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George Dunlap
384a6ddf3d
In order for a PV domain to set up DMA from a passed-through device to
one of its pages, the page must be mapped in the IOMMU. On the other
hand, before a PV page may be used as a "special" page type (such as a
pagetable or descriptor table), it _must not_ be writable in the IOMMU
(otherwise a malicious guest could DMA arbitrary page tables into the
memory, bypassing Xen's safety checks); and Xen's current rule is to
have such pages not in the IOMMU at all.
At the moment, in order to accomplish this, the code borrows HVM
domain's "physmap" concept: When a page is assigned to a guest,
guess_physmap_add_entry() is called, which for PV guests, will create
a writable IOMMU mapping; and when a page is removed,
guest_physmap_remove_entry() is called, which will remove the mapping.
Additionally, when a page gains the PGT_writable page type, the page
will be added into the IOMMU; and when the page changes away from a
PGT_writable type, the page will be removed from the IOMMU.
Unfortunately, borrowing the "physmap" concept from HVM domains is
problematic. HVM domains have a lock on their p2m tables, ensuring
synchronization between modifications to the p2m; and all hypercall
parameters must first be translated through the p2m before being used.
Trying to mix this locked-and-gated approach with PV's lock-free
approach leads to several races and inconsistencies:
* A race between a page being assigned and it being put into the
physmap; for example:
- P1: call populate_physmap() { A = allocate_domheap_pages() }
- P2: Guess page A's mfn, and call decrease_reservation(A). A is owned by the domain,
and so Xen will clear the PGC_allocated bit and free the page
- P1: finishes populate_physmap() { guest_physmap_add_entry() }
Now the domain has a writable IOMMU mapping to a page it no longer owns.
* Pages start out as type PGT_none, but with a writable IOMMU mapping.
If a guest uses a page as a page table without ever having created a
writable mapping, the IOMMU mapping will not be removed; the guest
will have a writable IOMMU mapping to a page it is currently using
as a page table.
* A newly-allocated page can be DMA'd into with no special actions on
the part of the guest; However, if a page is promoted to a
non-writable type, the page must be mapped with a writable type before
DMA'ing to it again, or the transaction will fail.
To fix this, do away with the "PV physmap" concept entirely, and
replace it with the following IOMMU discipline for PV guests:
- (type == PGT_writable) <=> in iommu (even if type_count == 0)
- Upon a final put_page(), check to see if type is PGT_writable; if so,
iommu_unmap.
In order to achieve that:
- Remove PV IOMMU related code from guest_physmap_*
- Repurpose cleanup_page_cacheattr() into a general
cleanup_page_mappings() function, which will both fix up Xen
mappings for pages with special cache attributes, and also check for
a PGT_writable type and remove pages if appropriate.
- For compatibility with current guests, grab-and-release a
PGT_writable_page type for PV guests in guest_physmap_add_entry().
This will cause most "normal" guest pages to start out life with
PGT_writable_page type (and thus an IOMMU mapping), but no type
count (so that they can be used as special cases at will).
Also, note that there is one exception to to the "PGT_writable => in
iommu" rule: xenheap pages shared with guests may be given a
PGT_writable type with one type reference. This reference prevents
the type from changing, which in turn prevents page from gaining an
IOMMU mapping in get_page_type(). It's not clear whether this was
intentional or not, but it's not something to change in a security
update.
This is XSA-288.
Reported-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
master commit: fe21b78ef9
master date: 2019-03-05 13:48:32 +0100
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http://www.xen.org/
What is Xen?
============
Xen is a Virtual Machine Monitor (VMM) originally developed by the
Systems Research Group of the University of Cambridge Computer
Laboratory, as part of the UK-EPSRC funded XenoServers project. Xen
is freely-distributable Open Source software, released under the GNU
GPL. Since its initial public release, Xen has grown a large
development community, spearheaded by xen.org (http://www.xen.org).
This file contains some quick-start instructions to install Xen on
your system. For more information see http:/www.xen.org/ and
http://wiki.xen.org/
Quick-Start Guide
=================
First, this is just a quick-start guide. For more comprehensive
information see the INSTALL file and the Xen wiki at
http://wiki.xenproject.org and in particular
http://wiki.xenproject.org/wiki/Getting_Started.
Second, there are a number of prerequisites for building a Xen source
release. Make sure you have all the following installed, either by
visiting the project webpage or installing a pre-built package
provided by your OS distributor:
* GNU Make v3.80 or later
* C compiler and linker:
- For x86:
- GCC 4.1.2_20070115 or later
- GNU Binutils 2.16.91.0.5 or later
or
- Clang/LLVM 3.5 or later
- For ARM:
- GCC 4.8 or later
- GNU Binutils 2.24 or later
* Development install of zlib (e.g., zlib-dev)
* Development install of Python v2.3 or later (e.g., python-dev)
* Development install of curses (e.g., libncurses-dev)
* Development install of openssl (e.g., openssl-dev)
* Development install of x11 (e.g. xorg-x11-dev)
* Development install of uuid (e.g. uuid-dev)
* Development install of yajl (e.g. libyajl-dev)
* Development install of libaio (e.g. libaio-dev) version 0.3.107 or
greater.
