Transcript 03 OS security

Operating system security
Tuomas Aura
CSE-C3400 Information security
Aalto University, autumn 2014
Outline
Access control models in operating systems:
1. Windows
2. Unix
3. Alternatives
Acknowledgements: This lecture material is partly
based on a joint course with Dieter Gollmann
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WINDOWS ACCESS CONTROL
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Windows Security Model
 Principals = users, machines, groups,…
 Objects = files, Registry keys, printers, …
 Each object has a discretionary access control list
(DACL)
 The active subjects are processes or threads
running as a principal (i.e. acting on its behalf)
 Each process (or thread) has an access token
 When is a process allowed to access an object?
– Object DACL is compared with the process’s access
token when creating a handle to the object
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Security indentifier
 Principal names: machine\principal or domain\principal
– aalto\aura, pc3\Administrators, plover\aura
– Each principal has a unique security identifier (SID)
– Names may change; SID is permanent
 User SIDs:
– S-1-5-21-961468069-954667678-1722769278-1002
= Alice
– S-1-5-21-961468069-954667678-1722769278-500
= Administrator
– Typical way to create unique user SIDs:
S-1-5 + machine or domain id + relative id
 There are some well-known SIDs:
– S-1-5-18 = Local System, S-1-1-0 = Everyone, S-1-5-domain-513 =
Domain Users, etc.
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Windows domains
 Windows machine has a Local Security Authority (LSA),
which can create local users and local groups (=aliases)
– Local principals are stored in Registry
 A Windows server can become a Domain Controller (DC),
and other machines can join its domain
 Domain administrators manage the domain users and
groups centrally at the DC
– Domain principals are stored in Active Directory (AD)
– Names: domain\principal or principal@domain
 DC provides authentication services to other machines
– Domain user can log into any domain-joined machine
– Kerberos protocol used for distributed authentication
 In large organizations, DCs and domains can form a
hierarchy
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Access token
 Each process has an access token (=security token)
 Token contains
– Login user account SID (the process “runs as” this user)
– SIDs of all groups in which the user is a member (recursively)
– All privileges assigned to these groups
 Privileges are special local access rights:
– Backup, audit security log, take ownership, trusted for
delegation, debugging, performance profiling, shutdown. etc.
 Groups may be built-in or defined by admins:
– Users, Administrators, Remote Desktop Users
– Sales, Security Lab, Researchers, Europe Employees
 Token never changes after it has been created
– Better reliability and efficiency, slower revocation
 Child process gets a copy of the parent’s token; it may be
restricted
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Objects
 Objects: files, folder, Registry and AD objects,
printers, processes...
– Objects can be containers for other objects 
container hierarchy
 Each object has a security descriptor, which
includes the discretionary access control list
(DACL)
 Object also has an owner (identified by SID),
who has the implicit right to read and write
the DACL
– This is discretionary access control
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Permissions
 Permissions are actions that apply to each object
class
 Some generic permissions are defined for all
objects: read, write, execute, all, delete, etc.
 Specific permissions are defined for each object
class: Append, AddSubDir, CreateThread, etc.
