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Default to GSSAPI (Kerberos) instead of DIGEST-MD5 for SASL

RFC 6331 documents a number of serious security weaknesses in
the SASL DIGEST-MD5 mechanism. As such, QEMU should not be
using or recommending it as a default mechanism for VNC auth
with SASL.

GSSAPI (Kerberos) is the only other viable SASL mechanism that
can provide secure session encryption so enable that by defalt
as the replacement. If users have TLS enabled for VNC, they can
optionally decide to use SCRAM-SHA-1 instead of GSSAPI, allowing
plain username and password auth.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
tags/v2.10.0-rc0
Daniel P. Berrange 2 years ago
parent
commit
c6a9a9f575
2 changed files with 60 additions and 44 deletions
  1. +29
    -21
      qemu-doc.texi
  2. +31
    -23
      qemu.sasl

+ 29
- 21
qemu-doc.texi View File

@@ -1732,37 +1732,45 @@ SASL service config /etc/sasl2/qemu.conf. If running QEMU as an
unprivileged user, an environment variable SASL_CONF_PATH can be used
to make it search alternate locations for the service config.

The default configuration might contain
If the TLS option is enabled for VNC, then it will provide session encryption,
otherwise the SASL mechanism will have to provide encryption. In the latter
case the list of possible plugins that can be used is drastically reduced. In
fact only the GSSAPI SASL mechanism provides an acceptable level of security
by modern standards. Previous versions of QEMU referred to the DIGEST-MD5
mechanism, however, it has multiple serious flaws described in detail in
RFC 6331 and thus should never be used any more. The SCRAM-SHA-1 mechanism
provides a simple username/password auth facility similar to DIGEST-MD5, but
does not support session encryption, so can only be used in combination with
TLS.

When not using TLS the recommended configuration is

@example
mech_list: digest-md5
sasldb_path: /etc/qemu/passwd.db
mech_list: gssapi
keytab: /etc/qemu/krb5.tab
@end example

This says to use the 'Digest MD5' mechanism, which is similar to the HTTP
Digest-MD5 mechanism. The list of valid usernames & passwords is maintained
in the /etc/qemu/passwd.db file, and can be updated using the saslpasswd2
command. While this mechanism is easy to configure and use, it is not
considered secure by modern standards, so only suitable for developers /
ad-hoc testing.
This says to use the 'GSSAPI' mechanism with the Kerberos v5 protocol, with
the server principal stored in /etc/qemu/krb5.tab. For this to work the
administrator of your KDC must generate a Kerberos principal for the server,
with a name of 'qemu/somehost.example.com@@EXAMPLE.COM' replacing
'somehost.example.com' with the fully qualified host name of the machine
running QEMU, and 'EXAMPLE.COM' with the Kerberos Realm.

A more serious deployment might use Kerberos, which is done with the 'gssapi'
mechanism
When using TLS, if username+password authentication is desired, then a
reasonable configuration is

@example
mech_list: gssapi
keytab: /etc/qemu/krb5.tab
mech_list: scram-sha-1
sasldb_path: /etc/qemu/passwd.db
@end example

For this to work the administrator of your KDC must generate a Kerberos
principal for the server, with a name of 'qemu/somehost.example.com@@EXAMPLE.COM'
replacing 'somehost.example.com' with the fully qualified host name of the
machine running QEMU, and 'EXAMPLE.COM' with the Kerberos Realm.
The saslpasswd2 program can be used to populate the passwd.db file with
accounts.

Other configurations will be left as an exercise for the reader. It should
be noted that only Digest-MD5 and GSSAPI provides a SSF layer for data
encryption. For all other mechanisms, VNC should always be configured to
use TLS and x509 certificates to protect security credentials from snooping.
Other SASL configurations will be left as an exercise for the reader. Note that
all mechanisms except GSSAPI, should be combined with use of TLS to ensure a
secure data channel.

@node gdb_usage
@section GDB usage

+ 31
- 23
qemu.sasl View File

@@ -1,36 +1,44 @@
# If you want to use the non-TLS socket, then you *must* include
# the GSSAPI or DIGEST-MD5 mechanisms, because they are the only
# ones that can offer session encryption as well as authentication.
# If you want to use VNC remotely without TLS, then you *must*
# pick a mechanism which provides session encryption as well
# as authentication.
#
# If you're only using TLS, then you can turn on any mechanisms
# If you are only using TLS, then you can turn on any mechanisms
# you like for authentication, because TLS provides the encryption
#
# Default to a simple username+password mechanism
# NB digest-md5 is no longer considered secure by current standards
mech_list: digest-md5
# If you are only using UNIX sockets then encryption is not
# required at all.
#
# NB, previously DIGEST-MD5 was set as the default mechanism for
# QEMU VNC. Per RFC 6331 this is vulnerable to many serious security
# flaws as should no longer be used. Thus GSSAPI is now the default.
#
# To use GSSAPI requires that a QEMU service principal is
# added to the Kerberos server for each host running QEMU.
# This principal needs to be exported to the keytab file listed below
mech_list: gssapi

# Before you can use GSSAPI, you need a service principle on the
# KDC server for libvirt, and that to be exported to the keytab
# file listed below
#mech_list: gssapi
# If using TLS with VNC, or a UNIX socket only, it is possible to
# enable plugins which don't provide session encryption. The
# 'scram-sha-1' plugin allows plain username/password authentication
# to be performed
#
# You can also list many mechanisms at once, then the user can choose
# by adding '?auth=sasl.gssapi' to their libvirt URI, eg
# qemu+tcp://hostname/system?auth=sasl.gssapi
#mech_list: digest-md5 gssapi
#mech_list: scram-sha-1

# You can also list many mechanisms at once, and the VNC server will
# negotiate which to use by considering the list enabled on the VNC
# client.
#mech_list: scram-sha-1 gssapi

# Some older builds of MIT kerberos on Linux ignore this option &
# instead need KRB5_KTNAME env var.
# For modern Linux, and other OS, this should be sufficient
#
# There is no default value here, uncomment if you need this
#keytab: /etc/qemu/krb5.tab
# This file needs to be populated with the service principal that
# was created on the Kerberos v5 server. If switching to a non-gssapi
# mechanism this can be commented out.
keytab: /etc/qemu/krb5.tab

# If using digest-md5 for username/passwds, then this is the file
# If using scram-sha-1 for username/passwds, then this is the file
# containing the passwds. Use 'saslpasswd2 -a qemu [username]'
# to add entries, and 'sasldblistusers2 -f [sasldb_path]' to browse it
sasldb_path: /etc/qemu/passwd.db


auxprop_plugin: sasldb

#sasldb_path: /etc/qemu/passwd.db

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