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Information Flow Control for Standard OS Abstractions

Explore access control mechanisms in operating systems, from kernel decision-making to process UID assignment and high-score game security. Learn about secrecy labels, integrity, and privilege for data protection and trust. Discover the complexities of information flow control rules and endorsements in system security.

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Information Flow Control for Standard OS Abstractions

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  1. Information Flow Control for Standard OS Abstractions Landon Cox March 21, 2018

  2. Access control and the kernel • How does kernel know if system call is allowed? • Looks are user id (uid) of process making the call • Looks at resources accessed by call (e.g., file) • Checks access-control policy associated with resource • Decides if policy allows uid to access resources • How is a uid normally assigned to a process? • On fork, child inherits parent’s uid

  3. MOO accounting problem Game client (uidx) • Multi-player game called Moo • Want to maintain high score in a file • Should players be able to update score? • Yes • Do we trust users to write file directly? • No, they could lie about their score High score Game client (uidy)

  4. MOO accounting problem Game client (uidx) • Multi-player game called Moo • Want to maintain high score in a file • Should players be able to update score? • Yes • Do we trust users to write file directly? • No, they could lie about their score • Could have a trusted process update scores • Why isn’t this enough? • Can’t be sure that reported score is genuine • Also want to ensure that score was computed correctly High score Trusted process (root) Game client (uidy)

  5. Access control • Simple inheritance of uids is not sufficient for • Tasks involving management of “user id” state • Logging in (login) • Changing passwords (passwd) • Solution: setuid • Executable files can have their setuid bit set • If setuid bit is set, process inherits uid of image file’s owner on exec

  6. MOO accounting problem Game client (uidx) • Multi-player game called Moo • Want to maintain high score in a file • How does setuid allow us to know that score is correct? • Game executable is owned by trusted entity • Game cannot be modified by normal users • Users can run executable though • High-score is also owned by trusted entity • This is a form of trustworthy computing • High-score maintainer knows exactly which code will update score • Ensures code integrity, even when run by untrusted users • Also data confidentiality, since data accessed by code with integrity High score Game client (uidy)

  7. Information Flow Control (IFC) • Goal: track which secrets a process has seen • Mechanism: each process gets a secrecylabel • Label summarizes which categories of data a process is assumed to have seen. • Examples: • { “Financial Reports” } • { “HR Documents” } • { “Financial Reports” and “HR Documents” } “tag” “label” Slide by Max Krohn

  8. Secrecy, Integrity, and Privilege How did endorsements work in the Moo prob? • Secrecy label (Sp) • Specifies what data P has read • “/usr/bin/login may read the password file” • Integrity label (Ip) • Used to endorse trustworthiness of P • “/usr/bin/login can only be updated by root” • Also limits what P can read • “/usr/bin/login can only read user libs and config files endorsed by root” • Ownership (Op) • Regulates how P can update Sp and Ip • Tags P can add to its labels (e.g., t+), i.e., endorse via integrity label • Tags P can remove from its labels (e.g., t-), i.e., declassify via secrecy label • Dp is the set of tags that P can both add and remove What is an endorsement? Why is this restriction necessary?

  9. Secrecy, Integrity, and Privilege • Secrecy • “At some point process p added data with tag s to its address space.” • s ∈ Sp∃(data) : p read data with tag s • Integrity • “All inputs to process p had tag i.” • i ∈ Ip∀(data) : p read data with tag i • Privilege • “p can remove tag s from Spand add tag i to Ip.” • s ∈ t- p is trusted to declassify s • i ∈ t+p is trusted to endorse i • t∈ Dpt ∈ t- and t ∈ t+

  10. Tags + Secrecy Labels change_label({Finance}); Process p Secrets P has viewed tag_t HR = create_tag(); change_label({}); Any process can add any tag to its label. change_label({Finance,HR}); Sp = { Finance, HR } Sp = { Finance } Sp = {} DIFC Rule: A process can create a new tag; gets ability to declassify it. change_label({Finance}); Dp = {} Dp = { HR } Same as Step 1. Tags P can add and remove from its label DIFC: Declassification in action. Finance Legal HR Universe of Tags: SecretProjects Slide by Max Krohn

