view svr-authpubkey.c @ 1665:7c17995bcdfb

Improve address logging on early exit messages (#83) Change 'Early exit' and 'Exit before auth' messages to include the IP address & port as part of the message. This allows log scanning utilities such as 'fail2ban' to obtain the offending IP address as part of the failure event instead of extracting the PID from the message and then scanning the log again for match 'child connection from' messages Signed-off-by: Kevin Darbyshire-Bryant <[email protected]>
author Kevin Darbyshire-Bryant <6500011+ldir-EDB0@users.noreply.github.com>
date Wed, 18 Mar 2020 15:28:56 +0000
parents cc0fc5131c5c
children ba6fc7afe1c5
line wrap: on
line source

/*
 * Dropbear - a SSH2 server
 * 
 * Copyright (c) 2002,2003 Matt Johnston
 * 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. */
/*
 * This file incorporates work covered by the following copyright and  
 * permission notice:
 *
 * 	Copyright (c) 2000 Markus Friedl.  All rights reserved.
 * 	
 * 	Redistribution and use in source and binary forms, with or without
 * 	modification, are permitted provided that the following conditions
 * 	are met:
 * 	1. Redistributions of source code must retain the above copyright
 * 	   notice, this list of conditions and the following disclaimer.
 * 	2. Redistributions in binary form must reproduce the above copyright
 * 	   notice, this list of conditions and the following disclaimer in the
 * 	   documentation and/or other materials provided with the distribution.
 * 	
 * 	THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
 * 	IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * 	OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
 * 	IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
 * 	INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * 	NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * 	DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * 	THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * 	(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * 	THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 * This copyright and permission notice applies to the code parsing public keys
 * options string which can also be found in OpenSSH auth2-pubkey.c file 
 * (user_key_allowed2). It has been adapted to work with buffers.
 *
 */

/* Process a pubkey auth request */

#include "includes.h"
#include "session.h"
#include "dbutil.h"
#include "buffer.h"
#include "signkey.h"
#include "auth.h"
#include "ssh.h"
#include "packet.h"
#include "algo.h"

#if DROPBEAR_SVR_PUBKEY_AUTH

#define MIN_AUTHKEYS_LINE 10 /* "ssh-rsa AB" - short but doesn't matter */
#define MAX_AUTHKEYS_LINE 4200 /* max length of a line in authkeys */

static int checkpubkey(const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen);
static int checkpubkeyperms(void);
static void send_msg_userauth_pk_ok(const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen);
static int checkfileperm(char * filename);

/* process a pubkey auth request, sending success or failure message as
 * appropriate */
void svr_auth_pubkey(int valid_user) {

	unsigned char testkey; /* whether we're just checking if a key is usable */
	char* algo = NULL; /* pubkey algo */
	unsigned int algolen;
	unsigned char* keyblob = NULL;
	unsigned int keybloblen;
	unsigned int sign_payload_length;
	buffer * signbuf = NULL;
	sign_key * key = NULL;
	char* fp = NULL;
	enum signkey_type type = -1;
        int auth_failure = 1;

	TRACE(("enter pubkeyauth"))

	/* 0 indicates user just wants to check if key can be used, 1 is an
	 * actual attempt*/
	testkey = (buf_getbool(ses.payload) == 0);

	algo = buf_getstring(ses.payload, &algolen);
	keybloblen = buf_getint(ses.payload);
	keyblob = buf_getptr(ses.payload, keybloblen);

	if (!valid_user) {
		/* Return failure once we have read the contents of the packet
		required to validate a public key. 
		Avoids blind user enumeration though it isn't possible to prevent
		testing for user existence if the public key is known */
		send_msg_userauth_failure(0, 0);
		goto out;
	}
#if DROPBEAR_PLUGIN
        if (svr_ses.plugin_instance != NULL) {
            char *options_buf;
            if (svr_ses.plugin_instance->checkpubkey(
                        svr_ses.plugin_instance,
                        &ses.plugin_session,
                        algo, 
                        algolen, 
                        keyblob, 
                        keybloblen,
                        ses.authstate.username) == DROPBEAR_SUCCESS) {
                /* Success */
                auth_failure = 0;

