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Kernel page-table isolation
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<h2 id="background-on-kaiser">Background on KAISER</h2> <p>The KPTI patches were based on KAISER (short for <i>Kernel Address Isolation to have Side-channels Efficiently Removed</i>),<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> a technique conceived in 2016<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> and published in June 2017 back when Meltdown was not known yet. KAISER makes it harder to defeat KASLR, a 2014 mitigation for a much less severe issue. </p><p>In 2014, the Linux kernel adopted <a href="/facts/Kernel_address_space_layout_randomization/K6lMxWTr">kernel address space layout randomization</a> (KASLR),<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> which makes it more difficult to exploit other kernel vulnerabilities,<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> which relies on kernel address mappings remaining hidden from user space.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> Despite prohibiting access to these kernel mappings, it turns out that there are several <a href="/facts/Side-channel_attack/2B1dfBty">side-channel attacks</a> in modern processors that can leak the location of this memory, making it possible to work around KASLR.<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a><a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a><a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a><a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p><p>KAISER addressed these problems in KASLR by eliminating some sources of address leakage.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> Whereas KASLR merely prevents address mappings from leaking, KAISER also prevents the data from leaking, thereby covering the Meltdown case.<a class="footnote-ref" id="fnref:24" href="#fn:24"><sup>24</sup></a> </p><p>KPTI is based on KAISER. Without KPTI enabled, whenever executing user-space code (applications), Linux would also keep its entire kernel memory mapped in <a href="/facts/Page_table/0IBU5cwg">page tables</a>, although protected from access. The advantage is that when the application makes a <a href="/facts/System_call/NW44AFJr">system call</a> into the kernel or an <a href="/facts/Interrupt/Mm6C4rpc">interrupt</a> is received, kernel page tables are always present, so most <a href="/facts/Context_switch/N0oJQ6CC">context switching</a>-related overheads (<a href="/facts/TLB_flush/vvwGkfvD">TLB flush</a>, page-table swapping, etc) can be avoided.<a class="footnote-ref" id="fnref:25" href="#fn:25"><sup>25</sup></a> </p> <h2 id="meltdown-vulnerability-and-kpti">Meltdown vulnerability and KPTI</h2> <p>In January 2018, the <a href="/facts/Meltdown_(security_vulnerability)/KVRY83cy">Meltdown</a> vulnerability was published, known to affect <a href="/facts/List_of_Intel_CPUs/gBqeUglP">Intel's x86 CPUs</a> and <a href="/facts/ARM_Cortex-A75/KltI0yeA">ARM Cortex-A75</a>.<a class="footnote-ref" id="fnref:26" href="#fn:26"><sup>26</sup></a><a class="footnote-ref" id="fnref:27" href="#fn:27"><sup>27</sup></a> It was a far more severe vulnerability than the KASLR bypass that KAISER originally intended to fix: It was found that <i>contents</i> of kernel memory could also be leaked, not just the locations of memory mappings, as previously thought. </p><p>KPTI (conceptually based on KAISER) prevents Meltdown by preventing most protected locations from being mapped to user space. </p><p><a href="/facts/AMD/CB1G7QTb">AMD</a> x86 processors are not currently known to be affected by Meltdown and don't need KPTI to mitigate them.<a class="footnote-ref" id="fnref:28" href="#fn:28"><sup>28</sup></a><a class="footnote-ref" id="fnref:29" href="#fn:29"><sup>29</sup></a> However, AMD processors are still susceptible to KASLR bypass when KPTI is disabled.<a class="footnote-ref" id="fnref:30" href="#fn:30"><sup>30</sup></a> </p> <h2 id="implementation">Implementation</h2> <p>KPTI fixes these leaks by separating user-space and kernel-space page tables entirely. One set of page tables includes both kernel-space and user-space addresses same as before, but it is only used when the system is running in kernel mode. The second set of page tables for use in user mode contains a copy of user-space and a minimal set of kernel-space mappings that provides the information needed to enter or exit system calls, interrupts and exceptions.