heimdal: always confirm PA-PKINIT-KX for anon PKINIT

Import upstream 38c797e1a.

Upstream notes:

    RFC8062 Section 7 requires verification of the PA-PKINIT-KX key
    excahnge when anonymous PKINIT is used.  Failure to do so can
    permit an active attacker to become a man-in-the-middle.

Reported by:	emaste
Obtained from:	upstream 38c797e1a
Security:	CVE-2019-12098
MFC after:	1 week

2 files changed, 93 insertions, 0 deletions

diff --git a/crypto/heimdal/lib/krb5/krb5_locl.h b/crypto/heimdal/lib/krb5/krb5_locl.h
index d0c68927ffbd..0ea132f94c82 100644
--- a/crypto/heimdal/lib/krb5/krb5_locl.h
+++ b/crypto/heimdal/lib/krb5/krb5_locl.h
@@ -240,6 +240,7 @@ struct _krb5_get_init_creds_opt_private {
 #define KRB5_INIT_CREDS_CANONICALIZE		1
 #define KRB5_INIT_CREDS_NO_C_CANON_CHECK	2
 #define KRB5_INIT_CREDS_NO_C_NO_EKU_CHECK	4
+#define KRB5_INIT_CREDS_PKINIT_KX_VALID		32
     struct {
         krb5_gic_process_last_req func;
         void *ctx;
diff --git a/crypto/heimdal/lib/krb5/pkinit.c b/crypto/heimdal/lib/krb5/pkinit.c
index 7164a118c34a..3c914bb31f35 100644
--- a/crypto/heimdal/lib/krb5/pkinit.c
+++ b/crypto/heimdal/lib/krb5/pkinit.c
@@ -1306,6 +1306,98 @@ pk_rd_pa_reply_enckey(krb5_context context,
     return ret;
 }
 
+/*
+ * RFC 8062 section 7:
+ *
+ *  The client then decrypts the KDC contribution key and verifies that
+ *  the ticket session key in the returned ticket is the combined key of
+ *  the KDC contribution key and the reply key.
+ */
+KRB5_LIB_FUNCTION krb5_error_code KRB5_LIB_CALL
+_krb5_pk_kx_confirm(krb5_context context,
+		    krb5_pk_init_ctx ctx,
+		    krb5_keyblock *reply_key,
+		    krb5_keyblock *session_key,
+		    PA_DATA *pa_pkinit_kx)
+{
+    krb5_error_code ret;
+    EncryptedData ed;
+    krb5_keyblock ck, sk_verify;
+    krb5_crypto ck_crypto = NULL;
+    krb5_crypto rk_crypto = NULL;
+    size_t len;
+    krb5_data data;
+    krb5_data p1 = { sizeof("PKINIT") - 1, "PKINIT" };
+    krb5_data p2 = { sizeof("KEYEXCHANGE") - 1, "KEYEXCHANGE" };
+
+    heim_assert(ctx != NULL, "PKINIT context is non-NULL");
+    heim_assert(reply_key != NULL, "reply key is non-NULL");
+    heim_assert(session_key != NULL, "session key is non-NULL");
+
+    /* PA-PKINIT-KX is optional unless anonymous */
+    if (pa_pkinit_kx == NULL)
+	return ctx->anonymous ? KRB5_KDCREP_MODIFIED : 0;
+
+    memset(&ed, 0, sizeof(ed));
+    krb5_keyblock_zero(&ck);
+    krb5_keyblock_zero(&sk_verify);
+    krb5_data_zero(&data);
+
+    ret = decode_EncryptedData(pa_pkinit_kx->padata_value.data,
+			       pa_pkinit_kx->padata_value.length,
+			       &ed, &len);
+    if (ret)
+	goto out;
+
+    if (len != pa_pkinit_kx->padata_value.length) {
+	ret = KRB5_KDCREP_MODIFIED;
+	goto out;
+    }
+
+    ret = krb5_crypto_init(context, reply_key, 0, &rk_crypto);
+    if (ret)
+	goto out;
+
+    ret = krb5_decrypt_EncryptedData(context, rk_crypto,
+				     KRB5_KU_PA_PKINIT_KX,
+				     &ed, &data);
+    if (ret)
+	goto out;
+
+    ret = decode_EncryptionKey(data.data, data.length,
+			       &ck, &len);
+    if (ret)
+	goto out;
+
+    ret = krb5_crypto_init(context, &ck, 0, &ck_crypto);
+    if (ret)
+	goto out;
+
+    ret = krb5_crypto_fx_cf2(context, ck_crypto, rk_crypto,
+			     &p1, &p2, session_key->keytype,
+			     &sk_verify);
+    if (ret)
+	goto out;
+
+    if (sk_verify.keytype != session_key->keytype ||
+	krb5_data_ct_cmp(&sk_verify.keyvalue, &session_key->keyvalue) != 0) {
+	ret = KRB5_KDCREP_MODIFIED;
+	goto out;
+    }
+
+out:
+    free_EncryptedData(&ed);
+    krb5_free_keyblock_contents(context, &ck);
+    krb5_free_keyblock_contents(context, &sk_verify);
+    if (ck_crypto)
+	krb5_crypto_destroy(context, ck_crypto);
+    if (rk_crypto)
+	krb5_crypto_destroy(context, rk_crypto);
+    krb5_data_free(&data);
+
+    return ret;
+}
+
 static krb5_error_code
 pk_rd_pa_reply_dh(krb5_context context,
 		  const heim_octet_string *indata,