* Development install of GLib v2.0 (e.g. libglib2.0-dev)
* Development install of Pixman (e.g. libpixman-1-dev)
* pkg-config
* bridge-utils package (/sbin/brctl)
* iproute package (/sbin/ip)
* GNU bison and GNU flex
* GNU gettext
* ACPI ASL compiler (iasl)
* Libc multiarch package (e.g. libc6-dev-i386 / glibc-devel.i686).
Required when building on a 64-bit platform to build
32-bit components which are enabled on a default build.
In addition to the above there are a number of optional build
prerequisites. Omitting these will cause the related features to be
disabled at compile time:
* Development install of Ocaml (e.g. ocaml-nox and
ocaml-findlib). Required to build ocaml components which
includes the alternative ocaml xenstored.
* cmake (if building vtpm stub domains)
* markdown
* figlet (for generating the traditional Xen start of day banner)
* systemd daemon development files
* Development install of libnl3 (e.g., libnl-3-200,
libnl-3-dev, etc). Required if network buffering is desired
when using Remus with libxl. See docs/README.remus for detailed
information.
* 16-bit x86 assembler, loader and compiler for qemu-traditional / rombios
(dev86 rpm or bin86 & bcc debs)
Second, you need to acquire a suitable kernel for use in domain 0. If
possible you should use a kernel provided by your OS distributor. If
no suitable kernel is available from your OS distributor then refer to
http://wiki.xen.org/wiki/XenDom0Kernels for suggestions for
suitable kernels to use.
If you are looking to compile a Dom0 kernel from source, please refer to
http://wiki.xen.org/wiki/XenParavirtOps.
[NB. Unless noted otherwise, all the following steps should be
performed with root privileges.]
1. Download and untar the source tarball file. This will be a
file named xen-unstable-src.tgz, or xen-$version-src.tgz.
You can also pull the current version from the git or mercurial
repositories at http://xenbits.xen.org/
# tar xzf xen-unstable-src.tgz
Assuming you are using the unstable tree, this will
untar into xen-unstable. The rest of the instructions
use the unstable tree as an example, substitute the
version for unstable.
2. cd to xen-unstable (or whatever you sensibly rename it to).
3. For the very first build, or if you want to destroy build trees,
perform the following steps:
# ./configure
# make world
# make install
See the documentation in the INSTALL file for more info.
This will create and install onto the local machine. It will build
the xen binary (xen.gz), the tools and the documentation.
You can override the destination for make install by setting DESTDIR
to some value.
4. To rebuild an existing tree without modifying the config:
# make dist
This will build and install xen, tools, and docs into the local dist/
directory.
You can override the destination for make install by setting DISTDIR
to some value.
make install and make dist differ in that make install does the
right things for your local machine (installing the appropriate
version of udev scripts, for example), but make dist includes all
versions of those scripts, so that you can copy the dist directory
to another machine and install from that distribution.
xenstore: xenstored and oxenstored
====================================
Xen uses a configuration database called xenstore [0] to maintain configuration
and status information shared between domains. A daemon is implemented as part
of xenstore to act as an interface for access to the database for dom0 and
guests. Two xenstored daemons are supported, one written in C which we refer
to as the xenstored (sometimes referred to as cxenstored), and another written
in Ocaml called oxenstored. Details for xenstore and the different
implementations can be found on the wiki's xenstore reference guide [1] and
the xenstored [2] page. You can choose which xenstore you want to enable as
default on a system through configure:
./configure --with-xenstored=xenstored
./configure --with-xenstored=oxenstored
By default oxenstored will be used if the ocaml development tools are found.
If you enable oxenstored the xenstored will still be built and installed,
the xenstored used can be changed through the configuration file:
/etc/sysconfig/xencommons
or
/etc/default/xencommons
You can change the preferred xenstored you want to use in the configuration
but since we cannot stop the daemon a reboot will be required to make the
change take effect.
[0] http://wiki.xen.org/wiki/XenStore
[1] http://wiki.xen.org/wiki/XenStoreReference
[2] http://wiki.xen.org/wiki/Xenstored
Python Runtime Libraries
========================
Various tools, such as pygrub, have the following runtime dependencies:
* Python 2.3 or later.
URL: http://www.python.org/
Debian: python
Intel(R) Trusted Execution Technology Support
=============================================
Intel's technology for safer computing, Intel(R) Trusted Execution Technology
(Intel(R) TXT), defines platform-level enhancements that provide the building
blocks for creating trusted platforms. For more information, see
http://www.intel.com/technology/security/.
Intel(R) TXT support is provided by the Trusted Boot (tboot) module in
conjunction with minimal logic in the Xen hypervisor.
Tboot is an open source, pre- kernel/VMM module that uses Intel(R) TXT to
perform a measured and verified launch of an OS kernel/VMM.
The Trusted Boot module is available from
http://sourceforge.net/projects/tboot. This project hosts the code in a
mercurial repo at http://tboot.sourceforge.net/hg/tboot.hg and contains
tarballs of the source. Instructions in the tboot README describe how
to modify grub.conf to use tboot to launch Xen.
There are optional targets as part of Xen's top-level makefile that will
download and build tboot: install-tboot, build-tboot, dist-tboot, clean-tboot.
These will download the latest tar file from the SourceForge site using wget,
then build/install/dist according to Xen's settings.