1 Copyright 2008 Sun Microsystems, Inc. All rights reserved.
2 Use is subject to license terms.
3
4 Sun's Alternative "Privilege Separation" for OpenSSH
5
6
7 Table of Contents
8
9 1. Introduction
10 2. What is "Privilege?"
11 3. Analysis of the SSH Protocols
12 3.1. Privileged Resources, Operations, in the SSH Protocols
13 4. OpenSSH's Privilege Separation
14 5. SUNWssh's Alternative Privilege Separation
15 6. Comparison of the OpenSSH and SUNWssh PrivSep Models
16 7. Future Directions
17 8. Guide to the AltPrivSep Source Code
18 A. References
19
20
21
22
23
24 1. Introduction
25
26 Implementations of SSH servers require some degree of privilege in
27 order to function properly. Often such implementations retain such
28 privilege throughout normal operation even while users are logged
29 in. This means that vulnerabilities in the implementation of the
30 protocols can be exploited in such ways as to escalate the privilege
31 that would normally be accorded to mer-mortal users.
32
33 The OpenSSH team introduced support for "privilege separation" in
34 the OpenSSH ssh server some years ago to minimize the extent of
35 extant, undiscovered vulnerabilities in the OpenSSH server source
36 code. The basic concept is to have a multi-process server
37 implementation where one process, the "monitor" is privileged and
38 implements a smaller protocol than the ssh protocols, and thus is,
39 hopefully, less likely to sport exploitable security bugs.
40
41 The ssh team at Sun agrees with the basic OpenSSH privilege
42 separation concept, but disagrees with its design.
43
44 Here we present our alternative to the OpenSSH design. We begin
45 with the question of just what is "privilege" and follow on with an
46 analysis of the SSH protocols vis-a-vis privilege. Then we briefly
47 describe the OpenSSH model, followed by an exposition of our
48 alternative model.
49
50
51 2. What is "Privilege?"
52
53 Privilege, in a traditional Unix sense, is that which the "root"
54 user can do that other users cannot directly do. In Solaris 10
55 there is a new approach to this sort of privilege with the aim of
56 running much of the operating system with the Least Privilege
57 required; root's privilege is broken down into many privileges and
58 these are managed through privilege sets. We won't go into the
59 details of Solaris 10's Least Privilege facility here.
60
61 But privilege is also access to data and resources that can be used
62 to escalate the privilege of those who have access to them. For
63 example: secret, or private cryptographic keys used in
64 authentication. Network security typically requires the use of
65 cryptographic keys for authentication.
66
67
68 3. Analysis of the SSH Protocols
69
70 There are two or, rather three SSH protocols:
71
72 - version 1
73 - version 1.5
74 - version 2
75
76 Version 1 and 1.5 are much the same, from our point of view; version
77 2 is significantly different from the other two.
78
79 Familiarity by the reader with the specifications for these
80 protocols is not assumed, but would be beneficial to the reader.
81
82 Quite roughly, these protocols consist of the following:
83
84 a) initial version exchange (for protocol version negotiation)
85 b) a binary encoding of message data
86 c) message syntaxes for the protocols' messages
87 d) specifications on use of cryptography for transport
88 privacy (encryption) and integrity protection
89 e) a key exchange protocol (which also authenticates servers to
90 clients)
91 f) a protocol for user authentication
92 g) a session protocol
93 h) a re-keying protocol (v2-only)
94
95 Some of these parts of the ssh protocols are quite complex, some
96 quite straightforward. Altogether implementation of the ssh
97 protocols requires a source code base of significant size.
98
99 The OpenSSH implementation relies on OpenSSL for cryptographic
100 service, on libz for compression service and miscellaneous other
101 libraries. Besides these OpenSSH consists of several tens of
102 thousands of lines of source code in C.
103
104 SUNWssh is based on OpenSSH, so it is comparable in size and
105 complexity to OpenSSH.
106
107 There is, then, plenty of space for security bugs in the OpenSSH,
108 and, therefore, also in the SUNWssh source code bases.
109
110 The OpenSSH team designed and implemented a "privilege separation"
111 feature in their ssh server to reduce the risk that a security bug
112 in OpenSSH could be successfully exploited and an attacker's
113 privilege escalated.
