OpenSolaris_b135/cmd/ssh/README.altprivsep
Copyright 2008 Sun Microsystems, Inc. All rights reserved.
Use is subject to license terms.
Sun's Alternative "Privilege Separation" for OpenSSH
Table of Contents
1. Introduction
2. What is "Privilege?"
3. Analysis of the SSH Protocols
3.1. Privileged Resources, Operations, in the SSH Protocols
4. OpenSSH's Privilege Separation
5. SUNWssh's Alternative Privilege Separation
6. Comparison of the OpenSSH and SUNWssh PrivSep Models
7. Future Directions
8. Guide to the AltPrivSep Source Code
A. References
1. Introduction
Implementations of SSH servers require some degree of privilege in
order to function properly. Often such implementations retain such
privilege throughout normal operation even while users are logged
in. This means that vulnerabilities in the implementation of the
protocols can be exploited in such ways as to escalate the privilege
that would normally be accorded to mer-mortal users.
The OpenSSH team introduced support for "privilege separation" in
the OpenSSH ssh server some years ago to minimize the extent of
extant, undiscovered vulnerabilities in the OpenSSH server source
code. The basic concept is to have a multi-process server
implementation where one process, the "monitor" is privileged and
implements a smaller protocol than the ssh protocols, and thus is,
hopefully, less likely to sport exploitable security bugs.
The ssh team at Sun agrees with the basic OpenSSH privilege
separation concept, but disagrees with its design.
Here we present our alternative to the OpenSSH design. We begin
with the question of just what is "privilege" and follow on with an
analysis of the SSH protocols vis-a-vis privilege. Then we briefly
describe the OpenSSH model, followed by an exposition of our
alternative model.
2. What is "Privilege?"
Privilege, in a traditional Unix sense, is that which the "root"
user can do that other users cannot directly do. In Solaris 10
there is a new approach to this sort of privilege with the aim of
running much of the operating system with the Least Privilege
required; root's privilege is broken down into many privileges and
these are managed through privilege sets. We won't go into the
details of Solaris 10's Least Privilege facility here.
But privilege is also access to data and resources that can be used
to escalate the privilege of those who have access to them. For
example: secret, or private cryptographic keys used in
authentication. Network security typically requires the use of
cryptographic keys for authentication.
3. Analysis of the SSH Protocols
There are two or, rather three SSH protocols:
- version 1
- version 1.5
- version 2
Version 1 and 1.5 are much the same, from our point of view; version
2 is significantly different from the other two.
Familiarity by the reader with the specifications for these
protocols is not assumed, but would be beneficial to the reader.
Quite roughly, these protocols consist of the following:
a) initial version exchange (for protocol version negotiation)
b) a binary encoding of message data
c) message syntaxes for the protocols' messages
d) specifications on use of cryptography for transport
privacy (encryption) and integrity protection
e) a key exchange protocol (which also authenticates servers to
clients)
f) a protocol for user authentication
g) a session protocol
h) a re-keying protocol (v2-only)
Some of these parts of the ssh protocols are quite complex, some
quite straightforward. Altogether implementation of the ssh
protocols requires a source code base of significant size.
The OpenSSH implementation relies on OpenSSL for cryptographic
service, on libz for compression service and miscellaneous other
libraries. Besides these OpenSSH consists of several tens of
thousands of lines of source code in C.
SUNWssh is based on OpenSSH, so it is comparable in size and
complexity to OpenSSH.
There is, then, plenty of space for security bugs in the OpenSSH,
and, therefore, also in the SUNWssh source code bases.
The OpenSSH team designed and implemented a "privilege separation"
feature in their ssh server to reduce the risk that a security bug
in OpenSSH could be successfully exploited and an attacker's
privilege escalated.
3.1. Privileged Resources, Operations, in the SSH Protocols
What privileges does an SSH server need then?
Observation with Solaris 10's ppriv(1) and truss(1) commands as well
as analysis of the ssh protocols leads to conclude as follows.
No privilege or privileged resources are needed to implement the
parts (a)-(d) mentioned in section 3.
For key exchange and server authentication (e) an ssh server requires:
- Access to the host's ssh private keys.
- Access to the host's GSS-API acceptor credentials. [SSHv2-only]
An ssh server requires practically all privileges for user
authentication (f) (at least PAM does), particularly
PRIV_PROC_SETID, for logging the user in.