                /* Options provided? */
                options_buf = ses.plugin_session->get_options(ses.plugin_session);
                if (options_buf) {
                    struct buf temp_buf = { 
                        .data = (unsigned char *)options_buf,
                        .len = strlen(options_buf),
                        .pos = 0,
                        .size = 0
                    };
                    int ret = svr_add_pubkey_options(&temp_buf, 0, "N/A");
                    if (ret == DROPBEAR_FAILURE) {
                        /* Fail immediately as the plugin provided wrong options */
                        send_msg_userauth_failure(0, 0);
                        goto out;
                    }
                }
            }
        }
#endif
	/* check if the key is valid */
        if (auth_failure) {
            auth_failure = checkpubkey(algo, algolen, keyblob, keybloblen) == DROPBEAR_FAILURE;
        }

        if (auth_failure) {
		send_msg_userauth_failure(0, 0);
		goto out;
	}

	/* let them know that the key is ok to use */
	if (testkey) {
		send_msg_userauth_pk_ok(algo, algolen, keyblob, keybloblen);
		goto out;
	}

	/* now we can actually verify the signature */
	
	/* get the key */
	key = new_sign_key();
	type = DROPBEAR_SIGNKEY_ANY;
	if (buf_get_pub_key(ses.payload, key, &type) == DROPBEAR_FAILURE) {
		send_msg_userauth_failure(0, 1);
		goto out;
	}

	/* create the data which has been signed - this a string containing
	 * session_id, concatenated with the payload packet up to the signature */
	assert(ses.payload_beginning <= ses.payload->pos);
	sign_payload_length = ses.payload->pos - ses.payload_beginning;
	signbuf = buf_new(ses.payload->pos + 4 + ses.session_id->len);
	buf_putbufstring(signbuf, ses.session_id);

	/* The entire contents of the payload prior. */
	buf_setpos(ses.payload, ses.payload_beginning);
	buf_putbytes(signbuf, 
		buf_getptr(ses.payload, sign_payload_length),
		sign_payload_length);
	buf_incrpos(ses.payload, sign_payload_length);

	buf_setpos(signbuf, 0);

	/* ... and finally verify the signature */
	fp = sign_key_fingerprint(keyblob, keybloblen);
	if (buf_verify(ses.payload, key, signbuf) == DROPBEAR_SUCCESS) {
		dropbear_log(LOG_NOTICE,
				"Pubkey auth succeeded for '%s' with key %s from %s",
				ses.authstate.pw_name, fp, svr_ses.addrstring);
		send_msg_userauth_success();
#if DROPBEAR_PLUGIN
                if ((ses.plugin_session != NULL) && (svr_ses.plugin_instance->auth_success != NULL)) {
                    /* Was authenticated through the external plugin. tell plugin that signature verification was ok */
                    svr_ses.plugin_instance->auth_success(ses.plugin_session);
                }
#endif
                
	} else {
		dropbear_log(LOG_WARNING,
				"Pubkey auth bad signature for '%s' with key %s from %s",
				ses.authstate.pw_name, fp, svr_ses.addrstring);
		send_msg_userauth_failure(0, 1);
	}
	m_free(fp);

out:
	/* cleanup stuff */
	if (signbuf) {
		buf_free(signbuf);
	}
	if (algo) {
		m_free(algo);
	}
	if (key) {
		sign_key_free(key);
		key = NULL;
	}
	/* Retain pubkey options only if auth succeeded */
	if (!ses.authstate.authdone) {
		svr_pubkey_options_cleanup();
	}
	TRACE(("leave pubkeyauth"))
}

/* Reply that the key is valid for auth, this is sent when the user sends
 * a straight copy of their pubkey to test, to avoid having to perform
 * expensive signing operations with a worthless key */
static void send_msg_userauth_pk_ok(const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen) {

	TRACE(("enter send_msg_userauth_pk_ok"))
	CHECKCLEARTOWRITE();

	buf_putbyte(ses.writepayload, SSH_MSG_USERAUTH_PK_OK);
	buf_putstring(ses.writepayload, algo, algolen);
	buf_putstring(ses.writepayload, (const char*)keyblob, keybloblen);

	encrypt_packet();
	TRACE(("leave send_msg_userauth_pk_ok"))

}

static int checkpubkey_line(buffer* line, int line_num, const char* filename,
		const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen) {
	buffer *options_buf = NULL;
	unsigned int pos, len;
	int ret = DROPBEAR_FAILURE;

	if (line->len < MIN_AUTHKEYS_LINE || line->len > MAX_AUTHKEYS_LINE) {
		TRACE(("checkpubkey_line: bad line length %d", line->len))
		goto out;
	}

	if (memchr(line->data, 0x0, line->len) != NULL) {
		TRACE(("checkpubkey_line: bad line has null char"))
		goto out;
	}