114
115
116 3.1. Privileged Resources, Operations, in the SSH Protocols
117
118 What privileges does an SSH server need then?
119
120 Observation with Solaris 10's ppriv(1) and truss(1) commands as well
121 as analysis of the ssh protocols leads to conclude as follows.
122
123 No privilege or privileged resources are needed to implement the
124 parts (a)-(d) mentioned in section 3.
125
126
127 For key exchange and server authentication (e) an ssh server requires:
128
129 - Access to the host's ssh private keys.
130
131 - Access to the host's GSS-API acceptor credentials. [SSHv2-only]
132
133
134 An ssh server requires practically all privileges for user
135 authentication (f) (at least PAM does), particularly
136 PRIV_PROC_SETID, for logging the user in.
137
138
139 Post-authentication an ssh server requires the following privileges:
140
141 - Those required for auditing a user's subsequent logout.
142
143 That is, PRIV_PROC_AUDIT.
144
145
146 - Those required for record keeping (i.e., utmpx/wtmpx logging).
147
148 That is, either open file descriptor for those files or
149 PRIV_FILE_DAC_WRITE or otherwise access to those files, perhaps
150 through a special user id or group id which would be granted
151 write access through the ACLs on those files.
152
153 Since SSHv2 allows clients to open many channels with
154 pseudo-terminals a server may need to open and close utmpx/wtmpx
155 records multiple times in the lifetime of an SSHv2 connection.
156
157
158 - Those required for accessing the host's ssh private keys for
159 SSHv2 re-keying. [SSHv2-only]
160
161 These keys can be (and are) loaded at server startup time,
162 requiring PRIV_FILE_DAC_READ, or access through file ACLs, at
163 that time, but not thence.
164
165
166 - Those required for accessing the host's GSS-API acceptor
167 credentials for SSHv2 re-keying.
168
169 These credentials may require a large set of privileges. The
170 Solaris 10 Kerberos V GSS-API mechanism, for example, requires
171 PRIV_FILE_DAC_READ (for access to the system keytab) and
172 PRIV_FILE_DAC_WRITE (for access to the Kerberos V replay cache).
173
174
175 It is worth pointing out that because of a wrinkle in the
176 specification of the SSHv2 protocol and various implementations,
177 access to a host's ssh private keys can allow one not only to
178 impersonate the host as a server (which is, in practice, difficult),
179 but also to impersonate the host as a client (which is quite easy to
180 do) using "hostbased" user authentication.
181
182 It is entirely possible to have one-process server implementation
183 that drops most privileges and access to privileged resources after
184 user authentication succeeds. Such an implementation would make
185 some privileges, such as PRIV_PROC_SETID, available to any attacker
186 that successfully exploited a security bug in the ssh server.
187
188 But such an implementation would also have to retain access to
189 resources needed for authenticating the server, which, as described
190 above, can be used to impersonate the server, in some cases with
191 ease.
192
193
194 4. OpenSSH's Privilege Separation
195
196 The OpenSSH privilege separation model is quite complex.
197
198 It consists of a monitor, which retains all privileges and access to
199 privileged resources, and two processes which run with much less
200 privilege: one process running as a special user, "sshd," for
201 hosting all phases of the SSH protocols up to and including
202 authentication, and one process running as the actual user that logs
203 in and which hosts all phases of the SSH protocols post-user-
204 authentication.
205
206 The monitor and its companion processes speak a private protocol
207 over IPC. This protocol is intended to be smaller and simpler than
208 the SSH wire protocols.
209
210 In practice the OpenSSH monitor protocols relating to user
211 authentication are neither smaller nor simpler than the SSH user
212 authentication protocols; and though they are different they also
213 transport much the same data, including RSA/DSA signatures,
214 usernames, PAM conversations, and GSS-API context and MIC tokens.
215
216 The key exchange protocols have been broken down into their
217 essentials and the monitor serves only services such as signing
218 server replies with private host keys.
219
220 Note also that the OpenSSH monitor protocol uses the same encodings
221 as the SSH protocols and uses the same implementation of those
222 encodings.
