Don't let authenticated client use arbitrary sender addresses.

The following policy is now implemented:

    * users can use their SASL login name as sender address;
    * alias and/or list owners can use the address as envelope sender;
    * domain postmasters can use arbitrary sender addresses under their
      domains;
    * domain owners can use arbitrary sender addresses under their domains,
      unless it is also an existing account name;
    * for known domains without owner or postmasters, other sender addresses
      are not allowed; and
    * arbitrary sender addresses under unknown domains are allowed.

1 files changed, 171 insertions, 0 deletions

diff --git a/roles/MSA/files/usr/local/bin/postfix-sender-login.pl b/roles/MSA/files/usr/local/bin/postfix-sender-login.pl
new file mode 100755
index 0000000..58cbf9d
--- /dev/null
+++ b/roles/MSA/files/usr/local/bin/postfix-sender-login.pl
@@ -0,0 +1,171 @@
+#!/usr/bin/perl -T
+
+#----------------------------------------------------------------------
+# socketmap lookup table returning the SASL login name(s) owning a given
+# sender address
+# Copyright © 2017 Guilhem Moulin <guilhem@fripost.org>
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#----------------------------------------------------------------------
+
+use warnings;
+use strict;
+
+use Errno 'EINTR';
+use Socket qw/PF_UNIX SOCK_STREAM SHUT_RDWR/;
+
+use Net::LDAPI ();
+use Net::LDAP::Util qw/ldap_explode_dn escape_dn_value escape_filter_value/;
+use Net::LDAP::Constant qw/LDAP_NO_SUCH_OBJECT/;
+use Authen::SASL ();
+
+# clean up PATH
+$ENV{PATH} = join ':', qw{/usr/bin /bin};
+delete @ENV{qw/IFS CDPATH ENV BASH_ENV/};
+
+# returned for forbidden envelope sender addresses
+my $POSTMASTER = 'postmaster@fripost.org';
+
+# fdopen(3) the file descriptor FD
+die "This service must be socket-activated.\n"
+    unless defined $ENV{LISTEN_PID} and $ENV{LISTEN_PID} == $$
+       and defined $ENV{LISTEN_FDS} and $ENV{LISTEN_FDS} == 1;
+open my $S, '+<&=', 3 or die "fdopen: $!";
+
+my $BASEDN  = 'ou=virtual,dc=fripost,dc=org';
+my $BUFSIZE = 65536; # try to read that many bytes at the time
+my $LDAPI   = 'ldapi://%2Fvar%2Fspool%2Fpostfix-msa%2Fprivate%2Fldapi/';
+sub process_request($);
+
+while(1) {
+    accept(my $conn, $S) or do {
+        # try again if accept(2) was interrupted by a signal
+        next if $! == EINTR;
+        die "accept: $!";
+    };
+    my $reply = process_request($conn);
+
+    # encode the reply as a netstring and send it back
+    # https://cr.yp.to/proto/netstrings.txt 
+    $reply = length($reply).':'.$reply.',';
+    my $len = length($reply);
+
+    for (my $i = 0; $i < $len;) {
+        my $n = syswrite($conn, $reply, $len-$i, $i) // do {
+            warn "Can't write: $!";
+            last;
+        };
+        $i += $n;
+    }
+    close $conn or warn "Can't close: $!";
+}
+
+#############################################################################
+
+sub process_request($) {
+    my $conn = shift;
+    my ($buf, $offset) = (undef, 0);
+
+    # keep reading until the request length is determined
+    do {
+        my $n = sysread($conn, $buf, $BUFSIZE, $offset) // return "TEMP can't read: $!";
+        return "TEMP EOF" if $n == 0;
+        $offset += $n;
+    } until ($buf =~ /\A(0|[1-9][0-9]*):/);
+
+    # keep reading until the whole request is buffered
+    my $strlen = length("$1") + 1; # [len]":"
+    my $len = $strlen + $1 + 1;    # [len]":"[string]","
+    while ($offset < $len) {
+        my $n = sysread($conn, $buf, $BUFSIZE, $offset) // return "TEMP can't read: $!";
+        return "TEMP EOF" if $n == 0;