<a class="footnote-ref" id="fnref:31" href="#fn:31"><sup>31</sup></a> </p><p>On processors that support the <a href="/facts/Process-context_identifier/vvwGkfvD">process-context identifiers</a> (PCID), a <a href="/facts/Translation_lookaside_buffer/vvwGkfvD">translation lookaside buffer</a> (TLB) flush can be avoided,<a class="footnote-ref" id="fnref:32" href="#fn:32"><sup>32</sup></a> but even then it comes at a significant performance cost, particularly in <a href="/facts/Syscall/NW44AFJr">syscall</a>-heavy and interrupt-heavy workloads.<a class="footnote-ref" id="fnref:33" href="#fn:33"><sup>33</sup></a> </p><p>The overhead was measured to be 0.28% according to KAISER's original authors;<a class="footnote-ref" id="fnref:34" href="#fn:34"><sup>34</sup></a> a Linux developer measured it to be roughly 5% for most workloads and up to 30% in some cases, even with the PCID optimization;<a class="footnote-ref" id="fnref:35" href="#fn:35"><sup>35</sup></a> for database engine <a href="/facts/PostgreSQL/LHdXuHfl">PostgreSQL</a> the impact on read-only tests on an Intel <a href="/facts/Skylake_(microarchitecture)/qpcDcebM">Skylake</a> processor was 7–17% (or 16–23% without PCID),<a class="footnote-ref" id="fnref:36" href="#fn:36"><sup>36</sup></a> while a full benchmark lost 13–19% (<a href="/facts/Coffee_Lake/oASqYsf2">Coffee Lake</a> vs. <a href="/facts/Broadwell_(microarchitecture)/32jqNWRS">Broadwell-E</a>).<a class="footnote-ref" id="fnref:37" href="#fn:37"><sup>37</sup></a> Many benchmarks have been done by <a href="/facts/Phoronix/UvK56oPf">Phoronix</a>,<a class="footnote-ref" id="fnref:38" href="#fn:38"><sup>38</sup></a><a class="footnote-ref" id="fnref:39" href="#fn:39"><sup>39</sup></a><a class="footnote-ref" id="fnref:40" href="#fn:40"><sup>40</sup></a> <a href="/facts/Redis/UNphSYpD">Redis</a> slowed by 6–7%.<a class="footnote-ref" id="fnref:41" href="#fn:41"><sup>41</sup></a> Linux kernel compilation slowed down by 5% on <a href="/facts/Haswell_(microarchitecture)/C19Wlzhm">Haswell</a>.<a class="footnote-ref" id="fnref:42" href="#fn:42"><sup>42</sup></a> </p><p>KPTI can partially be disabled with the "nopti" kernel boot option. Also provisions were created to disable KPTI if newer processors fix the information leaks.<a class="footnote-ref" id="fnref:43" href="#fn:43"><sup>43</sup></a> </p> <h2 id="external-links">External links</h2> <ul><li><a href="https://www.kernel.org/doc/html/v5.15/x86/pti.html">17. Page Table Isolation (PTI) - The Linux Kernel documentation</a></li> <li><a href="https://lkml.org/lkml/2017/12/18/1523">KPTI documentation patch</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Larabel, Michael (2018-01-03). "Further Analyzing The Intel CPU "x86 PTI Issue" On More Systems". Phoronix. <a href="https://www.phoronix.com/scan.php?page=article&item=linux-more-x86pti" target="_blank">https://www.phoronix.com/scan.php?page=article&item=linux-more-x86pti</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Corbet, Jonathan (2017-12-20). "The current state of kernel page-table isolation". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Cimpanu, Catalin (2018-01-03). "OS Makers Preparing Patches for Secret Intel CPU Security Bug". Bleeping Computer. <a href="https://www.bleepingcomputer.com/news/security/os-makers-preparing-patches-for-secret-intel-cpu-security-bug/" target="_blank">https://www.bleepingcomputer.com/news/security/os-makers-preparing-patches-for-secret-intel-cpu-security-bug/</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Spectre, Meltdown: Critical CPU Security Flaws Explained – ExtremeTech". ExtremeTech. 2018-01-04. Retrieved 2018-01-05. <a href="https://www.extremetech.com/computing/261439-spectre-meltdown-new-critical-security-flaws-explored-explained" target="_blank">https://www.extremetech.com/computing/261439-spectre-meltdown-new-critical-security-flaws-explored-explained</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Corbet, Jonathan (2017-11-15). "KAISER: hiding the kernel from user space". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Gruss, Daniel; Lipp, Moritz; Schwarz, Michael; Fellner, Richard; Maurice, Clémentine; Mangard, Stefan (2017-06-24). KASLR is Dead: Long Live KASLR (PDF). Engineering Secure Software and Systems 2017. <a href="https://gruss.cc/files/kaiser.pdf" target="_blank">https://gruss.cc/files/kaiser.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Corbet, Jonathan (2017-12-20). "Kernel page-table isolation merged". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Kroah-Hartman, Greg (2018-01-02). "Linux 4.14.11 Changelog". kernel.org. <a href="https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.11" target="_blank">https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.11</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Kroah-Hartman, Greg (2018-01-05). "Linux 4.9.75 Changelog". kernel.org. <a href="https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.9.75" target="_blank">https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.9.75</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Kroah-Hartman, Greg (2018-01-05). "Linux 4.4.110 Changelog". <a href="https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.4.110" target="_blank">https://cdn.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.4.110</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>@aionescu (2017-11-14). "Windows 17035 Kernel ASLR/VA Isolation In Practice" (Tweet) – via Twitter. <a href="https://x.com/aionescu/status/930412525111296000" target="_blank">https://x.com/aionescu/status/930412525111296000</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>"Apple has already partially implemented fix in macOS for 'KPTI' Intel CPU security flaw". AppleInsider. 3 January 2018. Retrieved 2018-01-03. <a href="http://appleinsider.com/articles/18/01/03/apple-has-already-partially-implemented-fix-in-macos-for-kpti-intel-cpu-security-flaw" target="_blank">http://appleinsider.com/articles/18/01/03/apple-has-already-partially-implemented-fix-in-macos-for-kpti-intel-cpu-security-flaw</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Coldewey, Devin (2018-01-04). "Kernel panic! What are Meltdown and Spectre, the bugs affecting nearly every computer and device?". TechCrunch. <a href="https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/" target="_blank">https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Gruss, Daniel; Lipp, Moritz; Schwarz, Michael; Fellner, Richard; Maurice, Clémentine; Mangard, Stefan (2017-06-24). KASLR is Dead: Long Live KASLR (PDF). Engineering Secure Software and Systems 2017. <a href="https://gruss.cc/files/kaiser.pdf" target="_blank">https://gruss.cc/files/kaiser.pdf</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Gruss, Daniel (2018-01-03). "#FunFact: We submitted #KAISER to #bhusa17 and got it rejected". Archived from the original on 2018-01-08. Retrieved 2018-01-08 – via Twitter. <a href="https://twitter.com/lavados/status/948536300830851072" target="_blank">https://twitter.com/lavados/status/948536300830851072</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>"Linux kernel 3.14, Section 1.7. Kernel address space randomization". kernelnewbies.org. 2014-03-30. Retrieved 2014-04-02. <a href="http://kernelnewbies.org/Linux_3.14#head-192cae48200fccde67b36c75cdb6c6d8214cccb3" target="_blank">http://kernelnewbies.org/Linux_3.14#head-192cae48200fccde67b36c75cdb6c6d8214cccb3</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Bhattacharjee, Abhishek; Lustig, Daniel (2017-09-29). Architectural and Operating System Support for Virtual Memory. Morgan & Claypool Publishers. p. 56. ISBN 978-1-62705-933-6. <a href="978-1-62705-933-6" target="_blank">978-1-62705-933-6</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>Kerner, Sean Michael (2018-01-03). "KPTI Intel Chip Flaw Exposes Security Risks". eWEEK. <a href="http://www.eweek.com/security/kpti-intel-chip-flaw-exposes-security-risks" target="_blank">http://www.eweek.com/security/kpti-intel-chip-flaw-exposes-security-risks</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Gruss, Daniel; Lipp, Moritz; Schwarz, Michael; Fellner, Richard; Maurice, Clémentine; Mangard, Stefan (2017-06-24). KASLR is Dead: Long Live KASLR (PDF). Engineering Secure Software and Systems 2017. <a href="https://gruss.cc/files/kaiser.pdf" target="_blank">https://gruss.cc/files/kaiser.pdf</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>Jang, Yeongjin; Lee, Sangho; Kim, Taesoo (2016). "Breaking Kernel Address Space Layout Randomization with Intel TSX" (PDF). Proceedings of the 2016 ACM SIGSAC Conference on Computer and Communications Security. CCS '16. New York, NY, USA: ACM. pp. 380–392. doi:10.1145/2976749.2978321. ISBN 978-1-4503-4139-4. <a href="978-1-4503-4139-4" target="_blank">978-1-4503-4139-4</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Gruss, Daniel; Maurice, Clémentine; Fogh, Anders; Lipp, Moritz; Mangard, Stefan (2016). "Prefetch Side-Channel Attacks" (PDF). Proceedings of the 2016 ACM SIGSAC Conference on Computer and Communications Security. CCS '16. New York, NY, USA: ACM. pp. 368–379. doi:10.1145/2976749.2978356. ISBN 978-1-4503-4139-4. S2CID 15973158. <a href="978-1-4503-4139-4" target="_blank">978-1-4503-4139-4</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>Hund, R.; Willems, C.; Holz, T. (May 2013). "Practical Timing Side Channel Attacks against Kernel Space ASLR" (PDF). 