223
224
225 5. SUNWssh's Alternative Privilege Separation
226
227 The Sun Microsystems ssh team believes that the OpenSSH team has
228 reached the point of diminishing returns in attempting to separate
229 processing of the user authentication protocols and that the OpenSSH
230 approach to privilege separation of the key exchange protocols has
231 led to a situation in which the monitor acts as an oracle, willing
232 to sign anything provided by the unprivileged processes that talk to
233 it.
234
235 The Sun ssh team proposes a somewhat different privilege separation
236 implementation that shares with the OpenSSH model the goal of
237 minimizing and simplifying the protocol spoken by the monitor, but
238 little source code.
239
240 We eschew any temptation to apply the privilege separation concept
241 to the version negotiation, initial key exchange and user
242 authentication phases of the ssh protocols (but see section 7).
243
244 Instead we focus on separating processing of auditing, record
245 keeping and re-keying from processing of the session protocols. We
246 also wish to avoid creating any oracles in the monitor.
247
248 This approach allows us to have a very simple monitor protocol. Our
249 monitor protocol consists of the following operations:
250
251 - record a new pseudo-terminal session
252 - record the end of a pseudo-terminal session
253 - process a re-key protocol messages
254 - get keys negotiated during re-keying to the session process to it
255 can use them
256
257 Logout auditing is done when the session process dies and so does
258 not require a monitor protocol message.
259
260 By processing all re-key protocol messages in the monitor we prevent
261 the creation of oracles in the monitor. This is so because the
262 monitor signs only material which it has generated and over which an
263 attacker would have little influence (through the attackers offered
264 DH public key, for example).
265
266 Odds and ends:
267
268 - If the monitor receives SIGHUP, SIGTERM or SIGINT it will call
269 fatal_cleanup(), and thence will forcibly shutdown(3SOCKET) the
270 ssh connection socket, causing its child to exit, and audit a
271 logout.
272
273 - The monitor does not attempt to update utmpx/wtmpx independently
274 of its child -- it depends on the child asking it to.
275
276 - The child now is unable to chown() ptys back to root. That's Ok,
277 other services on Solaris do the same and everything still works
278 because of grantpt(3C).
279
280 - The sshd server process (the one that will become a monitor)
281 forks a child process before the key exchange starts. The reason
282 for it is that if we forked after that we would end up using
283 PKCS#11 sessions initialized in the monitor unless
284 UseOpenSSLEngine was explicitly set to 'no'. Using any existing
285 PKCS#11 sessions or object handles over fork is what the PKCS#11
286 standard explicitly prohibits. To solve that, we would have to
287 rekey before fork and then newly initialize the engine in the
288 child, together with the new crypto contexts initialized with the
289 keys produced by the key re-exchange. And, that wouldn't help in
290 situations where the client does not support rekeying which also
291 includes the whole protocol version 1. The pre-fork solution is
292 simpler and also much faster. So, the key exchange and
293 authentication is fully done in the child server process while
294 the monitor waits aside to read the authentication context that
295 is needed for further operation. The child drops privileges after
296 the authentication finishes.
297
298 With the ssh client, the situation is slightly more complicated.
299 Given the fact that the user can request to go to the background
300 during the connection using the ~& sequence we must be prepared
301 to rekey before forking, to reinitialize the engine in the child
302 after that, and then set the new crypto contexts with the new
303 keys. If the server we are communicating with does not support
304 rekeying we will not use the engine at all. We expect this
305 situation to be extremely rare and will not offer any workaround
306 for that. This also includes the protocol version 1. However,
307 this version is already considered obsolete and should not be used
308 if possible.
309
310 6. Comparison of the OpenSSH and SUNWssh PrivSep Models
311
312 The OpenSSH server involves three processes which we will term
313 "pre-session," "session" and "monitor."