 Permissions are encoded as a 32-bit mask for fast
checking
 Object DACL specifies which principals (SIDs) have
which permissions
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Access control list (DACL)
ACE1
ACE2
ACE2
ACE4
+
+
+
Tuomaura
Diego
Lecturers
EVERYONE
Write
Full Control
Read, Write
Read
 DACL is a list of access control entries (ACE)
 Negative ACEs are placed before positive ones
 The above DACL grants read access but no
write access to the user Tuomaura
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Viewing the DACL and ACEs
 Right-click on a file;
select Properties/
Security

Note: Windows DACLs only exist in
NTFS, not in FAT or other file systems
 Click on Advanced
to see the entire
security descriptor
DACL
(list of ACEs)
Permissions in
the selected ACE
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Access check algorithm
 Process specifies the desired access (requested
permissions) when creating a handle to the
object
 Privileges or implicit owner permissions may
alone be sufficient for the requested access
 Otherwise, Windows checks the DACL as follows:
– Look for ACEs that match both (1) any SID in the
subject’s access token and (2) any desired access right
– If any negative ACE matches, deny access
– If positive ACEs are found for all requested
permissions, grant access
– If the end of DACL is reached, deny access
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Performance and reliability
 Group membership and privileges are determined at login
time
– User’s group SIDs are cached in the token of the login process,
and sub-processes get a copy
– Token will not change even if a group membership or privilege is
revoked from the user
 Desired access is compared against the token and DACL
when creating a handle to the object – not at access time
– Changing file DACL does not affect open file handles
 Consequences:
– Better performance because of fewer checks
– Better reliability because a process knows in advance whether it
has sufficient access rights for a task
– No immediate revocation of access rights
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ACE inheritance
+ Diego Read, Write
Flags: OBJECT_INHERIT
Folder
File A
+ Diego Read, Write
Flags: INHERITED_ACE
File B
- Diego Write
+ Diego Read, Write
Flags: INHERITED_ACE
 Container objects can have inheritable ACEs
 Inherited ACEs are copied to the end of contained-object DACLs;
ACEs on the contained object come first and thus can override the
inherited ones
 Inherited ACEs are updated if the original one changes
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Blocking inheritance
+ Diego Read Flags: OBJECT_INHERIT
Folder X
Folder Y
Folder Z
+ Diego Read
Flag: INHERITED_ ACE,
INHERIT_ONLY
File A
+ Diego Read, Write
Flag: INHERITED_ACE
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File B
Flag: DACL_
PROTECTED
How to see them
 Local users and aliases:
> net user
> net localgroup
– Run compmgmt.msc, see System Tools / Local user and Groups
 Domain users, groups and aliases:
> net user /domain
> net group /domain
> net localgroup /domain
(slow if it is a large domain!)
(groups)
(domain local groups*)
 Members of a group, e.g.:
> net group “Researchers“ /domain
 Domain user information:
> net user alice /domain
 Privileges:
– Run secpol.msc, see Local Policies / User Rights Assignment
 Permissions:
> icacls file.txt
> icacls mydir /T /C
*Not explained in this lecture
(recursive)
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UNIX ACCESS CONTROL
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Principals




The principals are users and groups
Users have username and user identifier (UID)
Groups have group name, group identifier (GID)
UID and GID are usually 16-bit numbers
0 = root
19057 = aura
100 = users
 Both names and identifiers are permanent;
difficult to change once selected
– UID values often differ from system to system
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User accounts
 User accounts are stored in /etc/passwd
 User account format:
username:password:UID:GID:name:homedir:shell
 Example:
root:7kSSI2k.Df:0:0:root:/root:/bin/bash
mail:x:8:12:mail:/var/spool/mail:
news:x:9:13:news:/var/spool/news:
ace:69geDfelkw:500:103:Alice:/home/ace:/bin/bash
carol:7fkKdefh3d:501:102:Carol:/home/carol:/bin/nologin
tuomas:*:502:102:Tuomas Aura:/home/tuomas:/bin/tcsh
al::503:102::/home/al:/bin/bash
diego:RT.QsZEEsxT92:10026:53:Dieter
Gollmann:/home/staff/dieter:/bin/bash
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Superuser
 The superuser is a special privileged principal
with UID zero and usually the user name root
 There are few restrictions on the superuser
– All security checks are turned off for the superuser
– The superuser can become any other user
 Examples:
– The superuser cannot write to a read-only file
system but can remount it as writeable
– The superuser cannot decrypt passwords (because
they are hash values) but can reset them
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Groups
 Users belong to one or more groups
 The file /etc/group contains a list of all groups;
entry format:
groupname:password:GID:list of users
 Example:




infosecwww:*:209:carol,al
Every user belongs to a primary group; the group ID
(GID) of the primary group is stored in /etc/passwd
Depending on the Unix OS, user can belong to only one
or many groups at the same time
Usually only superuser can add groups and members
