xref: /openssh-portable/auth2-hostbased.c (revision 39be3dc2)
1 /* $OpenBSD: auth2-hostbased.c,v 1.46 2021/01/27 10:05:28 djm Exp $ */
2 /*
3  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
4  *
5  * Redistribution and use in source and binary forms, with or without
6  * modification, are permitted provided that the following conditions
7  * are met:
8  * 1. Redistributions of source code must retain the above copyright
9  *    notice, this list of conditions and the following disclaimer.
10  * 2. Redistributions in binary form must reproduce the above copyright
11  *    notice, this list of conditions and the following disclaimer in the
12  *    documentation and/or other materials provided with the distribution.
13  *
14  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
15  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
16  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
17  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
18  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
19  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
20  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
21  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
22  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
23  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
24  */
25 
26 #include "includes.h"
27 
28 #include <sys/types.h>
29 
30 #include <stdlib.h>
31 #include <pwd.h>
32 #include <string.h>
33 #include <stdarg.h>
34 
35 #include "xmalloc.h"
36 #include "ssh2.h"
37 #include "packet.h"
38 #include "kex.h"
39 #include "sshbuf.h"
40 #include "log.h"
41 #include "misc.h"
42 #include "servconf.h"
43 #include "compat.h"
44 #include "sshkey.h"
45 #include "hostfile.h"
46 #include "auth.h"
47 #include "canohost.h"
48 #ifdef GSSAPI
49 #include "ssh-gss.h"
50 #endif
51 #include "monitor_wrap.h"
52 #include "pathnames.h"
53 #include "ssherr.h"
54 #include "match.h"
55 
56 /* import */
57 extern ServerOptions options;
58 
59 static int
userauth_hostbased(struct ssh * ssh)60 userauth_hostbased(struct ssh *ssh)
61 {
62 	Authctxt *authctxt = ssh->authctxt;
63 	struct sshbuf *b;
64 	struct sshkey *key = NULL;
65 	char *pkalg, *cuser, *chost;
66 	u_char *pkblob, *sig;
67 	size_t alen, blen, slen;
68 	int r, pktype, authenticated = 0;
69 
70 	/* XXX use sshkey_froms() */
71 	if ((r = sshpkt_get_cstring(ssh, &pkalg, &alen)) != 0 ||
72 	    (r = sshpkt_get_string(ssh, &pkblob, &blen)) != 0 ||
73 	    (r = sshpkt_get_cstring(ssh, &chost, NULL)) != 0 ||
74 	    (r = sshpkt_get_cstring(ssh, &cuser, NULL)) != 0 ||
75 	    (r = sshpkt_get_string(ssh, &sig, &slen)) != 0)
76 		fatal_fr(r, "parse packet");
77 
78 	debug_f("cuser %s chost %s pkalg %s slen %zu",
79 	    cuser, chost, pkalg, slen);
80 #ifdef DEBUG_PK
81 	debug("signature:");
82 	sshbuf_dump_data(sig, slen, stderr);
83 #endif
84 	pktype = sshkey_type_from_name(pkalg);
85 	if (pktype == KEY_UNSPEC) {
86 		/* this is perfectly legal */
87 		logit_f("unsupported public key algorithm: %s",
88 		    pkalg);
89 		goto done;
90 	}
91 	if ((r = sshkey_from_blob(pkblob, blen, &key)) != 0) {
92 		error_fr(r, "key_from_blob");
93 		goto done;
94 	}
95 	if (key == NULL) {
96 		error_f("cannot decode key: %s", pkalg);
97 		goto done;
98 	}
99 	if (key->type != pktype) {
100 		error_f("type mismatch for decoded key "
101 		    "(received %d, expected %d)", key->type, pktype);
102 		goto done;
103 	}
104 	if (sshkey_type_plain(key->type) == KEY_RSA &&
105 	    (ssh->compat & SSH_BUG_RSASIGMD5) != 0) {
106 		error("Refusing RSA key because peer uses unsafe "
107 		    "signature format");
108 		goto done;
109 	}
110 	if (match_pattern_list(pkalg, options.hostbased_accepted_algos, 0) != 1) {
111 		logit_f("key type %s not in HostbasedAcceptedAlgorithms",
112 		    sshkey_type(key));
113 		goto done;
114 	}
115 	if ((r = sshkey_check_cert_sigtype(key,
116 	    options.ca_sign_algorithms)) != 0) {
117 		logit_fr(r, "certificate signature algorithm %s",
118 		    (key->cert == NULL || key->cert->signature_type == NULL) ?
