Commit message: set sshpam_ctxt to NULL after free

Avoids use-after-free in monitor when privsep child is compromised.
Reported by Moritz Jodeit; ok dtucker@
CVE: CVE-2015-6564
CWE: CWE-264
Vulnerability description: Use-after-free vulnerability in the mm_answer_pam_free_ctx function in monitor.c in sshd in OpenSSH before 7.0 on non-OpenBSD platforms might allow local users to gain privileges by leveraging control of the sshd uid to send an unexpectedly early MONITOR_REQ_PAM_FREE_CTX request.

Project: openssh-portable
File: monitor.c

mm_answer_rsa_keyallowed(int sock, Buffer *m)
{
	BIGNUM *client_n;
	Key *key = NULL;
	u_char *blob = NULL;
	u_int blen = 0;
	int allowed = 0;

	debug3("%s entering", __func__);

	auth_method = "rsa";
	if (options.rsa_authentication && authctxt->valid) {
		if ((client_n = BN_new()) == NULL)
			fatal("%s: BN_new", __func__);
		buffer_get_bignum2(m, client_n);
		allowed = auth_rsa_key_allowed(authctxt->pw, client_n, &key);
		BN_clear_free(client_n);
	}
	buffer_clear(m);
	buffer_put_int(m, allowed);
	buffer_put_int(m, forced_command != NULL);

	/* clear temporarily storage (used by generate challenge) */
	monitor_reset_key_state();

	if (allowed && key != NULL) {
		key->type = KEY_RSA;	/* cheat for key_to_blob */
		if (key_to_blob(key, &blob, &blen) == 0)
			fatal("%s: key_to_blob failed", __func__);
		buffer_put_string(m, blob, blen);

		/* Save temporarily for comparison in verify */
		key_blob = blob;
		key_bloblen = blen;
		key_blobtype = MM_RSAUSERKEY;
	}
	if (key != NULL)
		key_free(key);

	mm_request_send(sock, MONITOR_ANS_RSAKEYALLOWED, m);

	monitor_permit(mon_dispatch, MONITOR_REQ_RSACHALLENGE, allowed);
	monitor_permit(mon_dispatch, MONITOR_REQ_RSARESPONSE, 0);
	return (0);
}

Project: openssh-portable
File: monitor.c

monitor_child_preauth(Authctxt *_authctxt, struct monitor *pmonitor)
{
	struct mon_table *ent;
	int authenticated = 0, partial = 0;

	debug3("preauth child monitor started");

	close(pmonitor->m_recvfd);
	close(pmonitor->m_log_sendfd);
	pmonitor->m_log_sendfd = pmonitor->m_recvfd = -1;

	authctxt = _authctxt;
	memset(authctxt, 0, sizeof(*authctxt));

	authctxt->loginmsg = &loginmsg;

	if (compat20) {
		mon_dispatch = mon_dispatch_proto20;

		/* Permit requests for moduli and signatures */
		monitor_permit(mon_dispatch, MONITOR_REQ_MODULI, 1);
		monitor_permit(mon_dispatch, MONITOR_REQ_SIGN, 1);
	} else {
		mon_dispatch = mon_dispatch_proto15;

		monitor_permit(mon_dispatch, MONITOR_REQ_SESSKEY, 1);
	}

	/* The first few requests do not require asynchronous access */
	while (!authenticated) {
		partial = 0;
		auth_method = "unknown";
		auth_submethod = NULL;
		authenticated = (monitor_read(pmonitor, mon_dispatch, &ent) == 1);

		/* Special handling for multiple required authentications */
		if (options.num_auth_methods != 0) {
			if (!compat20)
				fatal("AuthenticationMethods is not supported"
				    "with SSH protocol 1");
			if (authenticated &&
			    !auth2_update_methods_lists(authctxt,
			    auth_method, auth_submethod)) {
				debug3("%s: method %s: partial", __func__,
				    auth_method);
				authenticated = 0;
				partial = 1;
			}
		}

		if (authenticated) {
			if (!(ent->flags & MON_AUTHDECIDE))
				fatal("%s: unexpected authentication from %d",
				    __func__, ent->type);
			if (authctxt->pw->pw_uid == 0 &&
			    !auth_root_allowed(auth_method))
				authenticated = 0;
#ifdef USE_PAM
			/* PAM needs to perform account checks after auth */
			if (options.use_pam && authenticated) {
				Buffer m;

				buffer_init(&m);
				mm_request_receive_expect(pmonitor->m_sendfd,
				    MONITOR_REQ_PAM_ACCOUNT, &m);
				authenticated = mm_answer_pam_account(pmonitor->m_sendfd, &m);
				buffer_free(&m);
			}
#endif
		}
		if (ent->flags & (MON_AUTHDECIDE|MON_ALOG)) {
			auth_log(authctxt, authenticated, partial,
			    auth_method, auth_submethod);
			if (!partial && !authenticated)
				authctxt->failures++;
		}
	}

	if (!authctxt->valid)
		fatal("%s: authenticated invalid user", __func__);
	if (strcmp(auth_method, "unknown") == 0)
		fatal("%s: authentication method name unknown", __func__);

	debug("%s: %s has been authenticated by privileged process",
	    __func__, authctxt->user);

	mm_get_keystate(pmonitor);

	/* Drain any buffered messages from the child */
	while (pmonitor->m_log_recvfd != -1 && monitor_read_log(pmonitor) == 0)
		;

	close(pmonitor->m_sendfd);
	close(pmonitor->m_log_recvfd);
	pmonitor->m_sendfd = pmonitor->m_log_recvfd = -1;
}