Design, implementation, evaluation and verification of black-box OS abstractions and mechanisms for time protection: the prevention of timing channels.
Micro-architectural timing channels result from competing access to shared, finite hardware resources. Sharing may be time-multiplexed (intra-core) or concurrent (inter-core).
Unauthorised information leakage is a violation of a system's security policy (see the background page for further explanation). Enforcing security is a primary responsibility of the operating system (OS); yet no contemporary, general-purpose OS has the means for preventing this violation. Our aim is to change this.
Our formally verified seL4 microkernel provably prevents other security violations, including through covert storage channels. This makes timing channels the last, fundamentally unsolved security problem in operating systems, and seL4 the ideal platform for solving it.
Hence we are looking for black-box OS-level mechanisms for providing temporary isolation. For spatial isolation such mechanisms are well-established, the core mechanism is memory protection. Here we are working on providing the corresponding temporal isolation, which we accordingly call time protection.
Our earlier investigations have shown that complete time protection is not possible on present hardware, hence solving this problem requires a hardware-software co-design approach. In other words: we need a new, security-oriented, hardware-software contract.
Consequently, we are pursuing a number of directions concurrently, with the ultimate aim of verified time protection.
We have designed and implemented (in seL4) fundamental time-protection mechanisms. These consist of:
A new kernel clone mechanism that provides a policy-free way of setting up a system without any memory shared between security partitions (save a small number of kernel data structures that are accessed carefully to ensure deterministic execution). This eliminates any channels through a shared kernel image.
Kernel cloning provides a way of identifying security-domain switches, thus informing the kernel at which context switches additional scrubbing operations are needed. This is a policy-free way of minimising isolation overheads.
Security-partition switches that carefully reset all shared micro-architectural state, while making switch times completely deterministic, and in particular, independent of previous execution history.
Mechanisms for partitioning lower-level caches, where flushing cost would be high and would not help if those caches are shared across cores. This employs page colouring, and extends to kernel code and data.
Partitioning of interrupt sources, so the system is able to use interrupt-driven I/O, unlike classical separation kernels, which disable all interrupts and rely on high-overhead polled I/O.
We have demonstrated that these mechanisms are effective, to the degree that the hardware provides the right mechanisms for scrubbing or partitioning state. Please see the downloads page for accessing the relevant artefacts.
Having developed seL4 mechanisms able to provide time protection, we now are working on suitable abstractions that integrate with the existing seL4 model, in particular the new scheduling-context capabilities that provide a principled treatment of time as a resource, with the aim of supporting mixed-criticality real-time systems.
Our aim is a similarly principled model that combines both types of temporal isolation, temporal integrity for real time, and temporal confidentiality for security.
The ultimate aim is to be able to prove that our system provides complete time protection. This will require formally specifying the hardware-software contract (at as abstract a level as possible), including formally defining partitionable vs flushable hardware resources.
Specifically we are working on a operational specification of the required properties, so that time protection can be verified as a functional-correctness property without explicit reasoning about time. This approach will allow us to extend seL4's existing proofs of information flow security to also provide assurance about timing channels.
When we started this line of work a few years ago, we provided a formal proof of the absence of side channels using a lattice scheduler. However, this work is based on idealised hardware.
We also empirically investigated a number of defence mechanisms suitable for black-box enforcement. The evaluations showed that these mechanisms are effective on some hardware, but fail to close channels completely on other.
In order to better understand the problem we conducted a comprehensive survey of micro-architectural timing channels.
Following the discovery that our defence mechanisms were not fully effective, we conducted a thorough analysis of microarchitectural timing channels of multiple generations of Intel and ARM processors. We examined the mechanisms provided by the hardware and measured their effectiveness. Our results show that all microarchitectural state we examined can be used for timing channels, including two channels (TLB and branch target buffer) that have not been demonstrated before.
More importantly, our investigation uncovered on each examined platform at least one channel that cannot be closed with any state-flushing operation provided by the hardware. This is summarised in an arXiv paper, and we provide the complete, updated results.