Post-authentication an ssh server requires the following privileges:
- Those required for auditing a user's subsequent logout.
That is, PRIV_PROC_AUDIT.
- Those required for record keeping (i.e., utmpx/wtmpx logging).
That is, either open file descriptor for those files or
PRIV_FILE_DAC_WRITE or otherwise access to those files, perhaps
through a special user id or group id which would be granted
write access through the ACLs on those files.
Since SSHv2 allows clients to open many channels with
pseudo-terminals a server may need to open and close utmpx/wtmpx
records multiple times in the lifetime of an SSHv2 connection.
- Those required for accessing the host's ssh private keys for
SSHv2 re-keying. [SSHv2-only]
These keys can be (and are) loaded at server startup time,
requiring PRIV_FILE_DAC_READ, or access through file ACLs, at
that time, but not thence.
- Those required for accessing the host's GSS-API acceptor
credentials for SSHv2 re-keying.
These credentials may require a large set of privileges. The
Solaris 10 Kerberos V GSS-API mechanism, for example, requires
PRIV_FILE_DAC_READ (for access to the system keytab) and
PRIV_FILE_DAC_WRITE (for access to the Kerberos V replay cache).
It is worth pointing out that because of a wrinkle in the
specification of the SSHv2 protocol and various implementations,
access to a host's ssh private keys can allow one not only to
impersonate the host as a server (which is, in practice, difficult),
but also to impersonate the host as a client (which is quite easy to
do) using "hostbased" user authentication.
It is entirely possible to have one-process server implementation
that drops most privileges and access to privileged resources after
user authentication succeeds. Such an implementation would make
some privileges, such as PRIV_PROC_SETID, available to any attacker
that successfully exploited a security bug in the ssh server.
But such an implementation would also have to retain access to
resources needed for authenticating the server, which, as described
above, can be used to impersonate the server, in some cases with
ease.
4. OpenSSH's Privilege Separation
The OpenSSH privilege separation model is quite complex.
It consists of a monitor, which retains all privileges and access to
privileged resources, and two processes which run with much less
privilege: one process running as a special user, "sshd," for
hosting all phases of the SSH protocols up to and including
authentication, and one process running as the actual user that logs
in and which hosts all phases of the SSH protocols post-user-
authentication.
The monitor and its companion processes speak a private protocol
over IPC. This protocol is intended to be smaller and simpler than
the SSH wire protocols.
In practice the OpenSSH monitor protocols relating to user
authentication are neither smaller nor simpler than the SSH user
authentication protocols; and though they are different they also
transport much the same data, including RSA/DSA signatures,
usernames, PAM conversations, and GSS-API context and MIC tokens.
The key exchange protocols have been broken down into their
essentials and the monitor serves only services such as signing
server replies with private host keys.
Note also that the OpenSSH monitor protocol uses the same encodings
as the SSH protocols and uses the same implementation of those
encodings.
5. SUNWssh's Alternative Privilege Separation
The Sun Microsystems ssh team believes that the OpenSSH team has
reached the point of diminishing returns in attempting to separate
processing of the user authentication protocols and that the OpenSSH
approach to privilege separation of the key exchange protocols has
led to a situation in which the monitor acts as an oracle, willing
to sign anything provided by the unprivileged processes that talk to
it.
The Sun ssh team proposes a somewhat different privilege separation
implementation that shares with the OpenSSH model the goal of
minimizing and simplifying the protocol spoken by the monitor, but
little source code.
We eschew any temptation to apply the privilege separation concept
to the version negotiation, initial key exchange and user
authentication phases of the ssh protocols (but see section 7).
Instead we focus on separating processing of auditing, record
keeping and re-keying from processing of the session protocols. We
also wish to avoid creating any oracles in the monitor.
This approach allows us to have a very simple monitor protocol. Our
monitor protocol consists of the following operations:
- record a new pseudo-terminal session
- record the end of a pseudo-terminal session
- process a re-key protocol messages
- get keys negotiated during re-keying to the session process to it
can use them
Logout auditing is done when the session process dies and so does
not require a monitor protocol message.
