Ubuntu Manpage: guestfs-security - security of libguestfs

Provided by: libguestfs0_1.32.2-4ubuntu2.2_amd64

NAME

       guestfs-security - security of libguestfs

DESCRIPTION

This manual page discusses security implications of using libguestfs, particularly with untrusted or
malicious guests or disk images.

SECURITY OF MOUNTING FILESYSTEMS
You should never mount an untrusted guest filesystem directly on your host kernel (eg. using loopback or
kpartx).

When you mount a filesystem, mistakes in the kernel filesystem (VFS) can be escalated into exploits by
attackers creating a malicious filesystem. These exploits are very severe for two reasons. Firstly
there are very many filesystem drivers in the kernel, and many of them are infrequently used and not much
developer attention has been paid to the code. Linux userspace helps potential crackers by detecting the
filesystem type and automatically choosing the right VFS driver, even if that filesystem type is
unexpected. Secondly, a kernel-level exploit is like a local root exploit (worse in some ways), giving
immediate and total access to the system right down to the hardware level.

These exploits can be present in the kernel for a very long time (https://lwn.net/Articles/538898/).

Libguestfs provides a layered approach to protecting you from exploits:

untrusted filesystem
--------------------------------------
appliance kernel
--------------------------------------
qemu process running as non-root
--------------------------------------
sVirt [if using libvirt + SELinux]
--------------------------------------
host kernel

We run a Linux kernel inside a qemu virtual machine, usually running as a non-root user. The attacker
would need to write a filesystem which first exploited the kernel, and then exploited either qemu
virtualization (eg. a faulty qemu driver) or the libguestfs protocol, and finally to be as serious as the
host kernel exploit it would need to escalate its privileges to root. Additionally if you use the
libvirt back end and SELinux, sVirt is used to confine the qemu process. This multi-step escalation,
performed by a static piece of data, is thought to be extremely hard to do, although we never say 'never'
about security issues.

Callers can also reduce the attack surface by forcing the filesystem type when mounting (use
"guestfs_mount_vfs").

GENERAL SECURITY CONSIDERATIONS
Be careful with any files or data that you download from a guest (by "download" we mean not just the
"guestfs_download" command but any command that reads files, filenames, directories or anything else from
a disk image). An attacker could manipulate the data to fool your program into doing the wrong thing.
Consider cases such as:

• the data (file etc) not being present

• being present but empty

• being much larger than normal

• containing arbitrary 8 bit data

• being in an unexpected character encoding

• containing homoglyphs.

PROTOCOL SECURITY
The protocol is designed to be secure, being based on RFC 4506 (XDR) with a defined upper message size.
However a program that uses libguestfs must also take care - for example you can write a program that
downloads a binary from a disk image and executes it locally, and no amount of protocol security will
save you from the consequences.

INSPECTION SECURITY
Parts of the inspection API (see "INSPECTION") return untrusted strings directly from the guest, and
these could contain any 8 bit data. Callers should be careful to escape these before printing them to a
structured file (for example, use HTML escaping if creating a web page).

Guest configuration may be altered in unusual ways by the administrator of the virtual machine, and may
not reflect reality (particularly for untrusted or actively malicious guests). For example we parse the
hostname from configuration files like /etc/sysconfig/network that we find in the guest, but the guest
administrator can easily manipulate these files to provide the wrong hostname.

The inspection API parses guest configuration using two external libraries: Augeas (Linux configuration)
and hivex (Windows Registry). Both are designed to be robust in the face of malicious data, although
denial of service attacks are still possible, for example with oversized configuration files.

RUNNING UNTRUSTED GUEST COMMANDS
Be very cautious about running commands from the guest. By running a command in the guest, you are
giving CPU time to a binary that you do not control, under the same user account as the library, albeit
wrapped in qemu virtualization. More information and alternatives can be found in the section "RUNNING
COMMANDS".

CVE-2010-3851
https://bugzilla.redhat.com/642934

This security bug concerns the automatic disk format detection that qemu does on disk images.

A raw disk image is just the raw bytes, there is no header. Other disk images like qcow2 contain a
special header. Qemu deals with this by looking for one of the known headers, and if none is found then
assuming the disk image must be raw.

This allows a guest which has been given a raw disk image to write some other header. At next boot (or
when the disk image is accessed by libguestfs) qemu would do autodetection and think the disk image
format was, say, qcow2 based on the header written by the guest.

This in itself would not be a problem, but qcow2 offers many features, one of which is to allow a disk
image to refer to another image (called the "backing disk"). It does this by placing the path to the
backing disk into the qcow2 header. This path is not validated and could point to any host file (eg.
"/etc/passwd"). The backing disk is then exposed through "holes" in the qcow2 disk image, which of
course is completely under the control of the attacker.

In libguestfs this is rather hard to exploit except under two circumstances:

1. You have enabled the network or have opened the disk in write mode.

2. You are also running untrusted code from the guest (see "RUNNING COMMANDS").

The way to avoid this is to specify the expected disk format when adding disks (the optional "format"
option to "guestfs_add_drive_opts"). You should always do this if the disk is raw format, and it's a
good idea for other cases too. (See also "DISK IMAGE FORMATS").

For disks added from libvirt using calls like "guestfs_add_domain", the format is fetched from libvirt
and passed through.