+        $offset += $n;
+    }
+
+    # requests are of the form $name <space> $key, cf. socketmap_table(5)
+    my $i = index($buf, ' ', $strlen);
+    return "TEMP invalid input: $buf" unless $i > $strlen and substr($buf,-1) eq ',';
+    my $name = substr($buf, $strlen, $i-$strlen);
+    my $key = substr($buf, $i, -1);
+    return "TEMP invalid name: $name" unless $name eq 'sender_login';
+
+    $key =~ /\A(.+)@([^\@]+)\z/ or return "NOTFOUND "; # invalid sender address
+    my ($localpart, $domainpart) = ($1, $2);
+
+    my $ldap = Net::LDAPI::->new( $LDAPI )
+        // return "TEMP couldn't create Net::LDAPI object";
+    $ldap->bind( undef, sasl => Authen::SASL::->new(mechanism => 'EXTERNAL') )
+        or return "TEMP LDAP: couldn't bind";
+
+    my $reply = lookup_sender($ldap, $localpart, $domainpart);
+    $ldap->unbind();
+    return $reply;
+}
+
+sub lookup_sender($$$) {
+    my ($ldap, $l, $d) = @_;
+
+    my $filter = '(&(objectClass=FripostVirtualDomain)(fvd='.escape_filter_value($d).'))';
+    my $mesg = $ldap->search( base => $BASEDN, scope => 'one', deref => 'never'
+                            , filter => $filter
+                            , attrs => [qw/objectClass fripostOwner fripostPostmaster/]
+                            );
+    return "TEMP LDAP error: ".$mesg->error() if $mesg->code;
+    my $entry = $mesg->pop_entry() // return "NOTFOUND "; # not a domain we know
+    return "TEMP LDAP error: multiple entry founds" if defined $mesg->pop_entry(); # sanity check
+
+    # domain postmasters are allowed to use any sender address 
+    my @logins = $entry->get_value('fripostPostmaster', asref => 0);
+    my @owners = $entry->get_value('fripostOwner', asref => 0);
+
+    if (grep { $_ eq 'FripostVirtualAliasDomain' } $entry->get_value('objectClass', asref => 0)) {
+        # so are alias domain owners
+        push @logins, @owners;
+    } else {
+        my $dn = 'fvd='.escape_dn_value($d).','.$BASEDN;
+        my $filter = '(&(|(objectClass=FripostVirtualAlias)(objectClass=FripostVirtualList)(objectClass=FripostVirtualUser))(fvl='.escape_filter_value($l).'))';
+        my $mesg = $ldap->search( base => $dn, scope => 'one', deref => 'never'
+                                , filter => $filter
+                                , attrs => [qw/objectClass fripostOwner/]
+                                );
+        unless ($mesg->code == 0 and defined ($entry = $mesg->pop_entry())) {
+            # domains owners are allowed to use any unkwown localpart as sender address
+            push @logins, @owners;
+        } else {
+            return "TEMP LDAP error: multiple entry founds" if defined $mesg->pop_entry(); # sanity check
+            if (grep { $_ eq 'FripostVirtualUser' } $entry->get_value('objectClass', asref => 0)) {
+                push @logins, $entry->dn();
+            } else {
+                # alias/list owners can use the address as sender, and so are the domains owners
+                push @logins, @owners, $entry->get_value('fripostOwner', asref => 0);
+            }
+        }
+    }
+
+    # convert DNs to SASL login names
+    my %logins;
+    foreach my $dn (@logins) {
+        next unless defined $dn;
+        $dn = ldap_explode_dn($dn, casefold => 'lower');
+        next unless defined $dn and $#$dn == 4;
+        my $l = $dn->[0]->{fvl} // next;
+        my $d = $dn->[1]->{fvd} // next;
+        $logins{$l.'@'.$d} = 1;
+    }
+
+    # if the entry is found in LDAP but doesn't have an owner, only
+    # $POSTMASTER is allowed to use it as sender address
+    my $reply = %logins ? join(',', keys %logins) : $POSTMASTER;
+    return "OK $reply";
+}