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The Simplified Mandatory Access Control Kernel

Discover how Smack, a modern MAC system, enhances system security by controlling access to resources based on labels. Learn about its principles, jargon, and implementation details for improved protection.

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The Simplified Mandatory Access Control Kernel

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  1. The Simplified Mandatory Access Control Kernel Casey Schaufler January 2008

  2. Casey Schaufler • Ported Unix Version 6 to 32bit • Started Development of TSOL • Architect of Trusted Irix • B1, CAPP, LSPP evaluated • US NSA’s Trusix Group • POSIX P1003.1e/2c • TSIG

  3. Today’s Talk • Mandatory Access Control (MAC) • What MAC is good for • How Smack implements MAC • What Smack is good for • Details of Smack

  4. Mandatory Access Control • Concepts • Subject is an active entity • Object is a passive entity • Access is an operation preformed on an object by a subject

  5. Mandatory Access Control • Principles • User has no say in it • Based on system controlled attributes

  6. Mandatory Access Control • Jargon • MAC • Label • Bell & LaPadula • Multilevel Security • CIPSO

  7. Mandatory Access Control

  8. MAC Implementations • Bell & LaPadula Sensitivity • Multics, Unix • Type Enforcement • SELinux • Pathname Controls • AppArmor, TOMOYO

  9. Uses of MAC Systems • Security Checkbox • Sharing an expensive machine • Disjoint sets of users • B&L Catagories • Hierarchical use of shared data • B&L Levels

  10. Where Did Smack Come From? • Traditionally • Label relationships hard coded • Names map to label values • Mythtory:TopSecret,Skeeve,Ahz,Chumly • Level=4,Catagories=17,49,113 • Users only use names • Why use anything but names?

  11. Smack Label Mechanism • Labels and label names are the same • No implicit relationship between labels • List of explicit access relationships • Every subject gets a label • Every object gets a label • Objects get creating Subject’s label

  12. Subjects Access Objects • lstat() reads a file object’s attributes • kill() writes to a process object • send() writes to a process object • bind() is uninteresting

  13. System Labels • _ floor • ^ hat • * star • Objects Only • Any single special character ^ * _

  14. User Labels ^ * SEAsia Dap _

  15. Explicit Access Rules • Dap SEAsia r • Med Pop w SEAsia Dap Pop Med

  16. Access Rule Specification • /etc/smack/accesses • Subject Object [–rwxa] • /smack/load • Strict fixed format • /sbin/smackload • Writes to /smack/load

  17. Bell & LaPadula Levels • Secret more sensitive than Unclass • TopSecret more sensitive than Secret • Secret Unclass rx • TopSecret Secret rx • TopSecret Unclass rx • All relationships must be specified

  18. Bell & LaPadula Categories • Categories Skeeve and Ahz • Labels: • “Skeeve,Ahz” • “Skeeve” • “Ahz” • Skeeve,Ahz Skeeve rx • Skeeve,Ahz Ahz rx

  19. Biba Integrity • Floor is highest integrity • Hat is lowest Integrity

  20. Ring of Vigilance • SEAsia Dap r • Med SEAsia r • Dap Med r SEAsia Dap Med

  21. Messaging • Informant Reporter w • Reporter Editor w • Editor Reporter w

  22. Time of Day • At 17:00 • WorkerBee Game x • At 08:00 • WorkerBee Game –

  23. Implementation • Label Scheme • Access Checks • File Systems • Networking • The LSM • Audit

  24. Label Scheme • Labels are short text strings • Compared for equality • Stored in a list • secid • Optional CIPSO value • Never forgotten

  25. Access Checks • Rules written to /smack/load • Hard Coded Labels • Subject and object equal • Find the subject/object pair • Check the request against the rule

  26. File Systems • Use xattrs if supported • Hard coded behavior • smackfs, pipefs, sockfs, procfs, devpts • Superblock values • File system root • File system default • File system floor and hat • Not yet implemented

  27. Networking Model • Sender writes to receiver • Sender is subject, receiver is object • Socket, packet not policy components • William Janet w • Allows a UDP packet • Janet William r • Does not allow a UDP Packet

  28. Packet Labeling • Unlabeled packets get ambient label • CIPSO option on every local packet • CIPSO value from the label list • Set via /smack/cipso • CIPSO direct mapping • Level 250 • Label copied into category bits • Same CIPSO as SELinux

  29. The LSM • Provides a restrictive interface • Evolved in step with SELinux • Imperfectly defined • Networking • Audit • USB • Module Stacking

  30. Programming interfaces • getxattr(), setxattr() • SMACK64 • /proc/<pid>/attr/current

  31. Socket Interfaces • Socket Attributes • fgetxattr(), fsetxattr() • SMACK64.IPIN • SMACK64.IPOUT • Packet Attributes • SO_PEERSEC • TCP • SCM_SECURITY • UDP

  32. Administrative Interfaces • /smack/load • /smack/cipso • /smack/doi • /smack/direct • /smack/nltype

  33. What Have You Learned? • Smack is a modern implementation of old school Mandatory Access Control with the mistakes omitted. • Smack is designed for simplicity • Smack is designed as a kernel mechanism

  34. Special Thank You • Paul Moore – Network interfaces • Ahmed S. Darwish – Work on smackfs • And a host of reviewers, including • Stephen Smalley, Seth Arnold, • Joshua Brindle, Al Viro, • James Morris, Kyle Moffett, • Pavel Machek

  35. Contact Information • http://schaufler-ca.com • casey@schaufler-ca.com • rancidfat@yahoo.com

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