|Marcelo Orenes-Vera, Hyunsung Yun, Nils Wistoff, Gernot Heiser, Luca Benini, David Wentzlaff and Margaret Martonosi|
AutoCC: Automatic discovery of covert channels in time-shared hardware
International Symposium on Microarchitecture (MICRO), Toronto, ON, CA, October, 2023
||Robert Sison, Scott Buckley, Toby Murray, Gerwin Klein and Gernot Heiser|
Formalising the prevention of microarchitectural timing channels by operating systems
International Symposium on Formal Methods (FM), Lübeck, DE, March, 2023
|Scott Buckley, Robert Sison, Nils Wistoff, Curtis Millar, Toby Murray, Gerwin Klein and Gernot Heiser|
Proving the absence of microarchitectural timing channels
arXiv preprint arXiv:2310.17046, 2023
|Nils Wistoff, Moritz Schneider, Frank Gürkaynak, Gernot Heiser and Luca Benini|
Systematic prevention of on-core timing channels by full temporal partitioning
IEEE Transactions on Computers, Volume 72, Number 5, pp. 1420–1430, 2023
|Nils Wistoff, Moritz Schneider, Frank Gürkaynak, Luca Benini and Gernot Heiser|
Microarchitectural timing channels and their prevention on an open-source 64-bit RISC-V core
Design, Automation and Test in Europe (DATE), virtual, February, 2021
Best Paper Award
|Gernot Heiser, Toby Murray and Gerwin Klein|
Towards provable timing-channel prevention
ACM Operating Systems Review, Volume 54, Issue 1, pp. 1-7, August, 2020
||Nils Wistoff, Moritz Schneider, Frank Gürkaynak, Luca Benini and Gernot Heiser|
Prevention of microarchitectural covert channels on an open-source 64-bit RISC-V core
Workshop on Computer Architecture Research with RISC-V (CARRV), Valencia, Spain, May, 2020
|Gernot Heiser, Gerwin Klein and June Andronick|
seL4 in Australia: From research to real-world trustworthy systems
Communications of the ACM, Volume 63, Issue 4, pp. 72-75, April, 2020
Verified seL4 on secure RISC-V processors
at linux.conf.au, Gold Coast, January, 2020
Principled elimination of microarchitectural timing channels through operating-system enforced time protection
PhD Thesis, UNSW, Sydney, Australia, October, 2019
Security needs a better hardware-software contract
Invited talk at Design Automation Conference (DAC), Las Vegas, NV, USA, June, 2019
|Gernot Heiser, Gerwin Klein and Toby Murray|
Can we prove time protection?
Workshop on Hot Topics in Operating Systems (HotOS), pp. 23-29, Bertinoro, Italy, May, 2019
||Qian Ge, Yuval Yarom, Tom Chothia and Gernot Heiser|
Time protection: The missing OS abstraction
EuroSys Conference, Dresden, Germany, March, 2019
Best Paper Award
Security needs a new hardware-software contract
Keynote at HiPEAC Workshop Secure Hardware, Architectures, and Operating Systems (SeHAS), January, 2019
||Qian Ge, Yuval Yarom and Gernot Heiser|
No security without time protection: we need a new hardware-software contract
Asia-Pacific Workshop on Systems (APSys), Korea, August, 2018
Best Paper Award Complete timing-channel data for evaluated x86 and Arm platforms.
|Qian Ge, Yuval Yarom, David Cock and Gernot Heiser|
A survey of microarchitectural timing attacks and countermeasures on contemporary hardware
Journal of Cryptographic Engineering, Volume 8, Issue 1, pp. 1-27, April, 2018
For safety's sake: we need a new hardware-software contract!
IEEE Design and Test, Volume 35, Issue 2, pp. 27-30, March, 2018
|Qian Ge, Yuval Yarom, Frank Li and Gernot Heiser|
Your processor leaks information — and there's nothing you can do about it
arXiv preprint arXiv:1612.04474, 2017
|Fangfei Liu, Qian Ge, Yuval Yarom, Frank Mckeen, Carlos Rozas, Gernot Heiser and Ruby B Lee|
CATalyst: defeating last-level cache side channel attacks in cloud computing
IEEE Symposium on High-Performance Computer Architecture, pp. 406–418, Barcelona, Spain, March, 2016
|Yuval Yarom, Qian Ge, Fangfei Liu, Ruby B. Lee and Gernot Heiser|
Mapping the Intel last-level cache
The Cryptology ePrint Archive, September, 2015
|Fangfei Liu, Yuval Yarom, Qian Ge, Gernot Heiser and Ruby B Lee|
Last-level cache side-channel attacks are practical
IEEE Symposium on Security and Privacy, pp. 605–622, San Jose, CA, US, May, 2015
|David Cock, Qian Ge, Toby Murray and Gernot Heiser|
The last mile: An empirical study of some timing channels on seL4
ACM Conference on Computer and Communications Security, pp. 570–581, Scottsdale, AZ, USA, November, 2014
Leakage in trustworthy systems
PhD Thesis, UNSW, Sydney, Australia, August, 2014