Linux stub domains

Linux stub domains are used in QubesOS and OpenXT.

• (2018) Linux-based Device Model Stubdomains in Qubes OS, Marek Marczykowski-Górecki
• (2018) Xen Security Weather Report 2018, Lars Kurth
• (2017) MSI support for PCI device pass-through with stub domains, Simon Gaiser

Upstreaming

todo: add remaining patches

00/17 Add support for qemu-xen runnning in a Linux-based stubdomain:

• Aug 2018, v2 patch
• Jul 2018, v1 patch

05/17 libxl: Handle Linux stubdomain specific QEMU options:

• Jan 2019, v3 patch
• Aug 2018, v2 patch

08/17 xl: add stubdomain related options to xl config parser

• Jul 2018, v1 patch

Interfaces between stubdom QEMU and dom0

• (June 2019) qemu-stubdom<=>dom0 interface changes
• (June 2019) Requirements for PVH stubdoms

Xen Summit 2019 Design Session

Display architecture proposed by Brendan K

• migrate from display changer on Qubes and OpenXT surfman, to upstream Xen interfaces (from EPAM)
• use Linux 5.1 drm-front driver for displayif
• share common codebase for: EPAM, Qubes, OpenXT, Redfield

QMP discussion

Proposal:

- LibXL and block backend: change "xenstore stuff" to "QMP stuff"

Concerns:

1. QMP is not a simple protocol.
2. xenstore-based protocol is much simpler than a JSON-encapsulated protocol.
3. There have been multiple historical bugs in parsing of JSON in C.
4. If we extend QMP support in LibXL and are communicating with a potentially compromised QEMU process, it would be a security regression.

Possible mitigations for a potentially compromised QEMU

1. Reduce privileges given to the QEMU instance
2. Limit interfaces to the QEMU instance
3. Work with upstream QEMU to propose a new safer-than-JSON alternative marshalling format backend
4. Minimize what LibXL does with QMP messages: use an off-the-shelf JSON firewall/translator/filter in the untrusted guest, limited to the QEMU subset of JSON
5. Don't talk to QEMU after the device is started: mostly done today, but does not work for some corner cases: optical drive insert, device hotplug
6. Have an option to intentionally break all QMP communications

Other notes:

• Qubes uses dracut for build, OpenXT uses OE
• For OpenXT/Qubes, many QEMU functions can build-time disabled
• For Debian/SuSE distro, the generic QEMU binary has many functions enabled. It may be an option to ship "minimal QEMU" and "minimal kernel" configured binaries for distro stubdoms.

Actions:

• Marek to resubmit Linux stubdom patches to xen-devel, ask people to comment on which patches have consensus or open discussion items
• Ian J can review patches in mid-August
• Upstream Xen will merge the ones which are not contended
• Work towards consensus on contended patches

Additional Notes

QMP in Qubes and OpenXT

Connection between the qemu device model and the libxl toolstack requires a communication channel to be established between the separate domains where they are run, which is then used to deliver QMP traffic in both Qubes and OpenXT.

Qubes uses a vchan-socket-proxy:

https://github.com/QubesOS/qubes-vmm-xen-stubdom-linux/blob/master/rootfs/init#L147

https://github.com/QubesOS/qubes-vmm-xen-stubdom-linux/blob/master/helpers/vchan-socket-proxy.c

OpenXT uses a qemu character device that delivers via Argo to a helper process in dom0:

https://github.com/OpenXT/xenclient-oe/blob/9.0.2/recipes-extended/qemu-dm/qemu-dm/qmp-argo-char-driver.patch

https://github.com/OpenXT/xctools/blob/9.0.2/qmp_helper/src/qmp_helper.c

The Xen toolstack libxl is currently dropping QMP Events after parsing them from JSON, so filtering at the qemu source could reduce the volume of interactions over the channel:

https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=tools/libxl/libxl_qmp.c;h=9aabad74fabd7bfd27b2bac153973c612a46a3c2;hb=HEAD#l278