From b4c3ce6fb9b2aebbbe7d802ce48c691a9cabcf4f Mon Sep 17 00:00:00 2001
From: Nicolas Williams <nico@twosigma.com>
Date: Wed, 10 Mar 2021 16:49:04 -0600
Subject: [PATCH 1/2] CVE-2022-44640 HEIMDAL: asn1: Invalid free in ASN.1 codec

This is a 10.0 on the Common Vulnerability Scoring System (CVSS) v3.

Heimdal's ASN.1 compiler generates code that allows specially
crafted DER encodings of CHOICEs to invoke the wrong free function
on the decoded structure upon decode error.  This is known to impact
the Heimdal KDC, leading to an invalid free() of an address partly
or wholly under the control of the attacker, in turn leading to a
potential remote code execution (RCE) vulnerability.

This error affects the DER codec for all CHOICE types used in
Heimdal, though not all cases will be exploitable.  We have not
completed a thorough analysis of all the Heimdal components
affected, thus the Kerberos client, the X.509 library, and other
parts, may be affected as well.

This bug has been in Heimdal since 2005.  It was first reported by
Douglas Bagnall, though it had been found independently by the
Heimdal maintainers via fuzzing a few weeks earlier.

While no zero-day exploit is known, such an exploit will likely be
available soon after public disclosure.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14929

[abartlet@samba.org Adapted from Heimdal commit
ea5ec8f174920cb80ce2b168b49195378420449e for older Heimdal in Samba 4.15
by dropping fuzz-inputs file and EXPORTS entry for fuzzing]

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>

Conflict: NA
Reference: https://attachments.samba.org/attachment.cgi?id=17679
---
 source4/heimdal/lib/asn1/gen_decode.c | 12 ++++++------
 source4/heimdal/lib/asn1/gen_free.c   |  7 +++++++
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/source4/heimdal/lib/asn1/gen_decode.c b/source4/heimdal/lib/asn1/gen_decode.c
index 9d816d5400d7..bf2d93b806df 100644
--- a/source4/heimdal/lib/asn1/gen_decode.c
+++ b/source4/heimdal/lib/asn1/gen_decode.c
@@ -584,14 +584,14 @@ decode_type (const char *name, const Type *t, int optional,
 		    classname(cl),
 		    ty ? "CONS" : "PRIM",
 		    valuename(cl, tag));
+	    fprintf(codefile,
+		    "(%s)->element = %s;\n",
+		    name, m->label);
 	    if (asprintf (&s, "%s(%s)->u.%s", m->optional ? "" : "&",
 			  name, m->gen_name) < 0 || s == NULL)
 		errx(1, "malloc");
 	    decode_type (s, m->type, m->optional, forwstr, m->gen_name, NULL,
 		depth + 1);
-	    fprintf(codefile,
-		    "(%s)->element = %s;\n",
-		    name, m->label);
 	    free(s);
 	    fprintf(codefile,
 		    "}\n");
@@ -600,23 +600,23 @@ decode_type (const char *name, const Type *t, int optional,
 	if (have_ellipsis) {
 	    fprintf(codefile,
 		    "else {\n"
+		    "(%s)->element = %s;\n"
 		    "(%s)->u.%s.data = calloc(1, len);\n"
 		    "if ((%s)->u.%s.data == NULL) {\n"
 		    "e = ENOMEM; %s;\n"
 		    "}\n"
 		    "(%s)->u.%s.length = len;\n"
 		    "memcpy((%s)->u.%s.data, p, len);\n"
-		    "(%s)->element = %s;\n"
 		    "p += len;\n"
 		    "ret += len;\n"
 		    "len = 0;\n"
 		    "}\n",
+		    name, have_ellipsis->label,
 		    name, have_ellipsis->gen_name,
 		    name, have_ellipsis->gen_name,
 		    forwstr,
 		    name, have_ellipsis->gen_name,
-		    name, have_ellipsis->gen_name,
-		    name, have_ellipsis->label);
+		    name, have_ellipsis->gen_name);
 	} else {
 	    fprintf(codefile,
 		    "else {\n"
diff --git a/source4/heimdal/lib/asn1/gen_free.c b/source4/heimdal/lib/asn1/gen_free.c
index b9cae7533b17..74449fe6ca82 100644
--- a/source4/heimdal/lib/asn1/gen_free.c
+++ b/source4/heimdal/lib/asn1/gen_free.c
@@ -61,6 +61,13 @@ free_type (const char *name, const Type *t, int preserve)
     case TNull:
     case TGeneralizedTime:
     case TUTCTime:
+        /*
+         * This doesn't do much, but it leaves zeros where garbage might
+         * otherwise have been found.  Gets us closer to having the equivalent
+         * of a memset()-to-zero data structure after calling the free
+         * functions.
+         */
+        fprintf(codefile, "*%s = 0;\n", name);
 	break;
     case TBitString:
 	if (ASN1_TAILQ_EMPTY(t->members))
-- 
2.34.1