2013 IEEE Symposium on Security and Privacy. pp. 191–205. doi:10.1109/sp.2013.23. ISBN 978-0-7695-4977-4. S2CID 215754624. <a href="978-0-7695-4977-4" target="_blank">978-0-7695-4977-4</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Gruss, Daniel; Lipp, Moritz; Schwarz, Michael; Fellner, Richard; Maurice, Clémentine; Mangard, Stefan (2017-06-24). KASLR is Dead: Long Live KASLR (PDF). Engineering Secure Software and Systems 2017. <a href="https://gruss.cc/files/kaiser.pdf" target="_blank">https://gruss.cc/files/kaiser.pdf</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> <li id="fn:24"><p>"Meltdown" (PDF). <a href="https://meltdownattack.com/meltdown.pdf" target="_blank">https://meltdownattack.com/meltdown.pdf</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></p></li> <li id="fn:25"><p>Corbet, Jonathan (2017-11-15). "KAISER: hiding the kernel from user space". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></p></li> <li id="fn:26"><p>"Spectre, Meltdown: Critical CPU Security Flaws Explained – ExtremeTech". ExtremeTech. 2018-01-04. Retrieved 2018-01-05. <a href="https://www.extremetech.com/computing/261439-spectre-meltdown-new-critical-security-flaws-explored-explained" target="_blank">https://www.extremetech.com/computing/261439-spectre-meltdown-new-critical-security-flaws-explored-explained</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></p></li> <li id="fn:27"><p>Coldewey, Devin (2018-01-04). "Kernel panic! What are Meltdown and Spectre, the bugs affecting nearly every computer and device?". TechCrunch. <a href="https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/" target="_blank">https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></p></li> <li id="fn:28"><p>Coldewey, Devin (2018-01-04). "Kernel panic! What are Meltdown and Spectre, the bugs affecting nearly every computer and device?". TechCrunch. <a href="https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/" target="_blank">https://techcrunch.com/2018/01/03/kernel-panic-what-are-meltdown-and-spectre-the-bugs-affecting-nearly-every-computer-and-device/</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></p></li> <li id="fn:29"><p>"An Update on AMD Processor Security". AMD. 2018-01-04. <a href="https://www.amd.com/en/corporate/speculative-execution" target="_blank">https://www.amd.com/en/corporate/speculative-execution</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></p></li> <li id="fn:30"><p>Hund, R.; Willems, C.; Holz, T. (May 2013). "Practical Timing Side Channel Attacks against Kernel Space ASLR" (PDF). 2013 IEEE Symposium on Security and Privacy. pp. 191–205. doi:10.1109/sp.2013.23. ISBN 978-0-7695-4977-4. S2CID 215754624. <a href="978-0-7695-4977-4" target="_blank">978-0-7695-4977-4</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></p></li> <li id="fn:31"><p>Corbet, Jonathan (2017-11-15). "KAISER: hiding the kernel from user space". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></p></li> <li id="fn:32"><p>Corbet, Jonathan (2017-11-15). "KAISER: hiding the kernel from user space". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></p></li> <li id="fn:33"><p>Leyden, John; Williams, Chris (2018-01-02). "Kernel-memory-leaking Intel processor design flaw forces Linux, Windows redesign". The Register. <a href="https://www.theregister.co.uk/2018/01/02/intel_cpu_design_flaw/" target="_blank">https://www.theregister.co.uk/2018/01/02/intel_cpu_design_flaw/</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></p></li> <li id="fn:34"><p>Gruss, Daniel; Lipp, Moritz; Schwarz, Michael; Fellner, Richard; Maurice, Clémentine; Mangard, Stefan (2017-06-24). KASLR is Dead: Long Live KASLR (PDF). Engineering Secure Software and Systems 2017. <a href="https://gruss.cc/files/kaiser.pdf" target="_blank">https://gruss.cc/files/kaiser.pdf</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></p></li> <li id="fn:35"><p>Corbet, Jonathan (2017-11-15). "KAISER: hiding the kernel from user space". LWN.net. <a href="/wiki/Jonathan_Corbet" target="_blank">/wiki/Jonathan_Corbet</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></p></li> <li id="fn:36"><p>Freund, Andres (2018-01-02). "heads up: Fix for intel hardware bug will lead to performance regressions". PostgreSQL development mailing list (pgsql-hackers). <a href="https://www.postgresql.org/message-id/20180102222354.qikjmf7dvnjgbkxe%40alap3.anarazel.de" target="_blank">https://www.postgresql.org/message-id/20180102222354.qikjmf7dvnjgbkxe%40alap3.anarazel.de</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></p></li> <li id="fn:37"><p>Larabel, Michael (2018-01-02). "Initial Benchmarks Of The Performance Impact Resulting From Linux's x86 Security Changes". 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