For libguestfs tools, use the --format command line parameter as appropriate.

CVE-2011-4127
https://bugzilla.redhat.com/752375

This is a bug in the kernel which allowed guests to overwrite parts of the host's drives which they
should not normally have access to.

It is sufficient to update libguestfs to any version ≥ 1.16 which contains a change that mitigates the
problem.

CVE-2012-2690
https://bugzilla.redhat.com/831117

Old versions of both virt-edit and the guestfish "edit" command created a new file containing the changes
but did not set the permissions, etc of the new file to match the old one. The result of this was that
if you edited a security sensitive file such as /etc/shadow then it would be left world-readable after
the edit.

It is sufficient to update libguestfs to any version ≥ 1.16.

CVE-2013-2124
https://bugzilla.redhat.com/968306

This security bug was a flaw in inspection where an untrusted guest using a specially crafted file in the
guest OS could cause a double-free in the C library (denial of service).

It is sufficient to update libguestfs to a version that is not vulnerable: libguestfs ≥ 1.20.8, ≥ 1.22.2
or ≥ 1.23.2.

CVE-2013-4419
https://bugzilla.redhat.com/1016960

When using the guestfish(1) --remote or guestfish --listen options, guestfish would create a socket in a
known location (/tmp/.guestfish-$UID/socket-$PID).

The location has to be a known one in order for both ends to communicate. However no checking was done
that the containing directory (/tmp/.guestfish-$UID) is owned by the user. Thus another user could
create this directory and potentially hijack sockets owned by another user's guestfish client or server.

It is sufficient to update libguestfs to a version that is not vulnerable: libguestfs ≥ 1.20.12, ≥ 1.22.7
or ≥ 1.24.

Denial of service when inspecting disk images with corrupt btrfs volumes
It was possible to crash libguestfs (and programs that use libguestfs as a library) by presenting a disk
image containing a corrupt btrfs volume.

This was caused by a NULL pointer dereference causing a denial of service, and is not thought to be
exploitable any further.

See commit d70ceb4cbea165c960710576efac5a5716055486 for the fix. This fix is included in libguestfs
stable branches ≥ 1.26.0, ≥ 1.24.6 and ≥ 1.22.8, and also in RHEL ≥ 7.0. Earlier versions of libguestfs
are not vulnerable.

CVE-2014-0191
Libguestfs previously used unsafe libxml2 APIs for parsing libvirt XML. These APIs defaulted to allowing
network connections to be made when certain XML documents were presented. Using a malformed XML document
it was also possible to exhaust all CPU, memory or file descriptors on the machine.

Since the libvirt XML comes from a trusted source (the libvirt daemon) it is not thought that this could
have been exploitable.

This was fixed in libguestfs ≥ 1.27.9 and the fix was backported to stable versions ≥ 1.26.2, ≥ 1.24.9, ≥
1.22.10 and ≥ 1.20.13.

Shellshock (bash CVE-2014-6271)
This bash bug indirectly affects libguestfs. For more information see:
https://www.redhat.com/archives/libguestfs/2014-September/msg00252.html

CVE-2014-8484
CVE-2014-8485
These two bugs in binutils affect the GNU strings(1) program, and thus the "guestfs_strings" and
"guestfs_strings_e" APIs in libguestfs. Running strings on an untrusted file could cause arbitrary code
execution (confined to the libguestfs appliance).

In libguestfs ≥ 1.29.5 and ≥ 1.28.3, libguestfs uses the "strings" -a option to avoid BFD parsing on the
file.

CVE-2015-5745
https://bugzilla.redhat.com/show_bug.cgi?id=1251157

This is not a vulnerability in libguestfs, but because we always give a virtio-serial port to each guest
(since that is how guest-host communication happens), an escalation from the appliance to the host qemu
process is possible. This could affect you if:

• your libguestfs program runs untrusted programs out of the guest (using "guestfs_sh" etc), or

• another exploit was found in (for example) kernel filesystem code that allowed a malformed filesystem
to take over the appliance.

If you use sVirt to confine qemu, that would thwart some attacks.

Permissions of .ssh and .ssh/authorized_keys
https://bugzilla.redhat.com/1260778

The tools virt-customize(1), virt-sysprep(1) and virt-builder(1) have an --ssh-inject option for
injecting an SSH key into virtual machine disk images. They may create a ~user/.ssh directory and
~user/.ssh/authorized_keys file in the guest to do this.

In libguestfs < 1.31.5 and libguestfs < 1.30.2, the new directory and file would get mode 0755 and mode
0644 respectively. However these permissions (especially for ~user/.ssh) are wider than the permissions
that OpenSSH uses. In current libguestfs, the directory and file are created with mode 0700 and mode
0600.

AUTHORS

       Richard W.M. Jones ("rjones at redhat dot com")

COPYRIGHT

       Copyright (C) 2009-2016 Red Hat Inc.

LICENSE

       This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser
       General  Public License as published by the Free Software Foundation; either version 2 of the License, or
       (at your option) any later version.

       This library is distributed in the hope that it will be useful, but WITHOUT ANY  WARRANTY;  without  even
       the  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General
       Public License for more details.

       You should have received a copy of the GNU Lesser General Public License along with this library; if not,
       write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

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