#+title: Advanced Access Control
#+date: 2026-04-13 Mon
#+author: W. Kosior
#+email: wkosior@agh.edu.pl

* BSD securelevel
- FreeBSD, OpenBSD, macOS
- normally booted system (securelevel 1):
  - disallows writes to =/dev/mem=
  - raw writes to mounted filesystems forbidden
- securelevel 2
  - raw writes to filesystems forbidden
  - backwards clock changes forbidden
  - firewall rule changes forbidden
- assumption of compromised root account

* Apple System Integrity Protection ("rootless")
- not really "rootless"
- root powers restricted for usual root processes
- no write access to
  - =/System=
  - =/sbin=
  - =/bin=
  - =/usr=
  - =/Applications=

* DAC → MAC
- Discretionary Access Control
  - based on users and their permissions
  - identified with UNIX traditional model
- Mandatory Access Control
  - user's access limited to work on specific task
  - identified with later extensions to UNIX-like OSes
    - Linux kernel: post-2000
- DAC / MAC — naming from US government framework (70s)
  - meaningful wrt OS access controls?

* LSMs
- Linux Security Modules
  - not really "modules"
    - compiled-in extensions
  - supplement normal UNIX controls
  - major & minor LSMs
    - possibly multiple LSMs active simultaneously
    - at most one major LSM at a time

* LSMs, Cont.
- Linux Security Modules
  - not really "modules"
    - compiled-in extensions
  - supplement normal UNIX controls
  - major & minor LSMs (+ capabilities)
    - possibly multiple LSMs active simultaneously
    - at most one major LSM at a time
- =security=LIST_OF_MODULES= kernel arg
  - or compiled-in default list

* LSMs, Cont…
- Linux Security Modules
  - not really "modules"
    - compiled-in extensions
  - supplement normal UNIX controls
  - major & minor LSMs (+ capabilities)
    - possibly multiple LSMs active simultaneously
    - at most one major LSM at a time
- =security=LIST_OF_MODULES= kernel arg
  - or compile-in default list
- =/sys/kernel/security/=
- =/sys/kernel/security/lsm=
- =/proc/$PID/attr/$MODULE_NAME=

* Selected LSMs
- /Capabilities (formely not an LSM)/
- /Lockdown/
- Integrity Policy Enforcement
- Landlock
- LoadPin
- SafeSetID
- TOMOYO
- Yama
- AppArmor
- SMACK
- SELinux

* Selected LSMs, Cont.
- /Capabilities (formely not an LSM)/
- /Lockdown/
- Integrity Policy Enforcement (access control based on file origin)
- Landlock (sandboxing, inode-based)
- LoadPin (modules from RO devices only)
- SafeSetID (restricted =setuid()= as non-root)
- Yama (ptrace restrictions)
- AppArmor
- SMACK
- SELinux
- TOMOYO

* Selected LSMs, Cont…
- /Capabilities (formely not an LSM)/
- /Lockdown/
- Integrity Policy Enforcement (access control based on file origin)
- Landlock (sandboxing, inode-based)
- LoadPin (modules from RO devices only)
- SafeSetID (restricted =setuid()= as non-root)
- Yama (ptrace restrictions)
- *SMACK* (legacy major)
- *AppArmor* (legacy major)
- *SELinux* (legacy major)
- TOMOYO (former legacy major)

* Legacy Major LSMs
- *SMACK*
  - used in Tizen OS (Samsung TVs)
  - low complexity, policies based on file labels (xattrs)

* Legacy Major LSMs, Cont.
- *SMACK*
  - used in Tizen OS (Samsung TVs)
  - low complexity, policies based on file labels (xattrs)
- *AppArmor*
  - *presently* developed by Canonical
  - on by default in Ubuntu
  - moderately complex, policies based on paths (xattrs)

