Virtual Machines
Xen and Terra
Rajan Palanivel
Xen and Terra : Papers
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Xen and the art of virtualization.
-Univ. of Cambridge
Terra: A VM based platform for trusted
computing.
- Stanford Univ.
Virtual Machine and
Advantages
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2.
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Multiplexing the real machine in to multiple “virtual”
machines.
General architecture consist of a software layer
(Monitor) that exposes VMs and various “guest”
OSs run on these VMs.
Some Advantages:
Concurrent execution of different OS on the same
hardware and hence different applications.
Resource Isolation.
Upgrade OS software to a different version without
losing the ability to run older legacy OS and it’s
applications.
Types of Monitor.
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Monitor runs on a higher privilege level than the
guest OS.
Sensitive/Privileged instructions. (Ex: MOV)
Classified by the amount of guest OS instructions
that are executed by monitor or by the real
hardware.
CSIM (Complete software Interpreter machine),
Hybrid VM (HVM) and VMM.
VMM : Requires that “Statistically dominant subset of
the virtual processor instructions be executed on the
real processor” . (Type 1 and Type 2)
Xen –Type 1 VMM.
Type 1 and Type 2 VMMs
Guest
Process
Guest
Process
Guest
Process
Guest Operating System
Guest Operating System
Virtual Machine Monitor
Virtual Machine Monitor
Host Operating System
Hardware
Hardware
Type I VMM
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Runs directly on
hardware
Good performance
Guest
Process
Type II VMM
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Uses existing host
OS abstractions to
implement services
Poor performance
Full Virtualization
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4.
Full Virtualization: No modification required for the
guest OS (VMWare’s ESXServer).
Drawbacks : (esp. on x86)
Sensitive Instructions fail without traps.
Need dynamic rewrite of OS kernel.
Shadow system structures (performance issue to
sync virtual and shadow structures).
Guest OS may need both virtual and real resources.
(Time: TCP timeouts and RTT, Machine address for
super pages etc).
Xen: Para Virtualization
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Para Virtualization: Exposed hardware is
similar but not identical to the real
machine.
OS modifications required.
ABI not changed. (Guest apps run
without changes).
High performance.
Xen Hypervisor.
Xen
Xen – CPU
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Xen Hypervisor runs in ring 0.
Guest OS runs in a lower privilege level (ring 1).
Privileged and sensitive instructions are
paravirtualized by requiring them to be validated and
executed by hypervisor.
Guest OS protects itself from it’s other processes by
running in a separate address space (and separate
privilege level).
Trap/Exception handlers are registered with Xen for
validation. (Xen checks that the code segment of the
handlers will not run in ring 0).
Fast Exception handlers for system calls.
Xen – Memory Management
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Initial Memory allocation:
2.
Static Allocation for each domain.
Dynamic expansion/contraction possible.
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Virtualizing memory is Complicated in x86:
1.
1.
2.
x86-MMU handles TLB misses by searching
through the page table in the hardware.
(No Soft TLB support).
TLB flush on context switches. (No tagged
TLB support).
Xen – Memory Management
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Virtual address translation:
Page Tables: Allocated and managed by guest OSs
but restricted to read only access. Updates validated
and applied by Xen. (via hypercalls)
Xen: Associates a type and reference count with each
machine page frame. (PD, PT, LDT, GDT, RW).
Xen exists in the 64 MB section on top of every
address space (TLB flush prevented when
entering/leaving Xen Hypervisor).
Page fault Handling: (CR2 register) : Pre determined
location.
Control /Data / Timers
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Hypercalls : synchronous calls from domain to
Hypervisor. Domains do privileged operations
via hypercalls.
Events: asynchronous notifications delivered
via events from Xen to domains. (For
delivering h/w interrupts).
Data transfer through Descriptor rings.
(Producer – Consumer).
Time and Timers: Real, Virtual and wall-clock.
Xen - Network
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Xen provides a Virtual Firewall-router (VFR).
Each domain has one or more VIFs (virtual
Interfaces) attached logically to VFR.
VFR has rules of the form <pattern><action>.
Two I/O buffer descriptor rings. (Transmit and
Receive).
Trans: Domain updates the transmit descriptor ring.
Xen copies the descriptor and the packet header.
Header is inspected by VFR. Payload is not copied
(Scatter-gather). Pages are pinned till completion.
Recv: Xen multiplexes/firewalls using VFR and avoids
copy by page flipping.
I/O Ring (Transmit or Receive)
Control and Management.
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Management software runs on a special
guest OS (domain 0).
List of parameters to manage include
access control (for i/o devices), amount
of physical memory per domain, VFR
rules etc.
Mgmt software uses control interfaces
provided by Xen.
Xen – Relative Performance
Xen – Network Performance
Xen - Performance
Multiple Apache
processes in Linux
vs.
One Apache process in
each guest OS
Terra: Goals
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Goal is to run applications with wide range of
security requirements simultaneously.
Multiple Closed platforms on general purpose
hardware.
Software stack is tailored from hardware
interface up to meet the security
requirements of it’s applications.
Isolation and authentication.
Terra - TVMM
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Trusted VMM (TVMM).
Facilitates Open and Closed box VMs.
Open box VM runs regular commodity
applications.
Closed box VMs provide hardware memory
protection. (Isolation)
Cryptographic authentication (Attestation).
TVMM acts as a trusted party to authenticate
the software running in a VM to remote
parties.
Terra – Architecture.
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Two VM abstractions (Open and close).
Contents of closed box cannot be
inspected or manipulated by the
platform owner.
Provides Isolation, Extensibility,
Efficiency, Compatibility and Security.
Extra features by Terra: Root Secure,
Attestation and Trusted path.
Terra - Architecture
Terra – Attestation and VM
Identity
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Attestation: Application in a closed box VM
authenticates itself to remote parties.
Authenticates who built the hardware and
what layers of software are running on the
machine.
Building a certificate chain (H/w -> BIOS ->
boot loader -> TVMM -> VM -> Apps).
Terra uses a tamper resistant hardware
(TPM). (Embedded private key)
Terra – Attestation
VM
TVMM
Boot Loader
Firmware
Hardware
ENDORSE
API CALL
SIGNED
CERTIFICATE
Terra : Example Attestation
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TLS/SSL Session between Quicken and Remote
bank server.
Client sends the attestation certificate chain during
TLS Handshake.
Lowest certificate (of h/w) in the chain is from a trusted
authority.
All hashes in the chain are on the list of remote server’s list of
authorized software. (trustable BIOS, boot loader, TVMM)
Topmost certificate (containing the hash of quicken) is on the
list of authorized version.
If all the checks are valid, then TLS is completed and session
key exchanged.
Privacy
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Attestation process identifies the machine!
(privacy concerns).
Privacy CA (PCA): User’s machine send an
attested cert request to PCA and PCA issues
an anonymized cert for attestation.
Other issues : DRM etc. – Media servers may
release content only to platforms that would
prevent copying, expire the media after
certain time etc.
Terra – Device Driver Security.
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Drivers have the most security holes..
Hardware memory protection + chipset
protection can prevent drivers from DMA ing
to other address spaces.
NGSCB architecture:
Runs in a curtained memory protected from
DMA attacks and from untrusted OS.
Leverages device drivers of the untrusted
OS (running in an open box VM) via an
explicit interface in the untrusted Os kernel.
Q & A.
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QA