Transcript Access Control

CIT 380: Securing Computer
Systems
Access Control
CIT 380: Securing Computer Systems
Slide #1
UNIX Access Control Model
• UID
– integer user ID
– UID=0 is root
• GID
– integer group ID
– Users can belong to multiple groups
CIT 380: Securing Computer Systems
Slide #2
UNIX File Permissions
Three sets of permissions:
– User owner
– Group owner
– Other (everyone else)
Three permissions per group
– read
– write
– execute
• UID 0 can access regardless of permissions.
• Files: directories, devices (disks, printers), IPC
CIT 380: Securing Computer Systems
Slide #3
UNIX File Permissions
Best-match policy
– OS applies permission set that most closely
matches.
– You can be denied access by best match even if
you match another set.
Directories
– read = listing of directory
– execute = traversal of directory
– write = add or remove files from directory
CIT 380: Securing Computer Systems
Slide #4
Octal Permission Notation
Each set (u,g,o) is represented by an octal digit.
Each permission (r,w,x) is one bit within a digit.
ex: chmod 0644 file
u: rw, g: r, o: r
ex: chmod 0711 bin
u: rwx, g: x, o: x
CIT 380: Securing Computer Systems
4
read
setuid
2
write
setgid
1
execute sticky
Slide #5
Changing Ownership
newgrp
– Group owner of files is your default group.
– Changes default group to another group to
which you belong.
chgrp
– Changes group owner of existing file.
chmod
– Changes owner of existing file.
– Only root can use this command.
CIT 380: Securing Computer Systems
Slide #6
Default Permissions: umask
• Determines access permissions given to
newly created files
• Three-digit octal number
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–
–
–
Programs default to 0666
Umask modifies to: 0666 & ~umask
ex: umask=022 => file has mode 0644
ex: umask=066 => file has mode 0600
CIT 380: Securing Computer Systems
Slide #7
Access Control
• What is Access Control?
• Access Control Matrix Model
– Protection State Transitions
– Special Rights
– Principle of Attenuation of Privilege
• Groups and Roles
• Implementation of the Access Control Matrix
– Access Control Lists: by column (object).
– Capabilities: by row (subject).
– UNIX
• Hardware Protection
CIT 380: Securing Computer Systems
Slide #8
Why study Access Control?
• Center of gravity of computer security
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–
–
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Why do we authorize users?
What security features do OSes provide?
What’s the purpose of cryptography?
Access Control is pervasive.
• Access Control is where Computer Science
meets Security Engineering.
– We’ll start with theory (computer science)
– Then examine implementations (engineering)
CIT 380: Securing Computer Systems
Slide #9
Access Control is Pervasive
Application
Middleware
Operating System
Hardware
CIT 380: Securing Computer Systems
Slide #10
Access Control is Pervasive
1. Application
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Complex, custom security policy.
Ex: Amazon account: wish list, reviews, purchases
2. Middleware
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Database, system libraries, 3rd party software
Ex: Credit card authorization center
3. Operating System
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File ACLs, account permissions
4. Hardware
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Memory management, hardware device access.
CIT 380: Securing Computer Systems
Slide #11
Access Control Matrix
• Precisely describes protection state of system.
P
Q
• Sets of system states:
– P: Set of all possible states.
– Q: Set of allowed states, according to security policy.
– P-Q: Set of disallowed states.
• ACM describes the set of states Q.
CIT 380: Securing Computer Systems
Slide #12
Access Control Matrix
• As system changes, state changes.
– State transitions.
– Only concerned with protection state.
• ACM must be enforced by a mechanism that
limits state transitions to those that go from
one element of Q to another.
CIT 380: Securing Computer Systems
Slide #13
Access Control Matrix
objects (entities)
subjects
o1 … om s1 … sn
s1
s2
• Objects O = { o1,…,om }
– All protected entities.
