view svr-kex.c @ 1861:2b3a8026a6ce

Add re-exec for server This allows ASLR to re-randomize the address space for every connection, preventing some vulnerabilities from being exploitable by repeated probing. Overhead (memory and time) is yet to be confirmed. At present this is only enabled on Linux. Other BSD platforms with fexecve() would probably also work though have not been tested.
author Matt Johnston <matt@ucc.asn.au>
date Sun, 30 Jan 2022 10:14:56 +0800
parents 435cfb9ec96e
children 284c3837891c
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/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002,2003 Matt Johnston
 * Copyright (c) 2004 by Mihnea Stoenescu
 * All rights reserved.
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE. */

#include "includes.h"
#include "dbutil.h"
#include "algo.h"
#include "buffer.h"
#include "session.h"
#include "kex.h"
#include "ssh.h"
#include "packet.h"
#include "bignum.h"
#include "dbrandom.h"
#include "runopts.h"
#include "ecc.h"
#include "gensignkey.h"

static void send_msg_kexdh_reply(mp_int *dh_e, buffer *ecdh_qs);
#if DROPBEAR_EXT_INFO
static void send_msg_ext_info(void);
#endif

/* Handle a diffie-hellman key exchange initialisation. This involves
 * calculating a session key reply value, and corresponding hash. These
 * are carried out by send_msg_kexdh_reply(). recv_msg_kexdh_init() calls
 * that function, then brings the new keys into use */
void recv_msg_kexdh_init() {
	DEF_MP_INT(dh_e);
	buffer *ecdh_qs = NULL;

	TRACE(("enter recv_msg_kexdh_init"))
	if (!ses.kexstate.recvkexinit) {
		dropbear_exit("Premature kexdh_init message received");
	}

	switch (ses.newkeys->algo_kex->mode) {
#if DROPBEAR_NORMAL_DH
		case DROPBEAR_KEX_NORMAL_DH:
			m_mp_init(&dh_e);
			if (buf_getmpint(ses.payload, &dh_e) != DROPBEAR_SUCCESS) {
				dropbear_exit("Bad kex value");
			}
			break;
#endif
#if DROPBEAR_ECDH
		case DROPBEAR_KEX_ECDH:
#endif
#if DROPBEAR_CURVE25519
		case DROPBEAR_KEX_CURVE25519:
#endif
#if DROPBEAR_ECDH || DROPBEAR_CURVE25519
			ecdh_qs = buf_getstringbuf(ses.payload);
			break;
#endif
	}
	if (ses.payload->pos != ses.payload->len) {
		dropbear_exit("Bad kex value");
	}

	send_msg_kexdh_reply(&dh_e, ecdh_qs);

	mp_clear(&dh_e);
	if (ecdh_qs) {
		buf_free(ecdh_qs);
		ecdh_qs = NULL;
	}

	send_msg_newkeys();

#if DROPBEAR_EXT_INFO
	/* Only send it following the first newkeys */
	if (!ses.kexstate.donesecondkex && ses.allow_ext_info) {
		send_msg_ext_info();
	}
#endif

	ses.requirenext = SSH_MSG_NEWKEYS;
	TRACE(("leave recv_msg_kexdh_init"))
}


#if DROPBEAR_DELAY_HOSTKEY

static void svr_ensure_hostkey() {

	const char* fn = NULL;
	enum signkey_type type = ses.newkeys->algo_hostkey;
	void **hostkey = signkey_key_ptr(svr_opts.hostkey, type);
	int ret = DROPBEAR_FAILURE;

	if (hostkey && *hostkey) {
		return;
	}

	switch (type)
	{
#if DROPBEAR_RSA
		case DROPBEAR_SIGNKEY_RSA:
			fn = RSA_PRIV_FILENAME;
			break;
#endif
#if DROPBEAR_DSS
		case DROPBEAR_SIGNKEY_DSS:
			fn = DSS_PRIV_FILENAME;
			break;
#endif
#if DROPBEAR_ECDSA
		case DROPBEAR_SIGNKEY_ECDSA_NISTP256:
		case DROPBEAR_SIGNKEY_ECDSA_NISTP384:
		case DROPBEAR_SIGNKEY_ECDSA_NISTP521:
			fn = ECDSA_PRIV_FILENAME;
			break;
#endif
#if DROPBEAR_ED25519
		case DROPBEAR_SIGNKEY_ED25519:
			fn = ED25519_PRIV_FILENAME;
			break;
#endif
		default:
			dropbear_assert(0);
	}