314
315 The OpenSSH pre-session process implements:
316
317 - the ssh version string exchange
318 - the ssh message encoding/decoding
319 - most of the initial key exchange protocols
320 - transport protection
321 - part of the user authentication protocols
322
323 The OpenSSH session process implements:
324
325 - the ssh message encoding/decoding
326 - transport protection
327 - most of the re-keying protocols
328 - the session protocols
329
330 The OpenSSH monitor process implements:
331
332 - the ssh message encoding/decoding
333 - parts of the key exchange and re-key protocols (primarily signing
334 of server replies with host private keys)
335 - most of the user authentication protocols, specifically:
336
337 - evaluation of ~/.ssh/authorized_keys (for pubkey userauth)
338 - evaluation of known hosts files (for hostbased userauth)
339 - evaluation of .shosts/.rhosts files (for hostbased userauth)
340 - verification of signatures w/ public keys (pubkey, hostbased)
341 - PAM API calls, conversation function
342 - GSS-API calls
343
344 Note that any vulnerabilities in the parsing of authorized_keys,
345 known hosts and .shosts/rhosts files are as exploitable in the
346 monitor as in a server w/o privilege separation.
347
348 Similarly for any vulnerabilities in PAM modules and GSS-API
349 mechanisms.
350
351 The SUNWssh server involves two processes which we will term
352 "session" and "monitor."
353
354 The SUNWssh monitor process implements:
355
356 - the ssh version string exchange
357 - the ssh message encoding/decoding
358 - transport protection
359 - all of the key exchange and re-key protocols
360 - all of the user authentication protocols
361
362 The SUNWssh session process implements:
363
364 - the ssh message encoding/decoding
365 - transport protection
366 - the session protocols
367
368 Obviously all of these processes also implement their side of the
369 monitor protocols.
370
371 The OpenSSH 3.5p1 monitor protocol, on Solaris, has approximately 20
372 monitor request and corresponding response messages.
373
374 The SUNWssh monitor protocol has 5 monitor request and response
375 messages; additionally, the monitor processes standard re-key
376 messages (but note: the monitor and the session process IPC is
377 completely unencrypted), which amounts to about 14 more messages
378 altogether.
379
380 Much of the OpenSSH monitor protocol is a variation of the
381 on-the-wire ssh protocols, with some contents re-packaging. We
382 believe this does not afford the monitor much additional, if any
383 protection from attacks in the key exchange and user authentication
384 protocols.
385
386 The re-packaging that is done in the OpenSSH monitor protocol is
387 risky business. By separating the act of signing some blob of data
388 from computing that blob of data one can create an oracle; this is
389 exactly what happened in the OpenSSH case.
390
391 As you can see in the next section, the SUNWssh privilege separation
392 could evolve somewhat in the OpenSSH direction by saving the monitor
393 all transport protection work, but we cannot save the monitor much,
394 if any work relating to authentication or key exchange.
395
396
397 7. Future Directions
398
399 The SUNWssh server privilege separation implementation could stand
400 several improvements.
401
402 The first improvement would be to have a single system-wide monitor.
403 This would reduce resource consumption. The work needed to
404 implement such an enhancement is very similar to the work needed to
405 produce an SSH API and library, and it is not trivial. If this is
406 not done then at least dropping PRIV_PROC_SETID and instead setting
407 the saved-set-user-id in the monitor to that of the logged in user
408 would be nice.
409
410 The second enhancement would be to add a "none" host key algorithm
411 to SSHv2 and a corresponding option in SUNWssh to disallow re-keying
412 with any other host key algorithm. This would allow customers to
413 configure their server and monitor so that no re-key protocol
414 messages need be processed by the monitor.
415
416 A third enhancement would be to enhance the GSS-API mechanisms to
417 require fewer privileges. In practice this means overhauling the
418 Kerberos V mechanism's replay cache. This would allow the monitor
419 to run with fewer privileges.
420
421 Further, even without improving the Kerberos V mechanism's replay
422 cache it should be possible to drop at least PRIV_PROC_FORK/EXEC/
423 SESSION.
424
425 A fourth enhancement would to have the unprivileged process handle
426 all transport protection and proxy to the monitor all key exchange
427 and user authentication protocol messages. This is a variation on
428 the OpenSSH model, but without the re-packaging of ssh message
429 contents seen there. After authentication succeeds the monitor
430 could either change the unprivileged process' credentials (as can be
431 done with ppriv(1) or the unprivileged process would, as in OpenSSH,
432 pass the session keys/IVs/keystate to the monitor which would then
433 pass them to a new process, the session process, that would then run
434 as the logged in user.