* Legacy Major LSMs, Cont…
- *SMACK*
  - used in Tizen OS (Samsung TVs)
  - low complexity, policies based on file labels (xattrs)
- *AppArmor*
  - *presently* developed by Canonical
  - on by default in Ubuntu
  - moderately complex, policies based on paths (xattrs)
- *SELinux*
  - *initiated* by NSA
  - on by default in Fedora & Red Hat
  - high complexity, policies based on file labels (xattrs)

* SMACK
- objects
  - processes
  - files
  - IP packet sources/targets
- subjects
  - processes

* SMACK, Cont.
- objects
  - processes
  - files
  - IP packet sources/targets
- subjects
  - processes
- object / subject labels
- operations (actions of subjects on objects)
  - read
  - write
  - execute
  - append (on files)

* SMACK, Cont…
- objects
  - processes
  - files
  - IP packet sources/targets
- subjects
  - processes
- object / subject labels
- operations (actions of subjects on objects)
  - read
  - write
  - execute
  - append (on files)
- access rules (additional restrictions on top of DAC)
  - =label_X label_Y rwx=
  - =/sys/fs/smackfs/=

* SMACK Labels
- e.g., =Puppy3=, =my:label=, =a,b,c=
  - ASCII, except =/=, =\=, ='=​, ="=, and unprintables
  - no assumed structure
  - length up to 255 (or 23)
- processes
  - =CAP_MAC_ADMIN= process can change to any label
  - unprivileged process is restricted in label changes

* SMACK Labels, Cont.
- e.g., =Puppy3=, =my:label=, =a,b,c=
  - ASCII, except =/=, =\=, ='=​, ="=, and unprintables
  - no assumed structure
  - length up to 255 (or 23)
- processes
  - =CAP_MAC_ADMIN= process can change to any label
  - unprivileged process restricted in label changes
- files / directories
  - =security.SMACK64= xattr — resource's label
  - =security.SMACK64EXEC= xattr — process' label after =exec()=
    - similar to SET-UID binaries; files only
  - =security.SMACK64TRANSMUTE= — new file's label
    - similar to SET-GID directories; directories only

* SMACK Labels, Cont…
- e.g., =Puppy3=, =my:label=, =a,b,c=
  - ASCII, except =/=, =\=, ='=​, ="=, and unprintables
  - no assumed structure
  - length up to 255 (or 23)
- processes
  - =CAP_MAC_ADMIN= process can change to any label
  - unprivileged process restricted in label changes
- files / directories
  - =security.SMACK64= xattr — resource's label
    - normally set to file creator process' label
  - =security.SMACK64EXEC= xattr — process' label after =exec()=
    - similar to SET-UID binaries; files only
  - =security.SMACK64TRANSMUTE= — new child file's label
    - similar to SET-GID directories; dirs only
- network packets can use label from IP CIPSO option
  - configured through =/sys/fs/smackfs/netlabel=

* SMACK Access Control Rules
- =subjectlabel objectlabel permissions=, e.g.,
  - =tiger musli rx=
    - proces labeled =tiger= can read & execute objects labeled =musli=
  - =:) tulip,bulb rawt-=
    - =:)= can read & append & write to =tulip,bulb=
    - =SMACK64TRANSMUTE= of =tulip,bulb= dirs takes effect (=t=)
    - minus ("=-=") — placeholder (omittable)
  - =Americano Bands -=
    - =Americano= has no access to =Bands=

* SMACK Access Control Rules, Cont.
- =subjectlabel objectlabel permissions=, e.g.,
  - =tiger musli rx=
    - proces labeled =tiger= can read & execute objects labeled =musli=
  - =:) tulip,bulb raw-=
    - =:)= can read & append & write to =tulip,bulb=
    - minus ("=-=") — placeholder (omittable)
  - =Americano Bands -=
    - =Americano= has no access to =Bands=
- =/sys/fs/smackfs/load2= — configure rules
  - one rule per subject-object labels pair
  - =echo 's_lbl o_lbl rwx' > /sys/fs/smackfs/load2= overrides old
  - persistence typically in =/etc/smack/accesses=
- =/sys/fs/smackfs/access2= — check for access
- =/sys/fs/smackfs/load-self2= — extra restrictions, per-process