  11. Tags + Integrity Labels change_label({}); Process p Endorsements of P Any process can remove any tag from its label. Ip = {Apple} Dp = {} Tags P can add and remove from its label Finance Legal Universe of Tags: Apple Slide by Max Krohn

  12. Tags + Integrity Labels change_label({}); Process p Ip = {} Dp = {} Finance Legal Universe of Tags: Apple Slide by Max Krohn

  13. Tags + Integrity Labels change_label({}); Process p tag_t HR = create_tag(); change_label({Microsoft}); Ip = {} Dp = {} Finance Legal Universe of Tags: Apple Slide by Max Krohn

  14. Tags + Integrity Labels change_label({}); Process p tag_t HR = create_tag(); Ip = {} Dp = {} Finance Legal Universe of Tags: Apple Slide by Max Krohn

  15. Tags + Integrity Labels change_label({}); Process p tag_t HR = create_tag(); Ip = {} DIFC Rule: A process can create a new tag; gets ability to endorse w/ it. Dp = {HR} Finance Legal HR Universe of Tags: Apple Slide by Max Krohn

  16. Tags + Integrity Labels change_label({}); Process p tag_t HR = create_tag(); change_label({HR}); Ip = {} Dp = {HR} Finance Legal HR Universe of Tags: Apple Slide by Max Krohn

  17. Tags + Integrity Labels change_label({}); Process p tag_t HR = create_tag(); change_label({HR}); Ip = {HR} Dp = {HR} DIFC: Endorsement in action. Finance Legal HR Universe of Tags: Apple Slide by Max Krohn

  18. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p Sp = {} Dp = { HR, Admin } Admin Microsoft Bob’s code Alice’s code Universe of Integrity Tags

  19. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p Sp = { HR } Dp = { HR, Admin } Admin Microsoft Bob’s code Why is this allowed? Alice’s code Universe of Integrity Tags

  20. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p Sp = { HR } Dp = { HR, Admin } Admin Microsoft Bob’s code What is the effect? Alice’s code Universe of Integrity Tags

  21. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p q Sp = { HR } Dp = { HR, Admin } Admin Sq = { HR } Microsoft Bob’s code What is the effect? Can now receive data from HR processes Alice’s code Universe of Integrity Tags

  22. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p q Sp = {} Dp = { HR, Admin } Admin Sq = { HR } Microsoft Bob’s code Why is this allowed? Alice’s code Universe of Integrity Tags

  23. Privilege in action (secrecy) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p q Sp = {} Dp = { HR, Admin } Admin Sq = { HR } Microsoft Bob’s code What is the effect? Alice’s code Universe of Integrity Tags

  24. Privilege in action (secrecy) r Finance Universe of Secrecy Tags: HR Legal Sr = {} SecretProjects Process p q Sp = {} Dp = { HR, Admin } Admin Sq = { HR } Microsoft Bob’s code What is the effect? Declassifies HR data received from q Alice’s code Universe of Integrity Tags

  25. Privilege in action (integrity) Finance Universe of Secrecy Tags: HR Legal SecretProjects Process p Ip = {Admin} Dp = { HR, Admin } Admin Microsoft Bob’s code Admin+ makes p a certifier Alice’s code Universe of Integrity Tags

  26. Privilege in action (integrity) Fake vi Ip = {} Process p vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  27. Privilege in action (integrity) Fake vi Ip = {} Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  28. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR, Admin } q Ip = {} fork() Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  29. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR } q Why drop Admin+? Ip = {} fork() Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  30. Privilege in action (integrity) Should this work? Fake vi Ip = {Admin} Dp = { HR } q Ip = {} fork() exec(“vi”) Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  31. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR } q Ip = {} fork() load(“libc”) Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  32. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR } q Ip = {} fork() read(“/etc/rc”) Process p “Run vi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  33. Privilege in action (integrity) Fake vi Ip = {} Process p “Run Fakevi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  34. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR, Admin } q Ip = {} fork() Process p “Run Fakevi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  35. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR } q Ip = {} fork() Process p “Run Fakevi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  36. Privilege in action (integrity) Fake vi Ip = {Admin} Dp = { HR } q exec(“fakevi”) Ip = {} fork() Process p “Run Fakevi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  37. Privilege in action (integrity) Should this work? Fake vi Ip = {Admin} Dp = { HR } q exec(“fakevi”) Ip = {} fork() Process p “Run Fakevi” vi Ip = {Admin} Dp = { HR, Admin } Ip = {Admin} libc /etc/rc Ip = {Admin} Ip = {Admin}