119 		    "(null)" : key->cert->signature_type);
120 		goto done;
121 	}
122 
123 	if (!authctxt->valid || authctxt->user == NULL) {
124 		debug2_f("disabled because of invalid user");
125 		goto done;
126 	}
127 
128 	if ((b = sshbuf_new()) == NULL)
129 		fatal_f("sshbuf_new failed");
130 	/* reconstruct packet */
131 	if ((r = sshbuf_put_stringb(b, ssh->kex->session_id)) != 0 ||
132 	    (r = sshbuf_put_u8(b, SSH2_MSG_USERAUTH_REQUEST)) != 0 ||
133 	    (r = sshbuf_put_cstring(b, authctxt->user)) != 0 ||
134 	    (r = sshbuf_put_cstring(b, authctxt->service)) != 0 ||
135 	    (r = sshbuf_put_cstring(b, "hostbased")) != 0 ||
136 	    (r = sshbuf_put_string(b, pkalg, alen)) != 0 ||
137 	    (r = sshbuf_put_string(b, pkblob, blen)) != 0 ||
138 	    (r = sshbuf_put_cstring(b, chost)) != 0 ||
139 	    (r = sshbuf_put_cstring(b, cuser)) != 0)
140 		fatal_fr(r, "reconstruct packet");
141 #ifdef DEBUG_PK
142 	sshbuf_dump(b, stderr);
143 #endif
144 
145 	auth2_record_info(authctxt,
146 	    "client user \"%.100s\", client host \"%.100s\"", cuser, chost);
147 
148 	/* test for allowed key and correct signature */
149 	authenticated = 0;
150 	if (PRIVSEP(hostbased_key_allowed(ssh, authctxt->pw, cuser,
151 	    chost, key)) &&
152 	    PRIVSEP(sshkey_verify(key, sig, slen,
153 	    sshbuf_ptr(b), sshbuf_len(b), pkalg, ssh->compat, NULL)) == 0)
154 		authenticated = 1;
155 
156 	auth2_record_key(authctxt, authenticated, key);
157 	sshbuf_free(b);
158 done:
159 	debug2_f("authenticated %d", authenticated);
160 	sshkey_free(key);
161 	free(pkalg);
162 	free(pkblob);
163 	free(cuser);
164 	free(chost);
165 	free(sig);
166 	return authenticated;
167 }
168 
169 /* return 1 if given hostkey is allowed */
170 int
hostbased_key_allowed(struct ssh * ssh,struct passwd * pw,const char * cuser,char * chost,struct sshkey * key)171 hostbased_key_allowed(struct ssh *ssh, struct passwd *pw,
172     const char *cuser, char *chost, struct sshkey *key)
173 {
174 	const char *resolvedname, *ipaddr, *lookup, *reason;
175 	HostStatus host_status;
176 	int len;
177 	char *fp;
178 
179 	if (auth_key_is_revoked(key))
180 		return 0;
181 
182 	resolvedname = auth_get_canonical_hostname(ssh, options.use_dns);
183 	ipaddr = ssh_remote_ipaddr(ssh);
184 
185 	debug2_f("chost %s resolvedname %s ipaddr %s",
186 	    chost, resolvedname, ipaddr);
187 
188 	if (((len = strlen(chost)) > 0) && chost[len - 1] == '.') {
189 		debug2("stripping trailing dot from chost %s", chost);
190 		chost[len - 1] = '\0';
191 	}
192 
193 	if (options.hostbased_uses_name_from_packet_only) {
194 		if (auth_rhosts2(pw, cuser, chost, chost) == 0) {
195 			debug2_f("auth_rhosts2 refused user \"%.100s\" "
196 			    "host \"%.100s\" (from packet)", cuser, chost);
197 			return 0;
198 		}
199 		lookup = chost;
200 	} else {
201 		if (strcasecmp(resolvedname, chost) != 0)
202 			logit("userauth_hostbased mismatch: "
203 			    "client sends %s, but we resolve %s to %s",
204 			    chost, ipaddr, resolvedname);
205 		if (auth_rhosts2(pw, cuser, resolvedname, ipaddr) == 0) {
206 			debug2_f("auth_rhosts2 refused "
207 			    "user \"%.100s\" host \"%.100s\" addr \"%.100s\"",
208 			    cuser, resolvedname, ipaddr);
209 			return 0;
210 		}
211 		lookup = resolvedname;
212 	}
213 	debug2_f("access allowed by auth_rhosts2");
214 
215 	if (sshkey_is_cert(key) &&
216 	    sshkey_cert_check_authority(key, 1, 0, 0, lookup, &reason)) {
217 		error("%s", reason);
218 		auth_debug_add("%s", reason);
219 		return 0;
220 	}
221 
222 	host_status = check_key_in_hostfiles(pw, key, lookup,
223 	    _PATH_SSH_SYSTEM_HOSTFILE,
224 	    options.ignore_user_known_hosts ? NULL : _PATH_SSH_USER_HOSTFILE);
225 
226 	/* backward compat if no key has been found. */
227 	if (host_status == HOST_NEW) {
228 		host_status = check_key_in_hostfiles(pw, key, lookup,
229 		    _PATH_SSH_SYSTEM_HOSTFILE2,
230 		    options.ignore_user_known_hosts ? NULL :
231 		    _PATH_SSH_USER_HOSTFILE2);
232 	}
233 
234 	if (host_status == HOST_OK) {
235 		if (sshkey_is_cert(key)) {
236 			if ((fp = sshkey_fingerprint(key->cert->signature_key,
237 			    options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
238 				fatal_f("sshkey_fingerprint fail");
239 			verbose("Accepted certificate ID \"%s\" signed by "
240 			    "%s CA %s from %s@%s", key->cert->key_id,
241 			    sshkey_type(key->cert->signature_key), fp,
242 			    cuser, lookup);
243 		} else {
244 			if ((fp = sshkey_fingerprint(key,
245 			    options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
246 				fatal_f("sshkey_fingerprint fail");
247 			verbose("Accepted %s public key %s from %s@%s",
248 			    sshkey_type(key), fp, cuser, lookup);
249 		}
250 		free(fp);
251 	}
252 
253 	return (host_status == HOST_OK);
254 }
255 
256 Authmethod method_hostbased = {
257 	"hostbased",
258 	userauth_hostbased,
259 	&options.hostbased_authentication
260 };
261