• Subjects S = { s1,…,sn }
– Active entities, S  O
• Rights R = { r1,…,rk }
…
• Entries A[si, oj]  R
• A[si, oj] = { rx, …, ry }
means subject si has rights
rx, …, ry over object oj
sn
CIT 380: Securing Computer Systems
Slide #14
Access Control Matrix
• Subjects
– Users
– Processes (Programs)
• Objects
– Processes (Programs)
– Files
CIT 380: Securing Computer Systems
Slide #15
Access Control Matrix
• Rights
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–
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–
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Read (r)
Write (w)
Execute (x)
Append (a)
Owner (o)
Copy Rights (c)
CIT 380: Securing Computer Systems
Slide #16
Example: File/Process
• Processes p, q
• Files f, g
• Rights r, w, x, a, o
p
q
f
rwo
a
g
r
ro
CIT 380: Securing Computer Systems
p
rwxo
r
q
w
rwxo
Slide #17
Copy Right
• Allows possessor to give rights to another
• Often attached to a right, so only applies to
that right
– r is read right that cannot be copied
– rc is read right that can be copied
• Is copy flag copied when giving r rights?
– Depends on model, instantiation of model
CIT 380: Securing Computer Systems
Slide #18
Ownership Right
Usually allows possessor to change entries
in ACM column
– So owner of object can add, delete rights for
others
– May depend on what system allows
• Can’t give rights to specific (set of) users
• Can’t pass copy flag to specific (set of) users
CIT 380: Securing Computer Systems
Slide #19
Attenuation of Privilege
Principle: Subject may not give rights it
does not possess to another.
– Restricts addition of rights within a system
– Usually ignored for owner
• Why? Owner gives herself rights, gives them to
others, deletes her rights.
CIT 380: Securing Computer Systems
Slide #20
How can we implement the ACM?
Problem: scale
– Thousands of subjects.
– Millions of objects.
– Yet most entries are blank or default.
Solutions
– Group subjects together as a single entities
• Groups and Roles
– Implement by row: Capabilities
– Implement by column: Access Control Lists
CIT 380: Securing Computer Systems
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Groups and Roles
• Collect subjects together to express:
– Need to share objects.
– Security categories (e.g., admin, faculty,
student, guest)
• role: group that ties membership to function
• Problem: loss of granularity.
CIT 380: Securing Computer Systems
Slide #22
Capabilities
• Implement ACM by row.
• Access Control associated with subject.
• Example: UNIX file descriptors
– System checks ACL on file open, returns fd.
– Process subsequently uses fd to read and write file.
– If ACL changes, process still has access via fd.
User
ls
homedir
rootdir
james
rx
rw
r
CIT 380: Securing Computer Systems
Slide #23
Capability Questions
How to revoke rights to an object?
• Direct solution
– Check capabilities of every process.
– Remove those that grant access to object.
– Computationally expensive.
CIT 380: Securing Computer Systems
Slide #24
Access Control Lists (ACLs)
• Implement ACM by column.
• Access control by object.
• Example: UNIX ACLs
– Short “rwx” user/group/other.
– Long POSIX ACLs.
CIT 380: Securing Computer Systems
User
audit data
root
rw
james r
joe
Slide #25
ACL Questions
1. Which subjects can modify an object’s
ACL?
2. Do ACLs apply to privileged users?
3. Do ACLs support groups and wildcards?
4. How are ACL conflicts resolved?
5. What are default permissions?
6. How can a subject’s rights be revoked?
CIT 380: Securing Computer Systems
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Which subjects can modify an ACL?
• Create an own right for an ACL.
– Only subjects with own right can modify ACL.
• Creating an object also creates object’s ACL.
– Usually creator given own right at this time.
– Other default rights may be set at creation too.
CIT 380: Securing Computer Systems
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Which subjects can modify an ACL?
• Some systems allow anyone with access to
object to modify ACL.
– What are the security implications of sharing
access to a file on such a system?
CIT 380: Securing Computer Systems
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Do ACLs apply to privileged users?
• Many systems have privileged users.
– UNIX: root.
– Windows NT: administrator.
• Should ACLs apply to privileged users?
– Need read access to all objects for backups.
– What security problems are produced by
ignoring ACLs for privileged users?
CIT 380: Securing Computer Systems
Slide #29
What are the default permissions?
• Interaction of ACLs with base permissions.
– POSIX ACLs modify UNIX base permissions.
CIT 380: Securing Computer Systems
Slide #30
What are the default permissions?
• How are default ACLs determined?
– Subject
• Subject sets default permissions, like UNIX umask.
– Inheritance
• Objects in hierarchical system inherit ACLs of
parent object.
• Subjects inherit sets of default permissions from
their parent subjects.
CIT 380: Securing Computer Systems
Slide #31
How are rights revoked?
Removal of subject’s rights to object.
– Delete entries for subject from ACL.