Use the groups command to see your groups
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Subjects
 The subjects in Unix are processes; a process
has a process ID (PID)
 Processes can create new processes
 Processes have a real UID and an effective UID
(similarly for GID)
 Real UID/GID: inherited from the parent;
typically UID/GID of the user logged in
 Effective UID/GID: inherited from the parent
process or from the file being executed
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Example
Process
/bin/login
real
UID
effective
real
root
root
GID
effective
system system
User diego logs on; the login process verifies the password and (with its
superuser rights) changes its UID and GID (setuid(2), setguid(2)):
/bin/login
diego diego staff staff
The login process executes the user’s login shell:
/bin/bash
diego diego staff
staff
From the shell, the user executes a command, e.g. ls
/bin/ls
diego diego staff staff
The user executes command passwd to change his password:
/bin/passwd
diego root
staff system
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Objects
 The objects of access control are files,
directories and devices
– Organized in a tree-structured file system
 Directory is a file containing file names and
pointers to inode data structures
 Inode stores information about the object
owner user and group, and permissions
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Information about objects
 Example: directory listing with ls -l
-rw-r--r-- 1 diego staff 1617 Oct 28 11:01 my.tex
drwx------ 2 diego staff 512 Oct 25 17:44 ads/
 File type: first character
‘-’ file
‘d’ directory
‘b’ block device file
‘c’ character device file
‘s’ socket
‘l’ symbolic link
‘p’ FIFO pipe
 File permissions: nine characters
 Link counter: the number of links (i.e. directory
entries pointing) to the inode
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Information about objects
-rw-r--r-- 1 diego staff 1617 Oct 28 11:01 my.tex
drwx------ 2 diego staff 512 Oct 25 17:44 ads/
 Username of the owner: usually the user that has
created the file
 Group: a newly created file usually belongs to its
creator’s primary group
 File size, modification time, filename
 Owner and root can change permissions (chmod);
root can change the file owner and group (chown)
 User can change the file group to of its own groups
 Filename is stored in the directory, not in inode, so
that many names can be linked to the same inode
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File permissions
 Permission bits are grouped in three triples that
define read, write, and execute access for owner,
group, and other
 rw-r--r-- read and write access for the owner,
read access for group and other
 rwx------ read, write, and execute access for the
owner, no rights to group and other
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Octal representation
 File permissions can also be specified as octal
numbers
 Examples: rw-r--r-- is equivalent to 644;
rwxrwxrwx is equivalent to 777
 Conversion table:
0040 read by group
0020 write by group
0010 execute by group
0004 read by other
0002 write by other
0001 execute by other
4000 set UID on execution
2000 set GID on execution
1000 set sticky bit
0400 read by owner
0200 write by owner
0100 execute by owner
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SUID and SGID programs
 SUID programs run with the effective UID of
the owner of the executable file
 For a SUID program, the execute permission of
the owner is given as s instead of x:
-rws--x—x 3 root bin 16384 Nov 16 1996 passwd*
 SGID programs run with the effective GID of
the owner of the executable file
 For a SGID program, the execute permission of
the group is given as s instead of x
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Special modes for a directory
 Sticky bit on an executable file historically indicated that
the process should not be swapped to disk
 Sticky bit on a directory restricts the deletion of files in that
directory only to the file owners (and root)
– Job queues for printing etc., are often implemented as a worldwritable directories; anyone can add a file but not delete the
files of others
– /tmp
 SGID bit on a directory means that new files inherit their
group from the directory, not from the user who creates
the file
– Avoid running the print daemon as root: create a special group
for the print daemon process and the print queue directory
– Implement project directory where members can share files
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Unix access control — dicsussion
 Unix permissions have been standardized by IEEE as part of
the POSIX standards (DOI 10.1109/IEEESTD.1992.106983)
– Fairly universal across Unix systems
 Limitations and advantages?
– Files have only one owner and group
– Complex policies, e.g. access to several groups, are impractical
to implement
– Superuser needed for maintaining groups
– All access rights (e.g. shutdown, create user) must be mapped
to file access and to read, write and execute permissions
– Relatively simple and widely understood
– Relatively easy to check the protection state
 Unix versions have subtle differences and may implement
additional access control features
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ALTERNATIVES
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Access control lists in Unix
Q: Are Unix file permissions a kind of ACL?
