P1L5

Question 1

access control

Answer 1

who is allowed to access what

Question 2

2 parts of access control

Answer 2

1. decide who would have access

2. enforcement

Question 3

Access control metric (ACM)

Answer 3

• abstract state: rows-users, column-resources

- ACM[U,O] defines what access right user U have on object O

Question 4

confidentiality

Answer 4

• read related

Question 5

integrity

Answer 5

• write related

Question 6

discretionary access control (DAC)

Answer 6

access to a resource is at the discretion of its owner. Since a user can make a copy and share with the third person

Question 7

ACL (access control list)

Answer 7

handle access to object Oi (column wise)

Question 8

capability list (C-list)

Answer 8

• handle right of user Ui (row wise)

Question 9

where are ACL and C-list

Answer 9

in the trusted part of the system

Question 10

how do we perform access control check

Answer 10

transversal of ACL to see if an ACE (access control entry) exists.

Question 11

a capability in C-list

Answer 11

• unforgeable reference/handle for a resource

- user catalog of capabilities define defines what a certain user can access.

Question 12

hydra

Answer 12

store c-list in objs, resources themselves

Question 13

how sharing happens?

Answer 13

create new ACE, and add access right to that

Question 14

most OS use ACL or C-list? why?

Answer 14

ACL, which is good for accountability and revocation. C-list is only good for efficiency

Question 15

a movie ticket is a capability or access control entry?

Answer 15

capability (holder get access)

Question 16

when does access check for ACE stop?

Answer 16

neg/deny found or transverse the whole list

Question 17

how is access control implemented in unix

Answer 17

• each resource look like a file
• each file has an owner
• each file can possibly be accessed by owner, group or everyone
• permission r,w,x
• ACL implemented using bitmap, 9 bits

Question 18

how does OS implement ACL?

Answer 18

• process call open file
• openfile table get i (index) from i-node table and return to the process
• ACL bit is stored at the same location on i-node table. This will grant access and point to the file data.

Question 19

TOCTOU (time to check time to use) vulnerability

Answer 19

permission changed between checking and using

Question 20

in unix, you can share the file by sharing the file descriptor

Answer 20

false

Question 21

setuid bit set

Answer 21

the uid of the process will be the owner

Question 22

roll-based access control (RBAC)

Answer 22

user -> roll -> rights

Question 23

RBAC benefits

Answer 23

policy associates with roll (in case a user leave or new one comes in)

Question 24

fail safe

Answer 24

deny when in doubt