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linux_SElinux.txt
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[[{security.selinux,security.aaa,kernel.100]]
# SElinux Summary
- Ext.Refs
- <https://github.com/SELinuxProject/selinux/wiki>
- <https://github.com/SELinuxProject/setools/wiki>
- <https://github.com/SELinuxProject/cil/wiki>
- <https://github.com/SELinuxProject/refpolicy>
- (Book) "SElinux by example"
- (Book) <https://www.freetechbooks.com/the-selinux-notebook-the-foundations-t785.html>
- [Stop disabling SELinux](https://www.youtube.com/watch?v=cNoVgDqqJmM&feature=youtu.be)
```
| SE-LINUX GLOBAL SUMMARY ──────────────────────────────────────
|
| KERNEL
| OBJECTS CLASSES
| ↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑
|Represent type─of─resources
|handled by kernel(vs app) Each type─of─resource has a set of
|that must be protected HARDCODED list of actions defined
|by Mandatory access Rules for each se-linux kernel object class
| ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓ ↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓
| File class ←→ [append, create, lock, ... ]
| Process class ←→ [dyntransition, ptrace, fork, ... ]
| INET Socket class ←→ [bind, read, ... ... ]
| login class ←→ [action1, action2, ... ... ]
| user class ←→ [action1, action2, ... ... ]
| ...
|
|─TheG Process objects differ from the rest in the sense that
|they are the "active" kernel object triggering a new system call,
|probably as a result of a user intention to access some other
|resource (kernel object instance in SELinux parlance)
|
|─ File alike objects (Files, directories,...) must be first
| "manually" labelled through extended file─system attributes.
| Other object labelling is done by kernel automatically
|
| KERNEL─OBJECT─INSTANCE CONTEXT
|
| ┌── SELinux context ────┐
| user:role:" type ":level
|
| - All Kernel Object instances are labeled with
| a 4-tuple SELINUX-CONTEXT (user:role:type:level)
|
| The type is the core data used for MACO TYPE─ENFORCEMENT
```
```
┌─────────────────────┬────────────────────────────────────────┐
│ DAC+MAC FLOW SUMMARY│ │
├─────────────────────┘ │
│ USER ┌─────────→ ┌─────────┐ │
│ SPACE │ ┌───────→ │ PROCESS │ ←─────────────────┐ │
│ │ │ ┌─────→ │"SUBJECT"│ │ │
│ │ │ │ │ type1 │ │ │
│ │ │ │ └─────────┘ │ │
│ │ │ │ 1) ↓ │ │
│ ───────────── │ │ │ ───── system─call ───────────────── │ ───│
│ KERNEL │ │ │ │☝request(hardcoded) │ │
│ SPACE │ │ │ │ action1 over targeted │ │
│ │ │ │ │object (labeled with │ │
│ │ │ │ │SEContext type2 ) │ │
│ │ │ │ 2) ↓ │ │
│ │ │ │ Lookup │ │
│ │ │ │ data 8) │Sys.│
│ │ │ │ │ ┌────────┐ │call│
│ │ │ │ KO 3) ↓ │target │ │Res.│
│ │ │ └───────── Error ┌────│object ├──┘ │
│ │ │ Checks │ │ type2 │ │
│ │ │ │ │ └────────┘ │
│ │ │ │ OK │ 7) │
│ │ │ 4) ↓ │ OK :exec gaction1 │
│ │ │ KO DAC │on target object │
│ │ └───────── permission │ │
│ │ checks │ │
│ │ KO │ │ │
│ │ (audit) 7) ↓ │ │
│ └───────────── LSM ────┘ │
│ hooks │
│ 5) │ ^ 6) │
│ type1 is allowed│ │ OK ☜ MAC │
│ action1 over│ │ KO TYPE │
│ type2 ?↓ │ ENFORCEMENT │
│ ┌───────────┐ │
│ │ Security │ │
│ │Enhance(SE)│ │
│ │ Server │ │
│ └───────────┘ │
│ ☝ │
│ The server will first query the AVC(ache) and return │
│ OK/KO if a match is found. │
│ If nothing is found the server will: │
│ 1) work over the AV─hash table for matching rules │
│ 2) calculate the result, │
│ 3) put the result back in AVC(ache) │
│ 4) Return with OK/KO │
└──────────────────────────────────────────────────────────────┘
```
```
┌────────────────────────────┬───────────────────────────────┐
│ INITIAL PRE─SETUP SUMMARY: │ │
├────────────────────────────┘ │
│ Define SE-Linux System policy with a flow similar to: │
