From 5ae9bc98f23aeaa2ce17debe5a9b0cf1130e54ed Mon Sep 17 00:00:00 2001
From: Greg Hudson <ghudson@mit.edu>
Date: Mon, 7 Jun 2021 13:27:29 -0400
Subject: [PATCH] Fix some principal realm canonicalization cases

The no_hostrealm and subst_defrealm flags in struct canonprinc were
only applied when dns_canonicalize_hostname=fallback; in the other
cases, the initial krb5_sname_to_principal() result is treated as
canonical.  For no_hostrealm this limitation doesn't currently matter,
because all uses pass a principal with no realm as input.  However,
subst_defrealm is used to convert the referral realm to the default
realm in krb5_get_init_creds_keytab(), krb5_cc_cache_match(), and
gss_acquire_cred() when it needs to check the desired name against a
specified ccache.

In k5_canonprinc(), if the input principal is a
krb5_sname_to_principal() result and fallback isn't in effect, apply
subst_defrealm.  Document in os-proto.h that no_hostrealm doesn't
remove an existing realm and that krb5_sname_to_principal() may
already have looked one up.

ticket: 9011 (new)
(cherry picked from commit c077d0c6430c4ac163443aacc03d14d206a4cbb8)
---
 src/lib/krb5/os/os-proto.h | 13 +++++++++----
 src/lib/krb5/os/sn2princ.c | 24 +++++++++++++++++++++---
 2 files changed, 30 insertions(+), 7 deletions(-)

diff --git a/src/lib/krb5/os/os-proto.h b/src/lib/krb5/os/os-proto.h
index 7d5e7978f..a985f2aec 100644
--- a/src/lib/krb5/os/os-proto.h
+++ b/src/lib/krb5/os/os-proto.h
@@ -85,10 +85,15 @@ struct sendto_callback_info {
 
 /*
  * Initialize with all zeros except for princ.  Set no_hostrealm to disable
- * host-to-realm lookup, which ordinarily happens after canonicalizing the host
- * part.  Set subst_defrealm to substitute the default realm for the referral
- * realm after realm lookup (this has no effect if no_hostrealm is set).  Free
- * with free_canonprinc() when done.
+ * host-to-realm lookup, which ordinarily happens during fallback processing
+ * after canonicalizing the host part.  Set subst_defrealm to substitute the
+ * default realm for the referral realm after realm lookup.  Do not set both
+ * flags.  Free with free_canonprinc() when done.
+ *
+ * no_hostrealm only applies if fallback processing is in use
+ * (dns_canonicalize_hostname = fallback).  It will not remove the realm if
+ * krb5_sname_to_principal() already canonicalized the hostname and looked up a
+ * realm.  subst_defrealm applies whether or not fallback processing is in use.
  */
 struct canonprinc {
     krb5_const_principal princ;
diff --git a/src/lib/krb5/os/sn2princ.c b/src/lib/krb5/os/sn2princ.c
index c99b7da17..93c155932 100644
--- a/src/lib/krb5/os/sn2princ.c
+++ b/src/lib/krb5/os/sn2princ.c
@@ -271,18 +271,36 @@ krb5_error_code
 k5_canonprinc(krb5_context context, struct canonprinc *iter,
               krb5_const_principal *princ_out)
 {
+    krb5_error_code ret;
     int step = ++iter->step;
 
     *princ_out = NULL;
 
-    /* If we're not doing fallback, the input principal is canonical. */
-    if (context->dns_canonicalize_hostname != CANONHOST_FALLBACK ||
-        iter->princ->type != KRB5_NT_SRV_HST || iter->princ->length != 2 ||
+    /* If the hostname isn't from krb5_sname_to_principal(), the input
+     * principal is canonical. */
+    if (iter->princ->type != KRB5_NT_SRV_HST || iter->princ->length != 2 ||
         iter->princ->data[1].length == 0) {
         *princ_out = (step == 1) ? iter->princ : NULL;
         return 0;
     }
 
+    /* If we're not doing fallback, the hostname is canonical, but we may need
+     * to substitute the default realm. */
+    if (context->dns_canonicalize_hostname != CANONHOST_FALLBACK) {
+        if (step > 1)
+            return 0;
+        iter->copy = *iter->princ;
+        if (iter->subst_defrealm && iter->copy.realm.length == 0) {
+            ret = krb5_get_default_realm(context, &iter->realm);
+            if (ret)
+                return ret;
+            iter->copy = *iter->princ;
+            iter->copy.realm = string2data(iter->realm);
+        }
+        *princ_out = &iter->copy;
+        return 0;
+    }
+
     /* Canonicalize without DNS at step 1, with DNS at step 2. */
     if (step > 2)
         return 0;