By processing all re-key protocol messages in the monitor we prevent
the creation of oracles in the monitor. This is so because the
monitor signs only material which it has generated and over which an
attacker would have little influence (through the attackers offered
DH public key, for example).
Odds and ends:
- If the monitor receives SIGHUP, SIGTERM or SIGINT it will call
fatal_cleanup(), and thence will forcibly shutdown(3SOCKET) the
ssh connection socket, causing its child to exit, and audit a
logout.
- The monitor does not attempt to update utmpx/wtmpx independently
of its child -- it depends on the child asking it to.
- The child now is unable to chown() ptys back to root. That's Ok,
other services on Solaris do the same and everything still works
because of grantpt(3C).
- The sshd server process (the one that will become a monitor)
forks a child process before the key exchange starts. The reason
for it is that if we forked after that we would end up using
PKCS#11 sessions initialized in the monitor unless
UseOpenSSLEngine was explicitly set to 'no'. Using any existing
PKCS#11 sessions or object handles over fork is what the PKCS#11
standard explicitly prohibits. To solve that, we would have to
rekey before fork and then newly initialize the engine in the
child, together with the new crypto contexts initialized with the
keys produced by the key re-exchange. And, that wouldn't help in
situations where the client does not support rekeying which also
includes the whole protocol version 1. The pre-fork solution is
simpler and also much faster. So, the key exchange and
authentication is fully done in the child server process while
the monitor waits aside to read the authentication context that
is needed for further operation. The child drops privileges after
the authentication finishes.
With the ssh client, the situation is slightly more complicated.
Given the fact that the user can request to go to the background
during the connection using the ~& sequence we must be prepared
to rekey before forking, to reinitialize the engine in the child
after that, and then set the new crypto contexts with the new
keys. If the server we are communicating with does not support
rekeying we will not use the engine at all. We expect this
situation to be extremely rare and will not offer any workaround
for that. This also includes the protocol version 1. However,
this version is already considered obsolete and should not be used
if possible.
6. Comparison of the OpenSSH and SUNWssh PrivSep Models
The OpenSSH server involves three processes which we will term
"pre-session," "session" and "monitor."
The OpenSSH pre-session process implements:
- the ssh version string exchange
- the ssh message encoding/decoding
- most of the initial key exchange protocols
- transport protection
- part of the user authentication protocols
The OpenSSH session process implements:
- the ssh message encoding/decoding
- transport protection
- most of the re-keying protocols
- the session protocols
The OpenSSH monitor process implements:
- the ssh message encoding/decoding
- parts of the key exchange and re-key protocols (primarily signing
of server replies with host private keys)
- most of the user authentication protocols, specifically:
- evaluation of ~/.ssh/authorized_keys (for pubkey userauth)
- evaluation of known hosts files (for hostbased userauth)
- evaluation of .shosts/.rhosts files (for hostbased userauth)
- verification of signatures w/ public keys (pubkey, hostbased)
- PAM API calls, conversation function
- GSS-API calls
Note that any vulnerabilities in the parsing of authorized_keys,
known hosts and .shosts/rhosts files are as exploitable in the
monitor as in a server w/o privilege separation.
Similarly for any vulnerabilities in PAM modules and GSS-API
mechanisms.
The SUNWssh server involves two processes which we will term
"session" and "monitor."
The SUNWssh monitor process implements:
- the ssh version string exchange
- the ssh message encoding/decoding
- transport protection
- all of the key exchange and re-key protocols
- all of the user authentication protocols
The SUNWssh session process implements:
- the ssh message encoding/decoding
- transport protection
- the session protocols
Obviously all of these processes also implement their side of the
monitor protocols.
The OpenSSH 3.5p1 monitor protocol, on Solaris, has approximately 20
monitor request and corresponding response messages.
The SUNWssh monitor protocol has 5 monitor request and response
messages; additionally, the monitor processes standard re-key
messages (but note: the monitor and the session process IPC is
completely unencrypted), which amounts to about 14 more messages
altogether.
Much of the OpenSSH monitor protocol is a variation of the
on-the-wire ssh protocols, with some contents re-packaging. We
believe this does not afford the monitor much additional, if any
protection from attacks in the key exchange and user authentication
protocols.
The re-packaging that is done in the OpenSSH monitor protocol is
risky business. By separating the act of signing some blob of data
from computing that blob of data one can create an oracle; this is
exactly what happened in the OpenSSH case.