	/* compare the algorithm. +3 so we have enough bytes to read a space and some base64 characters too. */
	if (line->pos + algolen+3 > line->len) {
		goto out;
	}
	/* check the key type */
	if (strncmp((const char *) buf_getptr(line, algolen), algo, algolen) != 0) {
		int is_comment = 0;
		unsigned char *options_start = NULL;
		int options_len = 0;
		int escape, quoted;
		
		/* skip over any comments or leading whitespace */
		while (line->pos < line->len) {
			const char c = buf_getbyte(line);
			if (c == ' ' || c == '\t') {
				continue;
			} else if (c == '#') {
				is_comment = 1;
				break;
			}
			buf_incrpos(line, -1);
			break;
		}
		if (is_comment) {
			/* next line */
			goto out;
		}

		/* remember start of options */
		options_start = buf_getptr(line, 1);
		quoted = 0;
		escape = 0;
		options_len = 0;
		
		/* figure out where the options are */
		while (line->pos < line->len) {
			const char c = buf_getbyte(line);
			if (!quoted && (c == ' ' || c == '\t')) {
				break;
			}
			escape = (!escape && c == '\\');
			if (!escape && c == '"') {
				quoted = !quoted;
			}
			options_len++;
		}
		options_buf = buf_new(options_len);
		buf_putbytes(options_buf, options_start, options_len);

		/* compare the algorithm. +3 so we have enough bytes to read a space and some base64 characters too. */
		if (line->pos + algolen+3 > line->len) {
			goto out;
		}
		if (strncmp((const char *) buf_getptr(line, algolen), algo, algolen) != 0) {
			goto out;
		}
	}
	buf_incrpos(line, algolen);
	
	/* check for space (' ') character */
	if (buf_getbyte(line) != ' ') {
		TRACE(("checkpubkey_line: space character expected, isn't there"))
		goto out;
	}

	/* truncate the line at the space after the base64 data */
	pos = line->pos;
	for (len = 0; line->pos < line->len; len++) {
		if (buf_getbyte(line) == ' ') break;
	}	
	buf_setpos(line, pos);
	buf_setlen(line, line->pos + len);

	TRACE(("checkpubkey_line: line pos = %d len = %d", line->pos, line->len))

	ret = cmp_base64_key(keyblob, keybloblen, (const unsigned char *) algo, algolen, line, NULL);

	if (ret == DROPBEAR_SUCCESS && options_buf) {
		ret = svr_add_pubkey_options(options_buf, line_num, filename);
	}

out:
	if (options_buf) {
		buf_free(options_buf);
	}
	return ret;
}


/* Checks whether a specified publickey (and associated algorithm) is an
 * acceptable key for authentication */
/* Returns DROPBEAR_SUCCESS if key is ok for auth, DROPBEAR_FAILURE otherwise */
static int checkpubkey(const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen) {

	FILE * authfile = NULL;
	char * filename = NULL;
	int ret = DROPBEAR_FAILURE;
	buffer * line = NULL;
	unsigned int len;
	int line_num;
	uid_t origuid;
	gid_t origgid;

	TRACE(("enter checkpubkey"))

	/* check that we can use the algo */
	if (have_algo(algo, algolen, sshhostkey) == DROPBEAR_FAILURE) {
		dropbear_log(LOG_WARNING,
				"Pubkey auth attempt with unknown algo for '%s' from %s",
				ses.authstate.pw_name, svr_ses.addrstring);
		goto out;
	}

	/* check file permissions, also whether file exists */
	if (checkpubkeyperms() == DROPBEAR_FAILURE) {
		TRACE(("bad authorized_keys permissions, or file doesn't exist"))
		goto out;
	}

	/* we don't need to check pw and pw_dir for validity, since
	 * its been done in checkpubkeyperms. */
	len = strlen(ses.authstate.pw_dir);
	/* allocate max required pathname storage,
	 * = path + "/.ssh/authorized_keys" + '\0' = pathlen + 22 */
	filename = m_malloc(len + 22);
	snprintf(filename, len + 22, "%s/.ssh/authorized_keys", 
				ses.authstate.pw_dir);

#if DROPBEAR_SVR_MULTIUSER
	/* open the file as the authenticating user. */
	origuid = getuid();
	origgid = getgid();
	if ((setegid(ses.authstate.pw_gid)) < 0 ||
		(seteuid(ses.authstate.pw_uid)) < 0) {
		dropbear_exit("Failed to set euid");
	}
#endif

	authfile = fopen(filename, "r");

