Change default principal for krb5_set_password

For background, I'm trying to write a Kerberos PAM module with as little
code difference between Kerberos implementations as possible (since, among
other things, this makes it much easier to test).  I was previously always
using krb5_set_password to do password change, passing the client
principal in.  But with MIT Kerberos, passing in a client principal causes
it to use the new set password protocol rather than the old change
password protocol, which is not compatible with old KDCs.

The recommendation on the MIT side is to pass in NULL for the principal,
which causes it to fall back to the change password protocol.  But if one
passes NULL to the current Heimdal krb5_set_password function, it uses
krb5_get_default_principal to figure out the client principal.  This is
frequently going to be wrong in a PAM context.

krb5_set_password requires credentials for the password change service be
passed in, and those credentials are already associated with a client
principal.  I think this is a much better choice of default client
principal than krb5_get_default_principal.  The attached patch implements
that change, although I've not tested it.  (I haven't checked whether one
could take the easier route of not copying and freeing the principal and
instead just referencing the principal inside the credential structure.)

-- 
Russ Allbery ([email protected])             <http://www.eyrie.org/~eagle/>

From 22f00a87b45e25d6103ad798f574ade417a75d07 Mon Sep 17 00:00:00 2001
From: Russ Allbery <[email protected]>
Date: Thu, 22 Dec 2011 11:18:33 -0800
Subject: [PATCH 2/2] Set default principal for password change to credential
 client

The default client principal for krb5_set_password, if the principal
argument were NULL, was krb5_get_default_principal.  But krb5_set_password
requires credentials for the password change service be passed in, and
those credentials are already associated with a client principal that's
much more likely to be the correct choice for a default.  Use that
principal instead of krb5_get_default_principal.
---
 lib/krb5/changepw.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/lib/krb5/changepw.c b/lib/krb5/changepw.c
index 1e7cd0d..dee4e1c 100644
--- a/lib/krb5/changepw.c
+++ b/lib/krb5/changepw.c
@@ -744,7 +744,7 @@ krb5_set_password(krb5_context context,
     krb5_data_zero(result_string);
 
     if (targprinc == NULL) {
-	ret = krb5_get_default_principal(context, &principal);
+	ret = krb5_copy_principal(context, creds->client, &principal);
 	if (ret)
 	    return ret;
     } else
-- 
1.7.7.3