As you can see in the next section, the SUNWssh privilege separation
could evolve somewhat in the OpenSSH direction by saving the monitor
all transport protection work, but we cannot save the monitor much,
if any work relating to authentication or key exchange.
7. Future Directions
The SUNWssh server privilege separation implementation could stand
several improvements.
The first improvement would be to have a single system-wide monitor.
This would reduce resource consumption. The work needed to
implement such an enhancement is very similar to the work needed to
produce an SSH API and library, and it is not trivial. If this is
not done then at least dropping PRIV_PROC_SETID and instead setting
the saved-set-user-id in the monitor to that of the logged in user
would be nice.
The second enhancement would be to add a "none" host key algorithm
to SSHv2 and a corresponding option in SUNWssh to disallow re-keying
with any other host key algorithm. This would allow customers to
configure their server and monitor so that no re-key protocol
messages need be processed by the monitor.
A third enhancement would be to enhance the GSS-API mechanisms to
require fewer privileges. In practice this means overhauling the
Kerberos V mechanism's replay cache. This would allow the monitor
to run with fewer privileges.
Further, even without improving the Kerberos V mechanism's replay
cache it should be possible to drop at least PRIV_PROC_FORK/EXEC/
SESSION.
A fourth enhancement would to have the unprivileged process handle
all transport protection and proxy to the monitor all key exchange
and user authentication protocol messages. This is a variation on
the OpenSSH model, but without the re-packaging of ssh message
contents seen there. After authentication succeeds the monitor
could either change the unprivileged process' credentials (as can be
done with ppriv(1) or the unprivileged process would, as in OpenSSH,
pass the session keys/IVs/keystate to the monitor which would then
pass them to a new process, the session process, that would then run
as the logged in user.
8. Guide to the AltPrivSep Source Code
First, a brief introduction to the SUNWssh/OpenSSH source code.
The source code is organized as follows:
$SRC/cmd/ssh/etc/
|
+-> config files
$SRC/cmd/ssh/include/
|
+-> header files (note: none are installed/shipped)
$SRC/cmd/ssh/libopenbsd-compat/common/
|
+-> misc. portability source code
$SRC/cmd/ssh/libssh/common/
|
+-> implementation of encoding, transport protection,
various wrappers around cryptography, the key exchange
and host authentication protocols, the session
protocols, and misc. other code
cipher.c
mac.c
compress.c
packet.c
|
+-> transport protocol
buffer.c
bufaux.c
|
+-> encoding
channels.c
nchan.c
|
+-> session protocol
kex.c
kexdh.c
kexgex.c
|
+-> key exchange/re-key code common to ssh and sshd
kexdhs.c
kexgexs.c
kexgsss.c
|
+-> key exchange/re-key code (server only)
kexdhc.c
kexgexc.c
kexgssc.c
|
+-> key exchange/re-key code (client only)
dh.c
rsa.c
mpaux.c
ssh-rsa.c
ssh-dss.c
ssh-gss.c
|
+-> crypto wrappers/utilities
log.c
|
+-> logging, including debug logging, on stderr or
syslog
$SRC/cmd/ssh/ssh/
|
+-> ssh(1)
$SRC/cmd/ssh/sshd/
|
+-> sshd(1M), including auditing, implementation of user
authentication and the OpenSSH and SUNWssh monitors
sshd.c
|
+-> main()
auth*.c
|
+-> user authentication
serverloop.c
session.c
|
+-> session protocols
bsmaudit.[ch]
sshlogin.c
loginrec.c
|
+-> auditing and record-keeping
$SRC/cmd/ssh/<misc commands>/
|
+-> scp, sftp, sftp-server, ssh-agent, ssh-add, ...
