Block environment variable mutations from trusted PL/Perl.

Many process environment variables (e.g. PATH), bypass the containment
expected of a trusted PL.  Hence, trusted PLs must not offer features
that achieve setenv().  Otherwise, an attacker having USAGE privilege on
the language often can achieve arbitrary code execution, even if the
attacker lacks a database server operating system user.

To fix PL/Perl, replace trusted PL/Perl %ENV with a tied hash that just
replaces each modification attempt with a warning.  Sites that reach
these warnings should evaluate the application-specific implications of
proceeding without the environment modification:

  Can the application reasonably proceed without the modification?

    If no, switch to plperlu or another approach.

    If yes, the application should change the code to stop attempting
    environment modifications.  If that's too difficult, add "untie
    %main::ENV" in any code executed before the warning.  For example,
    one might add it to the start of the affected function or even to
    the plperl.on_plperl_init setting.

In passing, link to Perl's guidance about the Perl features behind the
security posture of PL/Perl.

Back-patch to v12 (all supported versions).

Andrew Dunstan and Noah Misch

Security: CVE-2024-10979

doc/src/sgml/plperl.sgml

@@ -1093,6 +1093,19 @@ $$ LANGUAGE plperl;
    be permitted to use this language.
   </para>
 
+  <warning>
+   <para>
+    Trusted PL/Perl relies on the Perl <literal>Opcode</literal> module to
+    preserve security.
+    Perl
+    <ulink url="https://perldoc.perl.org/Opcode#WARNING">documents</ulink>
+    that the module is not effective for the trusted PL/Perl use case.  If
+    your security needs are incompatible with the uncertainty in that warning,
+    consider executing <literal>REVOKE USAGE ON LANGUAGE plperl FROM
+    PUBLIC</literal>.
+   </para>
+  </warning>
+
   <para>
    Here is an example of a function that will not work because file
    system operations are not allowed for security reasons: