1 /*        $NetBSD: auth2-hostbased.c,v 1.24 2024/07/08 22:33:43 christos Exp $  */
2 /* $OpenBSD: auth2-hostbased.c,v 1.53 2024/05/17 00:30:23 djm Exp $ */
3 
4 /*
5  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
6  *
7  * Redistribution and use in source and binary forms, with or without
8  * modification, are permitted provided that the following conditions
9  * are met:
10  * 1. Redistributions of source code must retain the above copyright
11  *    notice, this list of conditions and the following disclaimer.
12  * 2. Redistributions in binary form must reproduce the above copyright
13  *    notice, this list of conditions and the following disclaimer in the
14  *    documentation and/or other materials provided with the distribution.
15  *
16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
17  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
18  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
19  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
21  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
22  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
23  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
24  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
25  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
26  */
27 
28 #include "includes.h"
29 __RCSID("$NetBSD: auth2-hostbased.c,v 1.24 2024/07/08 22:33:43 christos Exp $");
30 #include <sys/types.h>
31 
32 #include <stdlib.h>
33 #include <pwd.h>
34 #include <string.h>
35 #include <stdarg.h>
36 
37 #include "xmalloc.h"
38 #include "ssh2.h"
39 #include "packet.h"
40 #include "kex.h"
41 #include "sshbuf.h"
42 #include "log.h"
43 #include "misc.h"
44 #include "servconf.h"
45 #include "sshkey.h"
46 #include "hostfile.h"
47 #include "auth.h"
48 #include "canohost.h"
49 #ifdef GSSAPI
50 #include "ssh-gss.h"
51 #endif
52 #include "monitor_wrap.h"
53 #include "pathnames.h"
54 #include "ssherr.h"
55 #include "match.h"
56 
57 /* import */
58 extern ServerOptions options;
59 extern struct authmethod_cfg methodcfg_hostbased;
60 
61 static int
userauth_hostbased(struct ssh * ssh,const char * method)62 userauth_hostbased(struct ssh *ssh, const char *method)
63 {
64           Authctxt *authctxt = ssh->authctxt;
65           struct sshbuf *b;
66           struct sshkey *key = NULL;
67           char *pkalg, *cuser, *chost;
68           u_char *pkblob, *sig;
69           size_t alen, blen, slen;
70           int r, pktype, authenticated = 0;
71 
72           /* XXX use sshkey_froms() */
73           if ((r = sshpkt_get_cstring(ssh, &pkalg, &alen)) != 0 ||
74               (r = sshpkt_get_string(ssh, &pkblob, &blen)) != 0 ||
75               (r = sshpkt_get_cstring(ssh, &chost, NULL)) != 0 ||
76               (r = sshpkt_get_cstring(ssh, &cuser, NULL)) != 0 ||
77               (r = sshpkt_get_string(ssh, &sig, &slen)) != 0)
78                     fatal_fr(r, "parse packet");
79 
80           debug_f("cuser %s chost %s pkalg %s slen %zu",
81               cuser, chost, pkalg, slen);
82 #ifdef DEBUG_PK
83           debug("signature:");
84           sshbuf_dump_data(sig, slen, stderr);
85 #endif
86           pktype = sshkey_type_from_name(pkalg);
87           if (pktype == KEY_UNSPEC) {
88                     /* this is perfectly legal */
89                     logit_f("unsupported public key algorithm: %s",
90                         pkalg);
91                     goto done;
92           }
93           if ((r = sshkey_from_blob(pkblob, blen, &key)) != 0) {
94                     error_fr(r, "key_from_blob");
95                     goto done;
96           }
97           if (key == NULL) {
98                     error_f("cannot decode key: %s", pkalg);
99                     goto done;
100           }
101           if (key->type != pktype) {
102                     error_f("type mismatch for decoded key "
103                         "(received %d, expected %d)", key->type, pktype);
104                     goto done;
105           }
106           if (match_pattern_list(pkalg, options.hostbased_accepted_algos, 0) != 1) {
107                     logit_f("signature algorithm %s not in "
108                         "HostbasedAcceptedAlgorithms", pkalg);
109                     goto done;
110           }
111           if ((r = sshkey_check_cert_sigtype(key,
112               options.ca_sign_algorithms)) != 0) {
113                     logit_fr(r, "certificate signature algorithm %s",
114                         (key->cert == NULL || key->cert->signature_type == NULL) ?
115                         "(null)" : key->cert->signature_type);
116                     goto done;
117           }
118           if ((r = sshkey_check_rsa_length(key,
119               options.required_rsa_size)) != 0) {
120                     logit_r(r, "refusing %s key", sshkey_type(key));
121                     goto done;
122           }
123 
124           if (!authctxt->valid || authctxt->user == NULL) {
125                     debug2_f("disabled because of invalid user");
126                     goto done;
127           }
128 
129           if ((b = sshbuf_new()) == NULL)
130                     fatal_f("sshbuf_new failed");
131           /* reconstruct packet */
132           if ((r = sshbuf_put_stringb(b, ssh->kex->session_id)) != 0 ||
133               (r = sshbuf_put_u8(b, SSH2_MSG_USERAUTH_REQUEST)) != 0 ||
134               (r = sshbuf_put_cstring(b, authctxt->user)) != 0 ||
135               (r = sshbuf_put_cstring(b, authctxt->service)) != 0 ||
136               (r = sshbuf_put_cstring(b, method)) != 0 ||
137               (r = sshbuf_put_string(b, pkalg, alen)) != 0 ||
138               (r = sshbuf_put_string(b, pkblob, blen)) != 0 ||
139               (r = sshbuf_put_cstring(b, chost)) != 0 ||
140               (r = sshbuf_put_cstring(b, cuser)) != 0)
141                     fatal_fr(r, "reconstruct packet");
142 #ifdef DEBUG_PK
143           sshbuf_dump(b, stderr);
144 #endif
145 
146           auth2_record_info(authctxt,
147               "client user \"%.100s\", client host \"%.100s\"", cuser, chost);
148 
149           /* test for allowed key and correct signature */
150           authenticated = 0;
151           if (mm_hostbased_key_allowed(ssh, authctxt->pw, cuser,
152               chost, key) &&
153               mm_sshkey_verify(key, sig, slen,
154               sshbuf_ptr(b), sshbuf_len(b), pkalg, ssh->compat, NULL) == 0)
155                     authenticated = 1;
156 
157           auth2_record_key(authctxt, authenticated, key);
158           sshbuf_free(b);
159 done:
160           debug2_f("authenticated %d", authenticated);
161           sshkey_free(key);
162           free(pkalg);
163           free(pkblob);
164           free(cuser);
165           free(chost);
166           free(sig);
167           return authenticated;
168 }
169 
170 /* return 1 if given hostkey is allowed */
171 int
hostbased_key_allowed(struct ssh * ssh,struct passwd * pw,const char * cuser,char * chost,struct sshkey * key)172 hostbased_key_allowed(struct ssh *ssh, struct passwd *pw,
173     const char *cuser, char *chost, struct sshkey *key)
174 {
175           const char *resolvedname, *ipaddr, *lookup, *reason;
176           HostStatus host_status;
177           int len;
178           char *fp;
179 
180           if (auth_key_is_revoked(key))
181                     return 0;
182 
183           resolvedname = auth_get_canonical_hostname(ssh, options.use_dns);
184           ipaddr = ssh_remote_ipaddr(ssh);
185 
186           debug2_f("chost %s resolvedname %s ipaddr %s",
187               chost, resolvedname, ipaddr);
188 
189           if (((len = strlen(chost)) > 0) && chost[len - 1] == '.') {
190                     debug2("stripping trailing dot from chost %s", chost);
191                     chost[len - 1] = '\0';
192           }
193 
194           if (options.hostbased_uses_name_from_packet_only) {
195                     if (auth_rhosts2(pw, cuser, chost, chost) == 0) {
196                               debug2_f("auth_rhosts2 refused user \"%.100s\" "
197                                   "host \"%.100s\" (from packet)", cuser, chost);
198                               return 0;
199                     }
200                     lookup = chost;
201           } else {
202                     if (strcasecmp(resolvedname, chost) != 0)
203                               logit("userauth_hostbased mismatch: "
204                                   "client sends %s, but we resolve %s to %s",
205                                   chost, ipaddr, resolvedname);
206                     if (auth_rhosts2(pw, cuser, resolvedname, ipaddr) == 0) {
207                               debug2_f("auth_rhosts2 refused "
208                                   "user \"%.100s\" host \"%.100s\" addr \"%.100s\"",
209                                   cuser, resolvedname, ipaddr);
210                               return 0;
211                     }
212                     lookup = resolvedname;
213           }
214           debug2_f("access allowed by auth_rhosts2");
215 
216           if (sshkey_is_cert(key) &&
217               sshkey_cert_check_authority_now(key, 1, 0, 0, lookup, &reason)) {
218                     error("%s", reason);
219                     auth_debug_add("%s", reason);
220                     return 0;
221           }
222 
223           host_status = check_key_in_hostfiles(pw, key, lookup,
224               _PATH_SSH_SYSTEM_HOSTFILE,
225               options.ignore_user_known_hosts ? NULL : _PATH_SSH_USER_HOSTFILE);
226 
227           /* backward compat if no key has been found. */
228           if (host_status == HOST_NEW) {
229                     host_status = check_key_in_hostfiles(pw, key, lookup,
230                         _PATH_SSH_SYSTEM_HOSTFILE2,
231                         options.ignore_user_known_hosts ? NULL :
232                         _PATH_SSH_USER_HOSTFILE2);
233           }
234 
235           if (host_status == HOST_OK) {
236                     if (sshkey_is_cert(key)) {
237                               if ((fp = sshkey_fingerprint(key->cert->signature_key,
238                                   options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
239                                         fatal_f("sshkey_fingerprint fail");
240                               verbose("Accepted certificate ID \"%s\" signed by "
241                                   "%s CA %s from %s@%s", key->cert->key_id,
242                                   sshkey_type(key->cert->signature_key), fp,
243                                   cuser, lookup);
244                     } else {
245                               if ((fp = sshkey_fingerprint(key,
246                                   options.fingerprint_hash, SSH_FP_DEFAULT)) == NULL)
247                                         fatal_f("sshkey_fingerprint fail");
248                               verbose("Accepted %s public key %s from %s@%s",
249                                   sshkey_type(key), fp, cuser, lookup);
250                     }
251                     free(fp);
252           }
253 
254           return (host_status == HOST_OK);
255 }
256 
257 Authmethod method_hostbased = {
258           &methodcfg_hostbased,
259           userauth_hostbased,
260 };
261