KDC null deref due to referrals [CVE-2013-1417]

An authenticated remote client can cause a KDC to crash by making a
valid TGS-REQ to a KDC serving a realm with a single-component name.
The process_tgs_req() function dereferences a null pointer because an
unusual failure condition causes a helper function to return success.

While attempting to provide cross-realm referrals for host-based
service principals, the find_referral_tgs() function could return a
TGS principal for a zero-length realm name (indicating that the
hostname in the service principal has no known realm associated with
it).

Subsequently, the find_alternate_tgs() function would attempt to
construct a path to this empty-string realm, and return success along
with a null pointer in its output parameter.  This happens because
krb5_walk_realm_tree() returns a list of length one when it attempts
to construct a transit path between a single-component realm and the
empty-string realm.  This list causes a loop in find_alternate_tgs()
to iterate over zero elements, resulting in the unexpected output of a
null pointer, which process_tgs_req() proceeds to dereference because
there is no error condition.

Add an error condition to find_referral_tgs() when
krb5_get_host_realm() returns an empty realm name.  Also add an error
condition to find_alternate_tgs() to handle the length-one output from
krb5_walk_realm_tree().

The vulnerable configuration is not likely to arise in practice.
(Realm names that have a single component are likely to be test
realms.)  Releases prior to krb5-1.11 are not vulnerable.

Thanks to Sol Jerome for reporting this problem.

CVSSv2: AV:N/AC:M/Au:S/C:N/I:N/A:P/E:H/RL:O/RC:C

(cherry picked from commit 3c7f1c21ffaaf6c90f1045f0f5440303c766acc0)

ticket: 7668
version_fixed: 1.11.4
status: resolved
search_sprinc(kdc_realm_t *kdc_active_realm, krb5_kdc_req *req,
              krb5_flags flags, krb5_db_entry **server, const char **status)
{
    krb5_error_code ret;
    krb5_principal princ = req->server;
    krb5_principal reftgs = NULL;

    ret = db_get_svc_princ(kdc_context, princ, flags, server, status);
    if (ret == 0 || ret != KRB5_KDB_NOENTRY)
        goto cleanup;

    if (!is_cross_tgs_principal(req->server)) {
        ret = find_referral_tgs(kdc_active_realm, req, &reftgs);
        if (ret != 0)
            goto cleanup;
        ret = db_get_svc_princ(kdc_context, reftgs, flags, server, status);
        if (ret == 0 || ret != KRB5_KDB_NOENTRY)
            goto cleanup;

        princ = reftgs;
    }
    ret = find_alternate_tgs(kdc_active_realm, princ, server, status);

cleanup:
    if (ret != 0 && ret != KRB5KDC_ERR_SVC_UNAVAILABLE) {
        ret = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
        if (*status == NULL)
            *status = "LOOKING_UP_SERVER";
    }
    krb5_free_principal(kdc_context, reftgs);
    return ret;
}

search_sprinc(kdc_realm_t *kdc_active_realm, krb5_kdc_req *req,
              krb5_flags flags, krb5_db_entry **server, const char **status)
{
    krb5_error_code ret;
    krb5_principal princ = req->server;
    krb5_principal reftgs = NULL;

    ret = db_get_svc_princ(kdc_context, princ, flags, server, status);
    if (ret == 0 || ret != KRB5_KDB_NOENTRY)
        goto cleanup;

    if (!is_cross_tgs_principal(req->server)) {
        ret = find_referral_tgs(kdc_active_realm, req, &reftgs);
        if (ret != 0)
            goto cleanup;
        ret = db_get_svc_princ(kdc_context, reftgs, flags, server, status);
        if (ret == 0 || ret != KRB5_KDB_NOENTRY)
            goto cleanup;

        princ = reftgs;
    }
    ret = find_alternate_tgs(kdc_active_realm, princ, server, status);

cleanup:
    if (ret != 0 && ret != KRB5KDC_ERR_SVC_UNAVAILABLE) {
        ret = KRB5KDC_ERR_S_PRINCIPAL_UNKNOWN;
        if (*status == NULL)
            *status = "LOOKING_UP_SERVER";
    }
    krb5_free_principal(kdc_context, reftgs);
    return ret;
}
