates and deletes are already
REVOKE restricted.) Since we are going through an API server via REST
and/or GraphQL, the possibility is very unlikely, but we would prefer a
defense in depth approach in case an oversight somehow allowed arbitrary
query access to the database with the web user. For the most part, we're
fairly well locked down, but I just can't quite see how to restrict
aforementioned query access from inserting to the history in an ad-hoc
manner rather than the trigger-based predetermined insert pattern.

On Thu, Jun 20, 2019 at 8:01 AM Adrian Klaver <adrian.klaver@aklaver.com>
wrote:

On 6/20/19 3:30 PM, Miles Elam wrote:

Thanks for the reply, Adrian.

1. The audit tables (plural) are recording the historical data for a
table, ie., updates and deletes. All the same data as found in a given
table along with the role that performed the operation, the transaction
id, and the time range where this data was in active use.

2. Only thorough a web UI via an API service.

3. Should be limited to web app, but the data scientists may need direct
access in the near future.

PGAudit does not cover our use case. We are making a temporal table
system since PostgreSQL does not support one natively. For example:
"What would this query have returned yesterday at 4:27pm PT?" Access is
as expected for inserts but updates and deletes are logged to history
tables. We cannot use 3rd party extensions because we are on AWS managed
databases. We are following the model detailed here
(https://wiki.postgresql.org/wiki/SQL2011Temporal) with some modifications.

Given the model listed in the link, it's not clear how we can prevent
user tampering with history inserts. (History updates and deletes are
already REVOKE restricted.) Since we are going through an API server via
REST and/or GraphQL, the possibility is very unlikely, but we would
prefer a defense in depth approach in case an oversight somehow allowed
arbitrary query access to the database with the web user. For the most
part, we're fairly well locked down, but I just can't quite see how to
restrict aforementioned query access from inserting to the history in an
ad-hoc manner rather than the trigger-based predetermined insert pattern.

Some draft ideas:

1) A session table that among other things records the Web Token/user
combination. Then in the trigger(s) that INSERT into the history table
check that the user has a valid current token.

2) In the inner function use PG_CONTEXT:

https://www.postgresql.org/docs/11/plpgsql-control-structures.html#PLPGSQL-CALL-STACK

to determine whether the inner SECURITY DEFINER function is being called
directly or through the outer trigger function.

On Thu, Jun 20, 2019 at 8:01 AM Adrian Klaver <adrian.klaver@aklaver.com
<mailto:adrian.klaver@aklaver.com>> wrote:

On 6/19/19 3:07 PM, Miles Elam wrote:

Hi Adrian, thanks for responding.

How would I restrict access to the SECURITY DEFINER function? If

it can

be called by the trigger, it can be called by the user as well I

would

think. Same issue as access to the table itself only now with a
superuser intermediary, right?

Should have also mentioned, if you are not adverse to a third party
solution there is PGAudit:

https://www.pgaudit.org/

--
Adrian Klaver
adrian.klaver@aklaver.com <mailto:adrian.klaver@aklaver.com>

--
Adrian Klaver
adrian.klaver@aklaver.com