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OS Security

OS Security. CSE 525 Course Presentation Dhanashri Kelkar Department of Computer Science and Engineering OGI School of Science and Engineering. OS Security.

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OS Security

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  1. OS Security CSE 525 Course Presentation Dhanashri Kelkar Department of Computer Science and Engineering OGI School of Science and Engineering

  2. OS Security • C. Cowan, S. Beattie, C. Wright, G. Kroah-Hartman "RaceGuard: Kernel Protection From Temporary File Race Vulnerabilities",  USENIX Security Symposium 2001 • C. Wright, C. Cowan, J. Morris, S. Smalley, and G. Kroah-Hartman. Linux security modules: General security support for the linux kernel. In Linux Security Modules: General Security Support for the Linux Kernel, USENIX Security Symposium 2002. Dhanashri Kelkar – OGI School of Science and Engineering

  3. Introduction • A study of computer security • TOCTTOU: Time of check to time of use errors • Race in between file existence check and file creation • Used in temporary file creation • Non-atomicity problem • Preemptive operating system Dhanashri Kelkar – OGI School of Science and Engineering

  4. Temporary File Creation • mktemp() • filename = generateRandomName(); • statResult = stat(filename); • if(!statResult) then open(filename, O_CREAT) • else go to step 1 • What if there is context switch between steps 2 and 3? Dhanashri Kelkar – OGI School of Science and Engineering

  5. filename = generateRandomName(); statResult = stat(filename); if(!statResult) then open(filename, O_CREAT) ln /etc/passwd tmpfile Security Attack • Privileged program attempts to create temp file and attacker guesses the file name Dhanashri Kelkar – OGI School of Science and Engineering

  6. Safe Temporary File Creation • Safe mechanism: • filename = generateRandomName(); • open(filename, O_CREAT|O_EXCL) • Used by mkstemp() • Not commonly available and portable • Many popular programs use mktemp() Dhanashri Kelkar – OGI School of Science and Engineering

  7. RaceGuard • Kernel enhancement • detects attempts to exploit temporary file race conditions • does this with sufficient speed and precision Dhanashri Kelkar – OGI School of Science and Engineering

  8. Temporary File Creation • Victim Program • Seeks to create temp file • Probes for existence of the file • If not found, proceeds to create it • Attacker • Exploits by creating a symbolic or hard link • Points to a security sensitive file Dhanashri Kelkar – OGI School of Science and Engineering

  9. RaceGuard Design • Maintains per-process cache of temporary file races in each PCB (task_struct) • If probe result is non-existent then cache • If file exists and name matches cached name then race attack, abort open attempt • If file creation is without conflicts then clear entry from cache • To avoid false positive event Dhanashri Kelkar – OGI School of Science and Engineering

  10. RaceGuard Implementation • Three groups system calls: • To inform that a file system entry does not exist • To create file system entries • To create and remove processes Dhanashri Kelkar – OGI School of Science and Engineering

  11. Security Testing • Non-deterministic vulnerability • Doctored version of mktemp library call • Pause program • Give attacker more time to deploy race • Print file name to be created • Instead of guessing file name, provide it by printing • Attacked programs • RCS 5.7, rdist 6.1.5, sdiff GNU 2.7 shadow-utils 19990827 Dhanashri Kelkar – OGI School of Science and Engineering

  12. Compatibility Testing • Check whether RaceGuard breaks down existing programs without race attacks • Programs checked • Mozilla web/mail client • RedHat Linux bootup/shutdown scripts • CVS checkout • VMW (Virtual Machine Emulation) system • Some tweaking performed to make it work Dhanashri Kelkar – OGI School of Science and Engineering

  13. Performance Testing • Microbenchmarks: • Stat non-existent file: • w/o: 4.3 µS w/: 8.8 µS Overhead: 104% • Open non-existent file: • w/o: 1.5 µS w/: 1.44 µS Overhead: -4% • Fork: • w/o: 161 µS w/: 183 µS Overhead: 13% Dhanashri Kelkar – OGI School of Science and Engineering

  14. Performance Testing • Macrobenchmarks (Khernel-stone): Dhanashri Kelkar – OGI School of Science and Engineering

  15. Where Are We? • RaceGuard: • Particular computer security case • Try to avoid temporary file creation races • LSM: Linux Security Modules • Generic access control mechanism Dhanashri Kelkar – OGI School of Science and Engineering

  16. Linux Access Control Mechanism • Discretionary access control mechanism (DAC): • User decides who gets access • Mandatory access control mechanism (MAC): • System administrator decides who gets access • POSIX1.e • Many more: e.g. SELinux by NSA Dhanashri Kelkar – OGI School of Science and Engineering

  17. Problems w/ multiple access control mechanism • No mechanism as to which is better • Depends on usage • Unable to include all available security modules inside kernel • Kernel upgrade is needed for every new module • Solution: • Separate loadable kernel modules • Load module you want to use • Direct access to modules through syscalls Dhanashri Kelkar – OGI School of Science and Engineering

  18. Problems with loadable modules • No efficient mechanism for kernel modules to access kernel data • Modules rely on system calls • Highly inefficient Dhanashri Kelkar – OGI School of Science and Engineering

  19. Linux Security Modules Mechanism • Access calls are handled inside kernel • Kernel uses its default policy • If default policy grants access, kernel “consults” loaded module • Special hooks provided for consulting • Access is granted only if modules says “Go ahead” Dhanashri Kelkar – OGI School of Science and Engineering

  20. LSM Hook Mechanism • Global table called security_ops in kernel • Table divided into sub-tables • Each sub-table has pointers to functions that make access decisions • Default access-granting entries filled at kernel boot time • Each module responsible for filling up tables • Module registration Dhanashri Kelkar – OGI School of Science and Engineering

  21. Module Registration & Deregistration • Module registration fails if another LSM module already loaded and registered • To load new module previous module needs to be un-registered • Success of un-registration depends on policy set by previous module Dhanashri Kelkar – OGI School of Science and Engineering

  22. LSM Summary • LSM provides generic way to implement access control mechanism • Different access control mechanisms can reside as loadable modules • System administrator can use appropriate modules as per need Dhanashri Kelkar – OGI School of Science and Engineering

  23. Details Not Covered • Implementation details • Data storage needs of various security policies • Module stacking • Performance evaluation Dhanashri Kelkar – OGI School of Science and Engineering

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