  38. Communication Rule P Process p Process q Sp = {HR} Sq = {HR, Finance} • p can send to q iff SpÍ Sq Slide by Max Krohn

  39. Flume Communication Rule P ? ? Database (q) • q changes to Sq = { Alice } • p sends to q • q changes back to Sq= {} MoinMoin (p) MoinMoin (r) Sr = { Bob } Sp = { Alice } Sq = {} Dq = { Alice, Bob } Sq = { Alice } Dq = { Alice, Bob } SpÍ Sq Slide by Max Krohn

  40. Flume Communication Rule P P Database (q) • p can send to qiff: • In IFC: SpÍ Sq • In Flume: Sp – DpÍ SqÈDq MoinMoin (p) MoinMoin (r) Sr = { Bob } Sp = { Alice } Sq = {} Dq= { Alice, Bob } Senders get extra latitude Receivers get extra latitude Slide by Max Krohn

  41. Unexpected Program Behavior (Unreliable Communication) Process p Process q P “Fire Alice, Bob, Charlie, Doug, Eddie, Frank, George, Hilda, Ilya…” “I stopped reading” “I crashed” Slide by Max Krohn

  42. Unreliable communication Process p Process q stdout stdin Sp = {} Dp = { HR } Sq = {HR} P “Fire Alice, Bob, Charlie, Doug, Eddie, Frank, George, Hilda, Ilya…” ? “SLOW DOWN!!” “I crashed” Slide by Max Krohn

  43. New Abstraction: Endpoints Process p Process q e f Se = { HR } Sf = { HR } Sp = {} Dp = { HR } Sq = {HR} P • If SeÍ Sf, then allow e to send to f • If SfÍ Se, then allow f to send to e • If Sf = Se , then allow bidirectional flow “Fire Alice, Bob, Charlie, Doug, Eddie, Frank, George, Hilda, Ilya…” P “SLOW DOWN!!” “I crashed” Slide by Max Krohn

  44. Endpoints Declassify Data Data enters process p with secrecy { HR } But p keeps its label Sp = {} Process p e Se = { HR } Sp = {} Dp = { HR } Thus p needs HRÎ Dp Slide by Max Krohn

  45. Endpoint Invariant Export inf. • For any tag tÎSp and tÏ Se • Or any tag tÎSe and t Ï Sp • It must be that tÎDp Import inf. Process p e Se = { HR } Sp = { Finance } Dp = { Finance, HR} Slide by Max Krohn

  46. Endpoints’ Labels Are Independent g Sg = {} Process p Process q e f Se = { HR } Sf = { HR } Sp = {} Dp = { HR } Sq = {HR} Slide by Max Krohn

  47. Example App: MoinMoin Wiki Slide by Max Krohn

  48. How Problems Arise… if not self.request.user.may.read(pagename): return self.notAllowedFault() x43 LayoffPlans MoinMoin Wiki (100 kLOC) FreeTShirts Slide by Max Krohn

  49. MoinMoin + DIFC LayoffPlans Apache Web Server MoinMoin Wiki (100 kLOC) Declassifier 1 kLOC FreeTShirts Trusted Untrusted Slide by Max Krohn

  50. FlumeWiki Flume-Oblivious unconfined confined reliable IPC Web Client GET /LayoffPlans?user=Intern&PW=abcd LayoffPlans S={ HR } Apache Declassifier 1 kLOC MoinMoin (100 kLOC) FreeTShirts S={} file I/O Slide by Max Krohn

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