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EuroSec2011 Slide "Memory Deduplication as a Threat to the Guest OS" by Kuniyasu Suzaki

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EuroSec2011 Slide "Memory Deduplication as a Threat to the Guest OS" by Kuniyasu Suzaki

  1. 1. Memory Deduplication as a Threat to the Guest OS Kuniyasu Suzaki, Kengo Iijima, Toshiki Yagi, Cyrille Artho Research Center for Information Security EuroSec 2011 at Salzburg, April 10
  2. 2. Background 1/2 • IaaS (Infrastructure as a Service) type cloud computing hosts many virtual machines. – Examples: Amazon EC2, Rackspace, ... – Physical resources (Memory/Storage) are very important. • Image of guest OS are provided by IaaS venders. – Most contents are same. • Deduplication technique is utilized to share same contents and reduces consumption of physical resources.
  3. 3. Background 2/2 • Unfortunately memory deduplication has access time difference between non-deduplicated pages and deduplicated pages, caused by Copy-On-Write. • Attacker can use this phenomena for memory disclosure attack . • Our paper presents real attack to a Guest OS (Windows and Linux).
  4. 4. Contents 1) Background 2) Memory deduplication 3) Memory disclosure attack 4) Experiments 5) Discussion and Conclusion
  5. 5. Memory Deduplication 1/2 • Memory deduplication is a technique to share same contents page. – Mainly used for virtual machines. – Very effective when same guest OS runs on several virtual machines. • Many virtual machine monitors include deduplication • Implementations may differ Guest Pseudo Memory VM1 VM2 VM(n) Real Physical Memory
  6. 6. Memory Deduplication 2/2 • Content-aware deduplication – Transparent Page Sharing on Disco [OSDI97] – Satori on Xen[USENIX09] • Periodical memory-scan dedpulication – Content-Based Page Sharing on VMWare ESX [SOSP02] – Differential Engine on Xen[OSDI08] – KSM (Kernel Samepage Merging) [LinuxSymp09] • General-purpose memory deduplication for Linux. • Used mainly for KVM. • Our paper uses KSM with KVM virtual machine.
  7. 7. KSM: Kernel Samepage Merging • KSM has 3 states for pages. – Volatile : contents change frequently (not to be candidate) – Unshared: candidate pages for deduplication – Shared: deduplicated pages • Pages are scanned (default: 20msec) – All pages are not scanned at a time. – The maximum is 25% of the available memory. – The time to be deduplicated depends on the situation. • Write accesses to deduplicated pages are managed by Copy-On-Write.
  8. 8. Copy-On-Write (COW) • When a write access is issued to a deduplicated page, a fresh copy is created, which accepts write access. • Write access time difference between deduplicateted and non-deduplicated pages due to copying. • A kind of Covert Channel of COW Guest Pseudo Memory VM1 VM2 Write Access VM1 VM2 (victim) (attacker) (victim) (attacker) Real Physical Memory Re-created page cases access time difference
  9. 9. Memory Disclosure Attack • Attacker can use write access time difference to guess memory contents on a victim’s VM. Basic idea for memory disclosure attack • Allocate same contents pages on memory of attacker’s VM. • Wait to be deduplicaed • Issue write access to the pages. When the pages are deduplicated, the write access time is longer than normal. • Used to detect a process or an open file on victim’s VM
  10. 10. Implementation detail 1/2 (1) Get memory image of victim VM – To detect a process • An executable binary file is used, because a loader (ex. ld- linux.so) loads most parts of binary file without change. • The last page which is less than 4KB is not target of matching, because the tail of 4KB may include arbitrary contents on the memory. – To detect an opened file • Normal file is used. It is saved in page cache without change.
  11. 11. Implementation detail 2/2 (2) Compare time difference • Write one byte on each target page and measure the access time. • Compare the time with the access time measured already – zero-cleared (dedupicated) and random data (non-deduplicated). • Length of waiting time – Depends on size of memory for memory scan type.
  12. 12. Problems on implementation • Memory disclosure attack for memory deduplication looks simple, but there are some problems. – 4KB Alignment problem • Caused by memory allocation strategy – Self-reflection problem • Caused by memory management on cache and heap – Run time modification problem • Caused by ASLR, swap-out, etc
  13. 13. 4KB alignment problem • On victim’s VM – A binary file is loaded with 4KB alignment. – Opened file is cached with 4KB alignment. • On Attacker’s VM – Attacker loads the matching contents on his heap memory. – The contents MUST be loaded on 4KB alignment. • Attacker can use posix_memalign() to get aligned memory region.