– If ownership doesn’t control granting rights,
matters can be complex:
• If A has granted rights to B, what should happen to
B’s rights if you remove A’s rights?
Removal of subject’s rights to all objects.
– Very expensive (millions of objects.)
– Most systems don’t support.
– Why isn’t disabling subject’s account sufficient?
CIT 380: Securing Computer Systems
Slide #32
ACLs vs Capabilities
ACLs
Capabilities
• Slow: OS has to read
• Fast: OS always knows
ACL for each object
subject identity.
accessed.
• Easy to find/change
• Easy to find/change
rights on a particular
rights on a particular
subject.
object.
• Difficult to revoke
• Difficult to revoke
privileges to a subject
privileges for a specific
object.
subject.
CIT 380: Securing Computer Systems
Slide #33
Limitations of Classic ACLs
ACL control list only contains 3 entries
– Limited to one user.
– Limited to one group.
Root (UID 0) can do anything.
CIT 380: Securing Computer Systems
Slide #34
POSIX Extended ACLs
Supported by most UNIX/Linux systems.
– Slight syntax differences may exist.
getfacl
setfacl
– chmod 600 file
– setfacl -m user:gdoor:r-- file
– File unreadable by other, but ACL allows gdoor
CIT 380: Securing Computer Systems
Slide #35
Host-based Access Control
• /etc/hosts.allow and /etc/hosts.deny
• used by tcpd, sshd, other servers
• Identify subjects by
– hostname
– IP address
– network address/mask
• Allow before Deny
– use last rule in /etc/hosts.deny to deny all
CIT 380: Securing Computer Systems
Slide #36
Hardware Protection
Confidentiality
– Processes cannot read memory space of kernel
or of other processes without permission.
Integrity
– Processes cannot write to memory space of
kernel or of other processes without permission.
CIT 380: Securing Computer Systems
Slide #37
Hardware Protection
Availability
– One process cannot deny access to CPU or
other resources to kernel or other processes.
CIT 380: Securing Computer Systems
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Hardware Mechanisms: VM
• Each process has its own address space.
– Prevents processes from accessing memory of
kernel or other processes.
• Attempted violations produce page fault exceptions.
CIT 380: Securing Computer Systems
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Hardware Mechanisms: VM
• Each process has its own address space.
– Implemented using a page table.
– Page table entries contain access control info.
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•
•
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Read
Write
Execute (not separate on Intel CPUs)
Supervisor (only accessible in supervisor mode)
CIT 380: Securing Computer Systems
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VM Address Translation
CIT 380: Securing Computer Systems
Slide #41
Hardware Mechanisms: Rings
Protection Rings.
– Lower number rings have more rights.
– Intel CPUs have 4 rings
• Ring 0 is supervisor mode.
• Ring 3 is user mode.
• Most OSes do not use other rings.
– Multics used 64 protection rings.
• Different parts of OS ran in different rings.
• Procedures of same program could have different
access rights.
CIT 380: Securing Computer Systems
Slide #42
Hardware Mechanisms: System Timer
• Timer interrupt
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Programmable Interval Timer chip.
Happens every 1-100 OS, depending on OS.
Transfers control from process to OS.
Ensures no process can deny availability of
machine to kernel or other processes.
CIT 380: Securing Computer Systems
Slide #43
Why is Access Control hard?
• Complex Objects
– Identifying objects of interest.
• Is your choice of objects too coarse or fine-grained?
– Hierarchical structure like filesystem or XML
• Subjects are Complex
– Identifying subjects of interest.
– What are the relationships between subjects?
CIT 380: Securing Computer Systems
Slide #44
Why is Access Control hard?
• Access Control states change.
• Security objectives often unclear.
CIT 380: Securing Computer Systems
Slide #45
References
1. Anderson, Ross, Security Engineering, Wiley,
2001.
2. Bishop, Matt, Introduction to Computer Security,
Addison-Wesley, 2005.
3. Bovet, Daniel and Cesati, Marco, Understanding
the Linux Kernel, 2nd edition, O’Reilly, 2003.
4. Silberschatz, et. al., Database System Concepts,
4th edition, McGraw-Hill, 2002.
5. Silberschatz, et. al., Operating System Concepts,
7th edition, Wiley, 2005.
6. Viega, John, and McGraw, Gary, Building Secure
Software, Addison-Wesley, 2002.
CIT 380: Securing Computer Systems
Slide #46