	if (readhostkey(fn, svr_opts.hostkey, &type) == DROPBEAR_SUCCESS) {
		return;
	}

	if (signkey_generate(type, 0, fn, 1) == DROPBEAR_FAILURE) {
		goto out;
	}
	
	ret = readhostkey(fn, svr_opts.hostkey, &type);

	if (ret == DROPBEAR_SUCCESS) {
		char *fp = NULL;
		unsigned int len;
		buffer *key_buf = buf_new(MAX_PUBKEY_SIZE);
		buf_put_pub_key(key_buf, svr_opts.hostkey, type);
		buf_setpos(key_buf, 4);
		len = key_buf->len - key_buf->pos;
		fp = sign_key_fingerprint(buf_getptr(key_buf, len), len);
		dropbear_log(LOG_INFO, "Generated hostkey %s, fingerprint is %s",
			fn, fp);
		m_free(fp);
		buf_free(key_buf);
	}

out:
	if (ret == DROPBEAR_FAILURE)
	{
		dropbear_exit("Couldn't read or generate hostkey %s", fn);
	}
}
#endif
	
/* Generate our side of the diffie-hellman key exchange value (dh_f), and
 * calculate the session key using the diffie-hellman algorithm. Following
 * that, the session hash is calculated, and signed with RSA or DSS. The
 * result is sent to the client. 
 *
 * See the transport RFC4253 section 8 for details
 * or RFC5656 section 4 for elliptic curve variant. */
static void send_msg_kexdh_reply(mp_int *dh_e, buffer *ecdh_qs) {
	TRACE(("enter send_msg_kexdh_reply"))

	/* we can start creating the kexdh_reply packet */
	CHECKCLEARTOWRITE();

#if DROPBEAR_DELAY_HOSTKEY
	if (svr_opts.delay_hostkey)
	{
		svr_ensure_hostkey();
	}
#endif

#if DROPBEAR_FUZZ
	if (fuzz.fuzzing && fuzz.skip_kexmaths) {
		fuzz_fake_send_kexdh_reply();
		return;
	}
#endif

	buf_putbyte(ses.writepayload, SSH_MSG_KEXDH_REPLY);
	buf_put_pub_key(ses.writepayload, svr_opts.hostkey,
			ses.newkeys->algo_hostkey);

	switch (ses.newkeys->algo_kex->mode) {
#if DROPBEAR_NORMAL_DH
		case DROPBEAR_KEX_NORMAL_DH:
			{
			struct kex_dh_param * dh_param = gen_kexdh_param();
			kexdh_comb_key(dh_param, dh_e, svr_opts.hostkey);

			/* put f */
			buf_putmpint(ses.writepayload, &dh_param->pub);
			free_kexdh_param(dh_param);
			}
			break;
#endif
#if DROPBEAR_ECDH
		case DROPBEAR_KEX_ECDH:
			{
			struct kex_ecdh_param *ecdh_param = gen_kexecdh_param();
			kexecdh_comb_key(ecdh_param, ecdh_qs, svr_opts.hostkey);

			buf_put_ecc_raw_pubkey_string(ses.writepayload, &ecdh_param->key);
			free_kexecdh_param(ecdh_param);
			}
			break;
#endif
#if DROPBEAR_CURVE25519
		case DROPBEAR_KEX_CURVE25519:
			{
			struct kex_curve25519_param *param = gen_kexcurve25519_param();
			kexcurve25519_comb_key(param, ecdh_qs, svr_opts.hostkey);

			buf_putstring(ses.writepayload, param->pub, CURVE25519_LEN);
			free_kexcurve25519_param(param);
			}
			break;
#endif
	}

	/* calc the signature */
	buf_put_sign(ses.writepayload, svr_opts.hostkey, 
			ses.newkeys->algo_signature, ses.hash);

	/* the SSH_MSG_KEXDH_REPLY is done */
	encrypt_packet();

	TRACE(("leave send_msg_kexdh_reply"))
}

#if DROPBEAR_EXT_INFO
/* Only used for server-sig-algs on the server side */
static void send_msg_ext_info(void) {
	TRACE(("enter send_msg_ext_info"))

	buf_putbyte(ses.writepayload, SSH_MSG_EXT_INFO);
	/* nr-extensions */
	buf_putint(ses.writepayload, 1);

	buf_putstring(ses.writepayload, SSH_SERVER_SIG_ALGS, strlen(SSH_SERVER_SIG_ALGS));
	buf_put_algolist_all(ses.writepayload, sigalgs, 1);
	
	encrypt_packet();

	TRACE(("leave send_msg_ext_info"))
}
#endif