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Kernel page-table isolation

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Vorlage:Redirect Vorlage:Update Kernel page-table isolation (KPTI, previously called KAISER) is a hardening technique in the Linux kernel to workaround the Meltdown hardware security-related bug in modern Intel x86 CPUs by better isolating user-space and kernel-space memory.[1][2] KPTI was merged into Linux kernel version 4.15,[3][4] to be released in early 2018, and backported into Linux Kernel 4.14.11[5]. Microsoft Windows implemented an identical feature in Windows 10 build 17035 (RS4)[6]. Apple's macOS was similarly updated with KPTI in the 10.13.2 update.[7]

KAISER

Prior to KPTI, whenever executing user-space code (applications), Linux would also keep its entire kernel memory mapped in page tables, although protected from access. The advantage is that when the application makes a system call into the kernel or an interrupt is received, kernel page tables are always present, so most context switching-related overheads (TLB flush, page-table swapping, etc) can be avoided.[1]

In 2005, the Linux kernel adopted address space layout randomization (ASLR), and in 2014 kernel address space layout randomization (KASLR), which makes it more difficult to exploit kernel vulnerabilities,[8][9] which relies on kernel addresses remaining hidden from user space. Despite prohibiting access to these kernel mappings, it turns out that there are several side-channel attacks in current Intel x86 processors (as of December 2017) that can leak the location of this memory, making it possible to work around KASLR.[2][10][11][12] AMD x86 processors are not affected by these attacks and don't need KPTI to mitigate them.[13][14][15]Vorlage:Third-party inline

Implementation

KPTI fixes these leaks by separating user-space and kernel-space page tables entirely. On recent x86 processorsVorlage:What, a TLB flush can be avoided using the process context identifiers (PCID) feature, but even then it comes at a significant performance cost, particularly in syscall-heavy and interrupt-heavy workloads. The overheadVorlage:What was measured to be 0.28% according to KAISER's original authors,[2] but roughly 5% for most workloads by a Linux developer.[1]

KPTI can partially be disabled with the "pti=off" kernel boot option. Also provisions were created to disable KPTI if newer processors fix the information leaks.[3]

See also

References

Vorlage:Reflist

  1. ↑ a b c Jonathan Corbet: KAISER: hiding the kernel from user space In: LWN.net, 15 November 2017 
  2. ↑ a b c Daniel Gruss, Moritz Lipp, Michael Lipp, Richard Fellner, Clémentine Maurice, Stefan Mangard: KASLR is Dead: Long Live KASLR. Engineering Secure Software and Systems 2017. 24. Juni 2017 (gruss.cc [PDF]).
  3. ↑ a b Jonathan Corbet: Kernel page-table isolation merged In: LWN.net, 20 December 2017 
  4. ↑ Michael Larabel: KAISER Getting Ready To Better Protect The Linux Kernel In: Phoronix, 27 November 2017 
  5. ↑ Greg Kroah-Hartman: Linux 4.14.11 Changelog. In: kernel.org.
  6. ↑ Vorlage:Cite tweet
  7. ↑ Apple has already partially implemented fix in macOS for 'KPTI' Intel CPU security flaw. In: AppleInsider. Abgerufen am 3. Januar 2018 (amerikanisches Englisch).
  8. ↑ Alan Dang: The NX Bit And ASLR - Behind Pwn2Own: Exclusive Interview With Charlie Miller In: Tom's Hardware, 25 March 2009. Abgerufen am 29. Dezember 2017 (englisch). 
  9. ↑ Abhishek Bhattacharjee, Daniel Lustig: Architectural and Operating System Support for Virtual Memory. Morgan & Claypool Publishers, 2017, ISBN 978-1-62705-933-6, S. 56 (englisch, google.com).
  10. ↑ Yeongjin Jang, Sangho Lee, Taesoo Kim: Breaking Kernel Address Space Layout Randomization with Intel TSX. In: 2016 ACM SIGSAC Conference on Computer and Communications Security (= CCS '16). ACM, New York, NY, USA 2016, S. 380–392, doi:10.1145/2976749.2978321 (oregonstate.edu [PDF]).
  11. ↑ Daniel Gruss, Clémentine Maurice, Anders Fogh, Moritz Lipp, Stefan Mangard: Prefetch Side-Channel Attacks: Bypassing SMAP and Kernel ASLR. In: 2016 ACM SIGSAC Conference on Computer and Communications Security (= CCS '16). ACM, New York, NY, USA 2016, S. 368–379, doi:10.1145/2976749.2978356 (gruss.cc [PDF]).
  12. ↑ R. Hund, C. Willems, T. Holz: Practical Timing Side Channel Attacks against Kernel Space ASLR. In: 2013 IEEE Symposium on Security and Privacy. Mai 2013, S. 191–205, doi:10.1109/sp.2013.23 (ieee-security.org [PDF]).
  13. ↑ Tom Lendacky: Do not enable PTI on AMD processors, 26 December 2017 
  14. ↑ Thomas Gleixner: x86/cpu, x86/pti: Do not enable PTI on AMD processors, 3 January 2018 
  15. ↑ An Update on AMD Processor Security, 4 January 2018 
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