* SMACK Special Labels
- =*= — can be accessed by all, cannot access any
- =^= — can read / execute everything
- =_= — can be read / executed by everyone
- mount options → default label for unlabeled files

* SMACK Access Logic
- allow if subject has =CAP_MAC_OVERRIDE=
- deny if subject is labeled =*=, otherwise
- allow if this is a read / execution by =^=, otherwise
- allow if this is a read / execution on =_=, otherwise
- allow it object is labeled =*=, otherwise
- allow if =subject_label = object_label=
- deny if per-process rules configured and lack the permission
- allow if currently loaded (global) rules allow
- deny by default

* SMACK Access Types
- process -> file
- process -> process (signal, ptrace)
  - extra configuration regarding ptrace available
- process or remote subject -> process or remote subject
  - packet sending = writing to target process
  - can be configured differently
    - socket attributes

* SMACK Bringup Mode
- =CONFIG_SECURITY_SMACK_BRINGUP= kernel config option
- permit & log certain accesses normally blocked
  - rule with =b= "permission"
  - processes with label written to =/sys/fs/smackfs/unconfined=
- rule set development
  - run new application with =b= rules or unconfined label
  - see what accesses are logged
  - prepare the rules adequately
  - make rules effective

* SMACK Process Relabeling
- =/proc/self/attr/{,smack/}current= — current process label
  - readable
  - writeable with =CAP_MAC_ADMIN=
  - writeable *without* =CAP_MAC_ADMIN= *if*:
    - target label written to =relabel-self=
- =/sys/fs/smackfs/relabel-self= — available target labels list
  - writeable with =CAP_MAC_ADMIN=

* SMACK — Summary
- simple
  - can be configured from ground up by the administrator
- uses files' extended attributes
  - can handle filesystems without these
- can be incrementally configured
- used in embedded systems
- suitable for securing containers
- not included in many distro's kernel binaries

* AppArmor
- objects use labels *internally* (not visible to the administrator)
- rules in profiles, written in =/etc/apparmor.d=
  - usually per-application
- userspace utilities used to (re)load profiles
- accesses allowed or blocked by path

* AppArmor Profile Syntax
#+begin_example
# This is a comment.
# File should be named `/etc/apparmor.d/usr.local.bin.foo'.
# The following is not a comment.
#include <tunables/global>
/usr/local/bin/foo flags=(complain) {
  # `complain' above inhibits blocking of access.  Would-be
  # violations are just logged.  "Learning mode".

  #include <abstractions/base>

  capability mknod, # Note the necessary comma.
  mount /dev/sd*,
  /bin/local/bin/foo mixr,
  /var/lib/foo/*     rw,
  /mnt/foobar/*      rw,
  link /var/lib/foo -> /mnt/foobar/,
}
#+end_example

* AppArmor Permission Types
- =k= — lock file
- =l= — create hard links
- =m= — memory map as executable
- =r= — read
- =w= — write
- =x= — execute
  - qualifiers: =p=, =P=, =c=, =C=, =u=, =U=, =i=
    - uppercase letter = environment variable scrubbing
  - fallbacks from =[pPcC]= to =[iuU]= possible

* AppArmor Profile Changes
- subprofiles
- hats
  - Apache =mod_apparmor=, dedicated profiles for
    - virtualhosts
    - URLs
    - paths, CGI applications

* AppArmor — Summary
- does not rely on extended attributes
  - can optionally use them
- relatively complex
  - configuration shipped with distro
  - easy to extend / supplement
- according to some, suitable for web servers
- widely available (Debian, Ubuntu, (Open)Suse)

* SELinux
- subects and objects have contexts
  #+begin_example
  foobar_u:foobar_r:foobar_t:=
  ^^^^     ^^^^     ^^^^
  user     role     type
  #+end_example
- subjects & objects can be
  - processes
  - files & directories
  - ports
- context stored in xattrs
  - relabeling needed when configuring SELinux