435
436
437 8. Guide to the AltPrivSep Source Code
438
439
440 First, a brief introduction to the SUNWssh/OpenSSH source code.
441
442 The source code is organized as follows:
443
444 $SRC/cmd/ssh/etc/
445 |
446 +-> config files
447
448 $SRC/cmd/ssh/include/
449 |
450 +-> header files (note: none are installed/shipped)
451
452 $SRC/cmd/ssh/libopenbsd-compat/common/
453 |
454 +-> misc. portability source code
455
456 $SRC/cmd/ssh/libssh/common/
457 |
458 +-> implementation of encoding, transport protection,
459 various wrappers around cryptography, the key exchange
460 and host authentication protocols, the session
461 protocols, and misc. other code
462
463 cipher.c
464 mac.c
465 compress.c
466 packet.c
467 |
468 +-> transport protocol
469
470 buffer.c
471 bufaux.c
472 |
473 +-> encoding
474
475 channels.c
476 nchan.c
477 |
478 +-> session protocol
479
480 kex.c
481 kexdh.c
482 kexgex.c
483 |
484 +-> key exchange/re-key code common to ssh and sshd
485
486 kexdhs.c
487 kexgexs.c
488 kexgsss.c
489 |
490 +-> key exchange/re-key code (server only)
491
492 kexdhc.c
493 kexgexc.c
494 kexgssc.c
495 |
496 +-> key exchange/re-key code (client only)
497
498 dh.c
499 rsa.c
500 mpaux.c
501 ssh-rsa.c
502 ssh-dss.c
503 ssh-gss.c
504 |
505 +-> crypto wrappers/utilities
506
507 log.c
508 |
509 +-> logging, including debug logging, on stderr or
510 syslog
511
512
513 $SRC/cmd/ssh/ssh/
514 |
515 +-> ssh(1)
516
517 $SRC/cmd/ssh/sshd/
518 |
519 +-> sshd(1M), including auditing, implementation of user
520 authentication and the OpenSSH and SUNWssh monitors
521
522 sshd.c
523 |
524 +-> main()
525
526 auth*.c
527 |
528 +-> user authentication
529
530 serverloop.c
531 session.c
532 |
533 +-> session protocols
534
535 bsmaudit.[ch]
536 sshlogin.c
537 loginrec.c
538 |
539 +-> auditing and record-keeping
540
541 $SRC/cmd/ssh/<misc commands>/
542 |
543 +-> scp, sftp, sftp-server, ssh-agent, ssh-add, ...
544
545
546 The SUNWssh altprivsep adds two new source files:
547
548 $SRC/cmd/ssh/include/altprivsep.h
549 $SRC/cmd/ssh/sshd/altprivsep.c
550 |
551 +-> monitor start routine, altprivsep_packet_*() routines
552 for communication with the monitor, routines to help
553 with key exchanges, service procedures for the monitor,
554 etc...
555
556 and modifies the following:
557
558 $SRC/cmd/ssh/include/config.h
559 |
560 +> adds cpp define "ALTPRIVSEP"
561
562 $SRC/cmd/ssh/include/ssh2.h
563 |
564 +-> adds private message type "SSH2_PRIV_MSG_ALTPRIVSEP" (254)
565
566 $SRC/cmd/ssh/include/packet.h
567 |
568 +-> adds prototypes for several simple utility functions,
569 some of which are specifically meant to avoid having to
570 link altprivsep.c into ssh(1)
571
572 $SRC/cmd/ssh/libssh/common/kex.c
573 $SRC/cmd/ssh/libssh/common/packet.c
574 |
575 +-> implements the hooks needed to proxy re-key messages
576 to/from the monitor
577
578 $SRC/cmd/ssh/sshd/Makefile
579 |
580 +-> adds altprivsep.o to list of objects linked into sshd(1M)
581
582 $SRC/cmd/ssh/sshd/serverloop.c
583 |
584 +-> adds an event loop for the monitor
585 modifies the usual event loops for SSHv2
586
587 $SRC/cmd/ssh/sshd/session.c
588 |
589 +-> modifies do_login() and session_pty_cleanup2() to call
590 altprivsep_record_login/logout() instead of
591 record_login/logout().
