Re-enabling SET ROLE in security definer functions

Hi All,

We are seeking to enable SET ROLE in security-definer functions, since @
D.E Shaw there are scripts from the past that used this feature, and I think
you'd also agree that SET ROLE is security definer functions has it's uses.

As the code stands right now, I see that the only concern against
enabling SET ROLE in SecDef functions is that, that when the local-user-id
change context is exited, the GUC might be left out of sync.

We currently have two bits indicating separately whether we are in
context where (i) the CurrentUserId has changed, or (ii) Security concerns
do not allow certain operation. But we have only one flag for GUC that stops
us from performing any of SET ROLE or SET SESSION AUTHORIZATION while any of
the above two flags are set.

I propose that we have a separate GUC flag to indicate whether we are in
UserId-changed context. So, we disallow SET ROLE only when we are in
Security-Restricted context, and disallow SET SESSION AUTHORIZATION when we
are either in Security-Restricted context or in UserId-changed context.

So SET ROLE would be prohibited in maintenance operations, but allowed
in SecDef functions (only if they are not invoked on a stack where
maintenance operation was initiated earlier). And SET SESSION AUTHORIZATION
will be disallowed when we are in either of the UserId-changed or
Security-Restricted contexts.

To address the problem of GUC getting out of sync when a SecDef function
is exited, we can perform a check at the end, just before reverting to the
calling userid, that if the called function stack has used SET ROLE to
change the CurrentUserId, then we keep that user id to be in sync with GUC,
rather than sync the GUC with current settings. This keeps the current
semantics of GUC where if the called function (whether SecDef or not) used
SET to change a GUC parameter, then that setting prevails even after the
function has exited successfully.

Attaching patch to implement the above proposals.

I have given some thought to nesting of such call scenarios, and haven't
found one which could cause an issue with this approach. Hope I haven't
overlooked something.

NB: In the patch, the block surrounded with
"if(InSecurityRestrictedOperation())" in guc.c will never be called, since
the GUC parameter that it applies to (session_auth) is also marked as not
allowed while in UserId-changed context, and that condition is cecked in
previous block of code. This can be remedied by swapping the two relevant
"if" blocks. I did not do it to keep the patch simple and small.

Best regards,

PS: For some context, this started with an aim to enable SET ROLE command in
security definer functions, which D.E Shaw needed. This command is still not
enabled in SecDef functions, but it led to a security exposure followed by
the security fix; commit id: 31d0bddf77b9e2b5581816aa96d3a3
92ab7d8543.
See also:
http://gurjeet-tech.blogspot.com/2009/12/conversation-on-fixing-security-issue.html

On Sat, Dec 12, 2009 at 9:00 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Gurjeet Singh <gurjeet(dot)singh(at)enterprisedb(dot)com> writes:
> > SET ROLE is safe in any context since it can be used to switch to only to
> > those roles that the Session User is a member of, whereas SET SESSION
> > AUTHORIZATION is unsafe since it can be used to switch to any role in the
> > cluster iff the Authenticated User is a superuser.
>
> Maybe you had better read that statement again, and remember that the
> session user is typically a superuser in exactly the cases we are
> concerned about.
>
> regards, tom lane
>

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