  14. 14. Self-reflection problem 1/2 • Contents of opened file by attacker’s program are stored on page cache memory by Linux kernel. • Attacker loads the matching contents on his heap memory. • The heap memory contents is deduplicated to the page cache memory contents on a VM! [self-reflection problem] attacker’s program memory on attacker’s VM matching Un-control by target file … attacker’s program page open(); cache Contents stored … on page cache posix_memalign(); read(); heap Same contents Contents stored … on heap memory are deduplicated on self memory gettimeofday(); # write data to heap access time is delayed with gettimeofday(); self memory deduplication
  15. 15. Self-reflection problem 2/2 • Image on heap and page cache must be different. • We gzipped the target file and expand it at run time. – The contents on page cache is gzipped image. attacker’s program memory on attacker’s VM matching target Un-control by file with gziped … attacker’s program page gzopen(); cache Contents stored … on page cache posix_memalign(); different contents gzread(); heap Contents stored … on heap memory gettimeofday(); Deduplicated with # write data to heap memory on other VM gettimeofday();
  16. 16. Run time modification problem • Some parts of code are modified at run time, and cause – False-negative • ASLR (Address Space Layout Randomization) – Modern kernel has this security function. (after Linux2.6.12, after Windows VISTA) – Most text parts are not changed. Even if the pages are scattered, the unit is 4KB. The memory disclosure attack does not care ASLR. • Swap-out • Self modification code – Not used by normal applications. – False-positive • Pre-load mechanism • Common 4KB Code
  17. 17. Summary of memory disclosure attack • The memory disclosure attack is 4KB aligned exact matching. • Limitation – Attacker can know • existence of process. • opened or downloaded file. – Attacker doesn’t know • termination of process because of un-cleared page cache. • which virtual machine has the contents. – Noises cause false-negative and false-positive. • Include timing of deduplication
  18. 18. Experiments • The memory disclosure attack runs on KSM with KVM(0.12.4). The host linux kernel is 2.6.33.5. • Victim’s VM – Guest OS: Debian GNU Linux and WindowsXP – Target applications • apache2 and sshd on Debian GNU Linux • Internet Explore6 and FireFox on WindowsXP • Attacker’s VM – Guest OS: Debian GNU Linux – Assume to know invocation of application – Wait 5 minutes after target application is invoked
  19. 19. Attack for Apache2 and sshd on Linux Average (us) zero Random Apache2 Apache2 ELF file 365,308B (89*4KB) Before invocation 6.45 0.37 0.53 After invocation 6.24 0.40 7.56 Before invocation After invocation Average (us) zero random sshd sshd ELF file 438,852B (107*4KB) Before invocation 6.60 0.55 0.45 After invocation 6.51 0.42 7.50 Non-deduplicated Timimng? swap-out? Code modification? Before invocation After invocation
  20. 20. Attack for Firefox and IE6 on WindowsXP Average (us) zero random Firefox Firefox binary file 912,334B (222*4KB) Before invocation 6.45 0.43 1.92 After invocation 6.49 0.37 7.68 Deduplicated Common 4KB Code? Pre-load? Before invocation After invocation Average (us) zero random IE6 Before invocation 6.59 0.27 5.68 IE6 binary file 93,184B (22*4KB) After invocation 6.32 0.68 7.00 IE6 has special optimization? Before invocation After invocation
  21. 21. Detect downloaded file • The memory disclosure attack can also be applied to find an opened file on a victim’s VM. • We confirm that the “Google logo” file can be detected if page caching is enabled on Firefox. • This disclosure attack is dangerous, because it detects a view of Home Page, even if the network is encrypted by TLS/SSL.
  22. 22. Countermeasure • Satori[USENIX ATC’09] has a mechanism to refuse memory deduplication for each page. – Applications have to control its memory. • Overshadow [ASPLOS’08] and SP3 [Vee’08] encrypt memory. – Unfortunately it ruins effect of memory deduplication. • We plan to develop memory deduplication for read-only pages. – It requires to monitor page table of Guest OS.
  23. 23. Discussion • Security function (memory sanitization) on Guest OS helps the memory disclosure attack – Because it tells termination of process. • The memory disclosure attack can be used for security education. – Detect hidden process. – Data life time is longer than we expect [USENIX Security’05 and ATC’06]. • The memory disclosure attack does not violate any SLA on cloud computing. It just measure access time.
  24. 24. Related Work • Cross VM Side Channel Attack, “Hey, you, get off of my cloud” [CCS’09] – Monitor the behavior of a physically shared cache of multi- cores (hyper threads) • Cold-boot attack [USENIX Security’08] – a kind of physical side channel attack, which freezes physical memory and scans the data.
  25. 25. Conclusion • We presented memory disclose attack on memory deduplication which use write access time difference on Copy-On-Write. • Experiments detected – Existence of apache2 and sshd on Linux, – IE6 and Firefox on WindowsXP, – downloaded file in the Firefox browser. • Countermeasures and applicable usage are mentioned.

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