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Kevin Steves

2002-Jun-24 16:52 UTC

README.privsep

Hi,

This is included in the release now; any feedback?

Privilege separation, or privsep, is method in OpenSSH by which
operations that require root privilege are performed by a separate
privileged monitor process.  Its purpose is to prevent privilege
escalation by containing corruption to an unprivileged process.  
More information is available at:
	http://www.citi.umich.edu/u/provos/ssh/privsep.html

Privilege separation is now enabled by default; see the
UsePrivilegeSeparation option in sshd_config(5).

On systems which lack mmap or anonymous (MAP_ANON) memory mapping, 
compression must be disabled in order for privilege separation to 
function.

When privsep is enabled, during the pre-authentication phase sshd will
chroot(2) to "/var/empty" and change its privileges to the
"sshd" user
and its primary group.  You should do something like the following to
prepare the privsep preauth environment:

	# mkdir /var/empty
	# chown root:sys /var/empty
	# chmod 755 /var/empty
	# groupadd sshd
	# useradd -g sshd -c 'sshd privsep' -d /var/empty sshd

If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
	# ln /usr/lib/.ns.so /usr/lib/ns.so.1

/var/empty should not contain any files.

configure supports the following options to change the default
privsep user and chroot directory:

  --with-privsep-path=xxx Path for privilege separation chroot
  --with-privsep-user=user Specify non-privileged user for privilege separation

Privsep requires operating system support for file descriptor passing
and mmap(MAP_ANON).

PAM-enabled OpenSSH is known to function with privsep on Linux.  
It does not function on HP-UX with a trusted system
configuration.  PAMAuthenticationViaKbdInt does not function with
privsep.

Note that for a normal interactive login with a shell, enabling privsep
will require 1 additional process per login session.

Given the following process listing (from HP-UX):

     UID   PID  PPID  C    STIME TTY       TIME COMMAND
    root  1005     1  0 10:45:17 ?         0:08 /opt/openssh/sbin/sshd -u0
    root  6917  1005  0 15:19:16 ?         0:00 sshd: stevesk [priv]
 stevesk  6919  6917  0 15:19:17 ?         0:03 sshd: stevesk at 2
 stevesk  6921  6919  0 15:19:17 pts/2     0:00 -bash

process 1005 is the sshd process listening for new connections.
process 6917 is the privileged monitor process, 6919 is the user owned
sshd process and 6921 is the shell process.

$Id: README.privsep,v 1.8 2002/06/24 16:49:22 stevesk Exp $

Ben Lindstrom

2002-Jun-24 17:52 UTC

head link

README.privsep

Maybe we should state how to change the user and the chroot before the
step by step.  I can see someone changing the privsep user and whining
about things not work.

- Ben

On Mon, 24 Jun 2002, Kevin Steves wrote:
> Hi,
>
> This is included in the release now; any feedback?
>
> Privilege separation, or privsep, is method in OpenSSH by which
> operations that require root privilege are performed by a separate
> privileged monitor process.  Its purpose is to prevent privilege
> escalation by containing corruption to an unprivileged process.
> More information is available at:
> 	http://www.citi.umich.edu/u/provos/ssh/privsep.html
>
> Privilege separation is now enabled by default; see the
> UsePrivilegeSeparation option in sshd_config(5).
>
> On systems which lack mmap or anonymous (MAP_ANON) memory mapping,
> compression must be disabled in order for privilege separation to
> function.
>
> When privsep is enabled, during the pre-authentication phase sshd will
> chroot(2) to "/var/empty" and change its privileges to the
"sshd" user
> and its primary group.  You should do something like the following to
> prepare the privsep preauth environment:
>
> 	# mkdir /var/empty
> 	# chown root:sys /var/empty
> 	# chmod 755 /var/empty
> 	# groupadd sshd
> 	# useradd -g sshd -c 'sshd privsep' -d /var/empty sshd
>
> If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
> 	# ln /usr/lib/.ns.so /usr/lib/ns.so.1
>
> /var/empty should not contain any files.
>
> configure supports the following options to change the default
> privsep user and chroot directory:
>
>   --with-privsep-path=xxx Path for privilege separation chroot
>   --with-privsep-user=user Specify non-privileged user for privilege
separation
>
> Privsep requires operating system support for file descriptor passing
> and mmap(MAP_ANON).
>
> PAM-enabled OpenSSH is known to function with privsep on Linux.
> It does not function on HP-UX with a trusted system
> configuration.  PAMAuthenticationViaKbdInt does not function with
> privsep.
>
> Note that for a normal interactive login with a shell, enabling privsep
> will require 1 additional process per login session.
>
> Given the following process listing (from HP-UX):
>
>      UID   PID  PPID  C    STIME TTY       TIME COMMAND
>     root  1005     1  0 10:45:17 ?         0:08 /opt/openssh/sbin/sshd -u0
>     root  6917  1005  0 15:19:16 ?         0:00 sshd: stevesk [priv]
>  stevesk  6919  6917  0 15:19:17 ?         0:03 sshd: stevesk at 2
>  stevesk  6921  6919  0 15:19:17 pts/2     0:00 -bash
>
> process 1005 is the sshd process listening for new connections.
> process 6917 is the privileged monitor process, 6919 is the user owned
> sshd process and 6921 is the shell process.
>
> $Id: README.privsep,v 1.8 2002/06/24 16:49:22 stevesk Exp $
> _______________________________________________
> openssh-unix-dev at mindrot.org mailing list
> http://www.mindrot.org/mailman/listinfo/openssh-unix-dev
>