#if DROPBEAR_SVR_MULTIUSER
	if ((seteuid(origuid)) < 0 ||
		(setegid(origgid)) < 0) {
		dropbear_exit("Failed to revert euid");
	}
#endif

	if (authfile == NULL) {
		goto out;
	}
	TRACE(("checkpubkey: opened authorized_keys OK"))

	line = buf_new(MAX_AUTHKEYS_LINE);
	line_num = 0;

	/* iterate through the lines */
	do {
		if (buf_getline(line, authfile) == DROPBEAR_FAILURE) {
			/* EOF reached */
			TRACE(("checkpubkey: authorized_keys EOF reached"))
			break;
		}
		line_num++;

		ret = checkpubkey_line(line, line_num, filename, algo, algolen, keyblob, keybloblen);
		if (ret == DROPBEAR_SUCCESS) {
			break;
		}

		/* We continue to the next line otherwise */
		
	} while (1);

out:
	if (authfile) {
		fclose(authfile);
	}
	if (line) {
		buf_free(line);
	}
	m_free(filename);
	TRACE(("leave checkpubkey: ret=%d", ret))
	return ret;
}


/* Returns DROPBEAR_SUCCESS if file permissions for pubkeys are ok,
 * DROPBEAR_FAILURE otherwise.
 * Checks that the user's homedir, ~/.ssh, and
 * ~/.ssh/authorized_keys are all owned by either root or the user, and are
 * g-w, o-w */
static int checkpubkeyperms() {

	char* filename = NULL; 
	int ret = DROPBEAR_FAILURE;
	unsigned int len;

	TRACE(("enter checkpubkeyperms"))

	if (ses.authstate.pw_dir == NULL) {
		goto out;
	}

	if ((len = strlen(ses.authstate.pw_dir)) == 0) {
		goto out;
	}

	/* allocate max required pathname storage,
	 * = path + "/.ssh/authorized_keys" + '\0' = pathlen + 22 */
	len += 22;
	filename = m_malloc(len);
	strlcpy(filename, ses.authstate.pw_dir, len);

	/* check ~ */
	if (checkfileperm(filename) != DROPBEAR_SUCCESS) {
		goto out;
	}

	/* check ~/.ssh */
	strlcat(filename, "/.ssh", len);
	if (checkfileperm(filename) != DROPBEAR_SUCCESS) {
		goto out;
	}

	/* now check ~/.ssh/authorized_keys */
	strlcat(filename, "/authorized_keys", len);
	if (checkfileperm(filename) != DROPBEAR_SUCCESS) {
		goto out;
	}

	/* file looks ok, return success */
	ret = DROPBEAR_SUCCESS;
	
out:
	m_free(filename);

	TRACE(("leave checkpubkeyperms"))
	return ret;
}

/* Checks that a file is owned by the user or root, and isn't writable by
 * group or other */
/* returns DROPBEAR_SUCCESS or DROPBEAR_FAILURE */
static int checkfileperm(char * filename) {
	struct stat filestat;
	int badperm = 0;

	TRACE(("enter checkfileperm(%s)", filename))

	if (stat(filename, &filestat) != 0) {
		TRACE(("leave checkfileperm: stat() != 0"))
		return DROPBEAR_FAILURE;
	}
	/* check ownership - user or root only*/
	if (filestat.st_uid != ses.authstate.pw_uid
			&& filestat.st_uid != 0) {
		badperm = 1;
		TRACE(("wrong ownership"))
	}
	/* check permissions - don't want group or others +w */
	if (filestat.st_mode & (S_IWGRP | S_IWOTH)) {
		badperm = 1;
		TRACE(("wrong perms"))
	}
	if (badperm) {
		if (!ses.authstate.perm_warn) {
			ses.authstate.perm_warn = 1;
			dropbear_log(LOG_INFO, "%s must be owned by user or root, and not writable by others", filename);
		}
		TRACE(("leave checkfileperm: failure perms/owner"))
		return DROPBEAR_FAILURE;
	}

	TRACE(("leave checkfileperm: success"))
	return DROPBEAR_SUCCESS;
}

#if DROPBEAR_FUZZ
int fuzz_checkpubkey_line(buffer* line, int line_num, char* filename,
		const char* algo, unsigned int algolen,
		const unsigned char* keyblob, unsigned int keybloblen) {
	return checkpubkey_line(line, line_num, filename, algo, algolen, keyblob, keybloblen);
}
#endif

#endif