592
593 modifies do_exec_pty() so that the server waits for the
594 call to altprivsep_record_login() in child process to
595 complete before returning so that the server and the
596 child processes do not compete for monitor IPC I/O.
597
598 $SRC/cmd/ssh/include/log.h
599 $SRC/cmd/ssh/libssh/common/log.c
600 |
601 +-> adds an internal interface, set_log_txt_prefix() so that
602 the monitor's debug and log messages get prefixed with a
603 string ("monitor ") that indicates they are from the
604 monitor
605
606 $SRC/cmd/ssh/sshd/sshd.c
607 |
608 +-> modifies the body of code that follows the user
609 authentication phase of the ssh protocols so as to start
610 the monitor and move the relevant code into the monitor
611 or session processes as appropriate while dropping
612 privileges and access to privileged resources in the
613 session process
614
615 The monitor uses the packet.h interfaces to communicate with the
616 session process as though it were its ssh client peer, but always
617 uses the "none" cipher, mac and compression algorithms and installs
618 even handlers only for the relevant key exchange messages and the
619 private monitor message used for the other monitor services.
620
621 The monitor serves the following services:
622
623 - APS_MSG_NEWKEYS_REQ -> used to obtain keys/IVs after re-keys
624 - APS_MSG_RECORD_LOGIN -> used to update utmpx/wtmpx
625 - APS_MSG_RECORD_LOGOUT -> used to update utmpx/wtmpx
626
627 The session and monitor processes communicate over a pipe.
628
629 All monitor IPC I/O from the session process is blocking (though the
630 pipe is set to non-blocking I/O). The monitor protocol is entirely
631 synchronous and relies on the re-key protocols being entirely
632 synchronous also (which they are, unlike the session protocols).
633
634 The kex.c and packet.c files are minimally modified, primarily to
635 prevent the monitor from handling SSH_MSG_NEWKEYS messages as a
636 normal ssh server should, instead letting the session process
637 process SSH_MSG_NEWKEYS messages by requesting the new keys
638 negotiated with client from the monitor.
639
640 Note that for SSHv1 no on-the-wire messages are processed by the
641 monitor after authentication. In fact, the monitor thinks it's
642 running SSHv2, even if the on-the-wire protocol is v1.
643
644
645 A. References
646
647 The IETF SECSH Working Group:
648
649 http://www.ietf.org/html.charters/secsh-charter.html
650
651 The SSHv2 architecture, assigned numbers:
652
653 http://www.ietf.org/internet-drafts/draft-ietf-secsh-architecture-16.txt
654 http://www.ietf.org/internet-drafts/draft-ietf-secsh-assignednumbers-06.txt
655
656 New cipher modes for SSHv2:
657
658 http://www.ietf.org/internet-drafts/draft-ietf-secsh-newmodes-02.txt
659
660 The SSHv2 "transport," including initial key exchange and re-key
661 protocols, but excluding negotiable DH group size and GSS-API-based
662 key exchange:
663
664 http://www.ietf.org/internet-drafts/draft-ietf-secsh-transport-18.txt
665
666 Additional key exchange protocols for SSHv2:
667
668 http://www.ietf.org/internet-drafts/draft-ietf-secsh-gsskeyex-08.txt
669 http://www.ietf.org/internet-drafts/draft-ietf-secsh-dh-group-exchange-04.txt
670
671 Base user authentication spec for SSHv2 (includes none, password,
672 pubkey and hostbased user authentication):
673
674 http://www.ietf.org/internet-drafts/draft-ietf-secsh-userauth-21.txt
675
676 SSHv2 user authentication using PAM-style prompting:
677
678 http://www.ietf.org/internet-drafts/draft-ietf-secsh-auth-kbdinteract-06.txt
679
680 SSHv2 user authentication using the GSS-API:
681
682 http://www.ietf.org/internet-drafts/draft-ietf-secsh-gsskeyex-08.txt
683
684 SSHv2 "session" protocol (i.e., the protocol used for pty sessions,
685 port forwarding, agent forwarding, X display forwarding, etc...):
686
687 http://www.ietf.org/internet-drafts/draft-ietf-secsh-connect-19.txt