libguestfs/docs/guestfs-security.pod

=head1 NAME

guestfs-security - security of libguestfs

=head1 DESCRIPTION

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

=head1 REPORTING SECURITY PROBLEMS

If you wish to privately report a security issue, please follow the
Red Hat security procedure at
L<https://access.redhat.com/security/team/contact>

If the security problem is not so serious, you can simply file a bug
(see L</BUGS> below), or send an email to our mailing list
(L<https://www.redhat.com/mailman/listinfo/libguestfs>).  You do not
need to subscribe to the mailing list to send email, but there will be
a delay while the message is moderated.

=head1 GENERAL ISSUES

=head2 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
(L<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 L<guestfs(3)/guestfs_mount_vfs>).

=head2 General security considerations

Be careful with any files or data that you download from a guest (by
"download" we mean not just the L<guestfs(3)/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:

=over 4

=item *

the data (file etc) not being present

=item *

being present but empty

=item *

being much larger than normal

=item *

containing arbitrary 8 bit data

=item *

being in an unexpected character encoding

=item *

containing homoglyphs.

=back

=head2 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.

=head2 Inspection security

Parts of the inspection API (see L<guestfs(3)/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
F</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.

=head2 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 L<guestfs(3)/RUNNING COMMANDS>.

=head1 HISTORICAL SECURITY ISSUES IN LIBGUESTFS

=head2 CVE-2010-3851

L<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:

=over 4

=item 1.

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

=item 2.

You are also running untrusted code from the guest (see
L<guestfs(3)/RUNNING COMMANDS>).

=back

The way to avoid this is to specify the expected disk format when
adding disks (the optional C<format> option to
L<guestfs(3)/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 L<guestfs(3)/DISK IMAGE FORMATS>).

For disks added from libvirt using calls like
L<guestfs(3)/guestfs_add_domain>, the format is fetched from libvirt
and passed through.

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

=head2 CVE-2011-4127

L<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 E<ge> 1.16 which
contains a change that mitigates the problem.

=head2 CVE-2012-2690

L<https://bugzilla.redhat.com/831117>

Old versions of both virt-edit and the guestfish C<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
F</etc/shadow> then it would be left world-readable after the edit.

It is sufficient to update libguestfs to any version E<ge> 1.16.

=head2 CVE-2013-2124

L<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 E<ge> 1.20.8, E<ge> 1.22.2 or E<ge> 1.23.2.

=head2 CVE-2013-4419

L<https://bugzilla.redhat.com/1016960>

When using the L<guestfish(1)> I<--remote> or guestfish I<--listen>
options, guestfish would create a socket in a known location
(F</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 (F</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 E<ge> 1.20.12, E<ge> 1.22.7 or E<ge> 1.24.

=head2 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 S<E<ge> 1.26.0>, S<E<ge>
1.24.6> and S<E<ge> 1.22.8>, and also in RHEL S<E<ge> 7.0>.
Earlier versions of libguestfs are not vulnerable.

=head2 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 E<ge> 1.27.9 and the fix was backported
to stable versions E<ge> 1.26.2, E<ge> 1.24.9, E<ge> 1.22.10 and E<ge>
1.20.13.

=head2 Shellshock (bash CVE-2014-6271)

This bash bug indirectly affects libguestfs.  For more information
see:
L<https://www.redhat.com/archives/libguestfs/2014-September/msg00252.html>

=head2 CVE-2014-8484

=head2 CVE-2014-8485

These two bugs in binutils affect the GNU L<strings(1)> program, and
thus the L<guestfs(3)/guestfs_strings> and
L<guestfs(3)/guestfs_strings_e> APIs in libguestfs.  Running strings
on an untrusted file could cause arbitrary code execution (confined to
the libguestfs appliance).

In libguestfs E<ge> 1.29.5 and E<ge> 1.28.3, libguestfs uses the
C<strings> I<-a> option to avoid BFD parsing on the file.

=head2 CVE-2015-5745

L<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:

=over 4

=item *

your libguestfs program runs untrusted programs out of the guest
(using L<guestfs(3)/guestfs_sh> etc), or

=item *

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

=back

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

=head2 Permissions of F<.ssh> and F<.ssh/authorized_keys>

L<https://bugzilla.redhat.com/1260778>

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

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

=head2 CVE-2015-8869

L<https://bugzilla.redhat.com/CVE-2015-8869>

This vulnerability in OCaml might affect virt tools written in the
OCaml programming language.  It affects only 64 bit platforms.
Because this bug affects code generation it is difficult to predict
which precise software could be affected, and therefore our
recommendation is that you recompile libguestfs using a version of the
OCaml compiler where this bug has been fixed (or ask your Linux distro
to do the same).

=head1 SEE ALSO

L<guestfs(3)>,
L<guestfs-internals(3)>,
L<guestfs-release-notes(1)>,
L<guestfs-testing(1)>,
L<http://libguestfs.org/>.

=head1 AUTHORS

Richard W.M. Jones (C<rjones at redhat dot com>)

=head1 COPYRIGHT

Copyright (C) 2009-2016 Red Hat Inc.