The SUNWssh altprivsep adds two new source files:
$SRC/cmd/ssh/include/altprivsep.h
$SRC/cmd/ssh/sshd/altprivsep.c
|
+-> monitor start routine, altprivsep_packet_*() routines
for communication with the monitor, routines to help
with key exchanges, service procedures for the monitor,
etc...
and modifies the following:
$SRC/cmd/ssh/include/config.h
|
+> adds cpp define "ALTPRIVSEP"
$SRC/cmd/ssh/include/ssh2.h
|
+-> adds private message type "SSH2_PRIV_MSG_ALTPRIVSEP" (254)
$SRC/cmd/ssh/include/packet.h
|
+-> adds prototypes for several simple utility functions,
some of which are specifically meant to avoid having to
link altprivsep.c into ssh(1)
$SRC/cmd/ssh/libssh/common/kex.c
$SRC/cmd/ssh/libssh/common/packet.c
|
+-> implements the hooks needed to proxy re-key messages
to/from the monitor
$SRC/cmd/ssh/sshd/Makefile
|
+-> adds altprivsep.o to list of objects linked into sshd(1M)
$SRC/cmd/ssh/sshd/serverloop.c
|
+-> adds an event loop for the monitor
modifies the usual event loops for SSHv2
$SRC/cmd/ssh/sshd/session.c
|
+-> modifies do_login() and session_pty_cleanup2() to call
altprivsep_record_login/logout() instead of
record_login/logout().
modifies do_exec_pty() so that the server waits for the
call to altprivsep_record_login() in child process to
complete before returning so that the server and the
child processes do not compete for monitor IPC I/O.
$SRC/cmd/ssh/include/log.h
$SRC/cmd/ssh/libssh/common/log.c
|
+-> adds an internal interface, set_log_txt_prefix() so that
the monitor's debug and log messages get prefixed with a
string ("monitor ") that indicates they are from the
monitor
$SRC/cmd/ssh/sshd/sshd.c
|
+-> modifies the body of code that follows the user
authentication phase of the ssh protocols so as to start
the monitor and move the relevant code into the monitor
or session processes as appropriate while dropping
privileges and access to privileged resources in the
session process
The monitor uses the packet.h interfaces to communicate with the
session process as though it were its ssh client peer, but always
uses the "none" cipher, mac and compression algorithms and installs
even handlers only for the relevant key exchange messages and the
private monitor message used for the other monitor services.
The monitor serves the following services:
- APS_MSG_NEWKEYS_REQ -> used to obtain keys/IVs after re-keys
- APS_MSG_RECORD_LOGIN -> used to update utmpx/wtmpx
- APS_MSG_RECORD_LOGOUT -> used to update utmpx/wtmpx
The session and monitor processes communicate over a pipe.
All monitor IPC I/O from the session process is blocking (though the
pipe is set to non-blocking I/O). The monitor protocol is entirely
synchronous and relies on the re-key protocols being entirely
synchronous also (which they are, unlike the session protocols).
The kex.c and packet.c files are minimally modified, primarily to
prevent the monitor from handling SSH_MSG_NEWKEYS messages as a
normal ssh server should, instead letting the session process
process SSH_MSG_NEWKEYS messages by requesting the new keys
negotiated with client from the monitor.
Note that for SSHv1 no on-the-wire messages are processed by the
monitor after authentication. In fact, the monitor thinks it's
running SSHv2, even if the on-the-wire protocol is v1.
A. References
The IETF SECSH Working Group:
http://www.ietf.org/html.charters/secsh-charter.html
The SSHv2 architecture, assigned numbers:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-architecture-16.txt
http://www.ietf.org/internet-drafts/draft-ietf-secsh-assignednumbers-06.txt
New cipher modes for SSHv2:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-newmodes-02.txt
The SSHv2 "transport," including initial key exchange and re-key
protocols, but excluding negotiable DH group size and GSS-API-based
key exchange:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-transport-18.txt
Additional key exchange protocols for SSHv2:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-gsskeyex-08.txt
http://www.ietf.org/internet-drafts/draft-ietf-secsh-dh-group-exchange-04.txt
Base user authentication spec for SSHv2 (includes none, password,
pubkey and hostbased user authentication):
http://www.ietf.org/internet-drafts/draft-ietf-secsh-userauth-21.txt
SSHv2 user authentication using PAM-style prompting:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-auth-kbdinteract-06.txt
SSHv2 user authentication using the GSS-API:
http://www.ietf.org/internet-drafts/draft-ietf-secsh-gsskeyex-08.txt
SSHv2 "session" protocol (i.e., the protocol used for pty sessions,
port forwarding, agent forwarding, X display forwarding, etc...):
http://www.ietf.org/internet-drafts/draft-ietf-secsh-connect-19.txt