Jim Knoble

2002-Jun-24 23:25 UTC

head link

README.privsep

Circa 2002-Jun-24 09:52:28 -0700 dixit Kevin Steves:

: This is included in the release now; any feedback?

  [...]

: When privsep is enabled, during the pre-authentication phase sshd will
: chroot(2) to "/var/empty" and change its privileges to the
"sshd" user
: and its primary group.  You should do something like the following to
: prepare the privsep preauth environment:
:
:       # mkdir /var/empty
:       # chown root:sys /var/empty

I would rather say here:

          chown 0 /var/empty
          chgrp 0 /var/empty

since several systems differ in which group is GID 0 (root, wheel,
sys), and since a few systems differ in the syntax that chown accepts
for specifying both UID and GID together ('chown uid:gid' versus
'chown
uid.gid').  Recommending the above syntax avoids the problem entirely.

:       # chmod 755 /var/empty
:       # groupadd sshd
:       # useradd -g sshd -c 'sshd privsep' -d /var/empty sshd

I'd also recommend '-s /dev/null' here, e.g.:

          useradd -g sshd -c 'sshd privsep' -d /var/empty \
            -s /dev/null sshd

since '/sbin/nologin' cannot be guaranteed to be present, nor is
'/bin/false' always a binary program (i've seen some cases where
it's a
shell script).

  [...]

: Privsep requires operating system support for file descriptor passing
: and mmap(MAP_ANON).
:
: PAM-enabled OpenSSH is known to function with privsep on Linux.

Would it be appropriate here to note that setting 'Compression no' in
/etc/sshd_config is necessary on Linux systems with 2.2.x or older
kernels?

--
jim knoble  |  jmknoble at pobox.com  |  http://www.pobox.com/~jmknoble/
(GnuPG fingerprint: 31C4:8AAC:F24E:A70C:4000::BBF4:289F:EAA8:1381:1491)
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Tim Rice

2002-Jun-25 01:56 UTC

head link

README.privsep

On Mon, 24 Jun 2002, Kevin Steves wrote:
> Hi,
>
> If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
> 	# ln /usr/lib/.ns.so /usr/lib/ns.so.1
Drop these two lines. If we are not going to do something like the patch
below to fix the initgroups problem on UnixWare and some Linux, then
add some lines like

On UnixWare, OpenUNIX, and some Linux systems you will have to
	# mkdir /var/empty/etc
	# touch /var/empty/etc/group


--- session.c.orig	Mon Jun 24 07:29:13 2002
+++ session.c	Mon Jun 24 18:49:49 2002
@@ -1180,6 +1180,7 @@
 			exit(1);
 		}
 		/* Initialize the group list. */
+		if (strcmp(pw->pw_name, SSH_PRIVSEP_USER))
 		if (initgroups(pw->pw_name, pw->pw_gid) < 0) {
 			perror("initgroups");
 			exit(1);

-- 
Tim Rice				Multitalents	(707) 887-1469
tim at multitalents.net

Kevin Steves

2002-Jun-25 23:17 UTC

head link

README.privsep

On Mon, Jun 24, 2002 at 06:56:36PM -0700, Tim Rice
wrote:> On Mon, 24 Jun 2002, Kevin Steves wrote:
> 
> > Hi,
> >
> > If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
> > 	# ln /usr/lib/.ns.so /usr/lib/ns.so.1
> 
> Drop these two lines. If we are not going to do something like the patch
> below to fix the initgroups problem on UnixWare and some Linux, then
> add some lines like
> 
> On UnixWare, OpenUNIX, and some Linux systems you will have to
> 	# mkdir /var/empty/etc
> 	# touch /var/empty/etc/group
we have this now:
http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/sshd.c.diff?r1=1.250&r2=1.251

the ln stuff can go away now?

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