Most Unix systems support additionally one of:
 Posix ACL
(standardization abandoned but widely implemented,
see POSIX 1003.1e Draft 17)
 Network File System NFSv4 ACL
(RFC 3550, section 5.11)
– Similar to Windows ACL with minor differences
 Linux:
– Many file systems (e.g. EXT3, EXT4, XFS) can be compiled
with POSIX ACL support
– NFS client and server map between POSIX and NFSv4 ACLs
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ACLs in Mac OS X
 Mac OS X implement both POSIX permissions and NFSv4 ACLs
– Similar to Windows ACLs, but little used in OS X
 Most files and folders do not have an ACL
 When you set permissions in the graphical user interface and there
are entries for more than one user or group, an ACL is created
 ACLs are mostly used mainly to prevent accidental deletion of
important folders:
$ ls -led Movies
drwx------+ 2 aura staff 68 14 Syy 20:45 Movies
0: group:everyone deny delete
$ rm -r Movies
rm: Movies: Permission denied
$ chmod +a# 0 "aura allow delete" Movies
$ rm -r Movies
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Security-Enhanced Linux (SELinux)
 Mandatory access control for Linux servers
– Originally developed by NSA, goal is security certification
 Policy that cannot be changed by users
– Modular policy language, complex policy definitions
– Policy is compiled and installed by admin, not changed often
 Hybrid access-control model:
1. Type enforcement
– Subjects and objects have a type, rules based on the type
2. Multi-level security (e.g. Bell-LaPadula)
–
Level = sensitivity + category
3. Role-based access control (RBAC)
 Each subject and object is labelled with security context
user:role:type:level
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SELinux discussion
 SELinux is mainly used to lock down a server in
case it has software vulnerabilities or Trojans
– Types or categories are used for sandboxing processes
 Developing and maintaining a new SELinux policy
is a lot of work!
– Original NSA reference policy is not usable
– Tresys reference policy has commercial support
– Red Hat Enterprise Linux tries to provide policies for
common services
– There are tools for policy validation and for learning
policies by observing the system in operation
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Mobile Operating Systems
 Android, iOS, Windows Phone
 Subjects are apps, objects are phone features and services
 Apps are given permissions
– E.g. read contacts, send SMS, read camera roll, read GPS
– User gives permissions at install time or when the permission is first
needed
– By default, apps can typically only access Internet and their own data
 Apps are isolated from each other
– Data can be shared only through special services that require
permissions, e.g. photo library
 No data sharing between apps through file system
– Often , cloud file systems like Dropbox are used instead (Is that good?)
 Permissions are actually capabilities
 Systems differ in the granularity of permissions and in user
interaction
 Learn much more in the course CSE-E5480 Mobile Systems Security
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Reading material
 Dieter Gollmann: Computer Security, 2nd ed.,
chapter 6–7; 3rd ed. chapters 7–8
 Matt Bishop: Introduction to computer security,
chapter 25
 Ross Anderson: Security Engineering, 2nd ed.,
chapter 4
 Online:
– John R. Michener, Understanding Windows File And Registry
Permissions, MSDN Magazine, Nov 2008
http://msdn.microsoft.com/en-us/magazine/cc982153.aspx
– Windows Development Reference, MSDN:
http://msdn.microsoft.com/enus/library/windows/desktop/aa374876(v=vs.85).aspx
– Wayne Pollock, Unix File and Directory Permissions and Modes
http://content.hccfl.edu/pollock/AUnix1/FilePermissions.htm
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Exercises: Windows
 How can Unix file permissions be expressed with Windows ACLs?
 Assume Fred is member of group Lecturers. Who gets access to an object
with the DACLs:
1. [-,Fred,READ], [+, Lecturers,READ] ?
2. [-, Lecturers,READ], [+,Fred,READ] ?
3. [+,Fred,READ], [-, Lecturers,READ] ?
 When a new object is created, what goes to its security descriptor?
 Access tokens are objects themselves. How does access control for the
tokens work?
 What is the time-of-check-to-time-of-use (TOCTTOU) issue? Where does
this create potential problems in the Windows file system?
 There is no Windows API for giving file ownership to others.
Administrators have backup and restore privileges. What trick can they
use to change file owner?
 Changing permissions on a top-level folder in the NTFS file system is a very
slow operation. You can try by creating new user and giving it read access
to C:\. This is actually a performance optimization. Explain why.
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Exercises: Unix
 Create a subdirectory in your home directory and put a file abc.txt
in this subdirectory. Set permission bits on the subdirectory so that
the owner has only execute access. Try to
–
–
–
–
list the subdirectory
display the contents of abc.txt
create a copy of abc.txt in the subdirectory
make the subdirectory the current directory with cd
 Repeat the same experiment first with read permission and then
with write permission on the subdirectory. Try to understand what
you observe.
 Find out how permissions are used to protect a files on a web
server, a shared temp directory, print queue directory, or shared
directory for a project group.
 Write and configure a SUID program in C that allows other users to
write log messages to a file which they otherwise cannot access.
What if there is a buffer overflow vulnerability or other bugs in your
code?
 Devices in Unix are mapped to special files under /dev. How would
you protect a terminal (tty) device from other users?
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