│ │
│ create modules → load into → Init bitmap─"matrix" │
│ kernel context1─to─[context2,action] │
│ ↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑ │
│ Access Vector (AV) │
└────────────────────────────────────────────────────────────┘
```
```
┌──────────────────────────┬───────┐
│ SELinux bootup sequence │ │
├──────────────────────────┘ │
│ kernel start → │
│ load policy into memory │
│ (organized in modules) │
│ │
│ semodule command manages the │
│install, remove, reload, upgrade, │
│enable or disable fo modules │
│Ex. lists modules currently loaded│
│~ sudo semodule ─l │
│→abrt 1.2.0 │
│→accountsd 1.0.6 │
│... │
└──────────────────────────────────┘
```
```
┌─────────────────────────────────┐
│QUERY SOURCE TO TARGET RELATIONS:│
├─────────────────────────────────┴─────────────────────────────────────────────────────┐
│#~ sesearch --allow \ ← Show allowed │
│ --source httpd_t │
│ --target httpd_sys_content_t │
│ --class file ← kernel selinux object class │
│ → Found 4 semantic av rules: │
│ → allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ;│
│ → allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ;│
│ → allow httpd_t httpd_content_type : file { ioctl read getattr lock open } ;│
│ → allow httpd_t httpdcontent : file { ioctl read write create .... } ;│
└─────────────────────────────────┴─────────────────────────────────────────────────────┘
```
```
│ DISPLAYING SELINUX CONTEXT ATTRIBUTES ────────
|
| - Use "-Z" flag to shell commands
|
| - FILE SYSTEM OBJECT (files, dirs,..) ───────
| $ ls -ldZ
| /usr/sbin/httpd → httpd_exec_t
| /var/www/html/ → httpd_sys_content_t
| /etc/apache2/ → httpd_config_t
| /var/log/httpd/ → httpd_log_t
| /etc/init.d/httpd → httpd_initrc_exec_t
| - SUBJECTS (running processes) ──────────────
| $ ps axZ │ grep [h]ttpd
| unconfined_u:system_r: httpd_ t:s0 ...
| - SOCKET ────────────────────────────────────
| $ sudo netstat ─tnlpZ │ grep httpd
| ... unconfined_u:system_r: httpd_ t:s0 ..
| - PORT ──────────────────────────────────────
| $ sudo semanage port ─l │ grep http
| http_cache_port_t tcp 3128,8080, ...
| ...
| http_port_t tcp 80, 443
```
```
| sestatus(8) ─────────────────────────────────────
| Return various status info: ex:
| $ sudo sestatus
| → SELinux (enforcing) status: enabled
| → SELinuxfs mount : /sys/fs/selinux
| → SELinux root directory : /etc/selinux
| → Loaded policy name : targeted
| → Current mode : permissive
| → Mode from config file : error (Success)
| → Policy MLS status : enabled
| → Policy deny_unknown status: allowed
| → Max kernel policy version : 28
```
```
| show AVC stats ───────────────────────────────
| $ sudo avcstat
| lookups hits misses allocs reclaims frees
| 6688846 5637360 1051486 051486 968960 1050979
```
```
| SHELL SCRIPTS SUPORT ───────────────────────
|
| getenforce(8) Returns status like:
| "permissive" │ "enforcing"
|
| selinuxenabled exits with
| 0 exit if enabled
| 1 exit if not
```
```
| ┌─────────────────┬────────┬────────────────────────────────────────────────────────────────────────┐
| │ TERMINOLOGY │ACRONYM │ DESCRIPTION │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Access Vector │AV │ - bit map representing a set of permissions such as open, read, ... │
| │ │ │ - Each policy defines a different AV. │
| │ │ │ - Actually is implemented as a hash table where the key is the tuple │
| │ │ │ (source-type, targeted-type, targeted-kernel-object-class) │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Access Vector │AVC │ ─ SELinux Security Server can take a time to calculate │
| │ Cache │ │ access decissions based on SE─rules. │
| │ │ │ ─ The AVC stores such decissions to speed up following │
| │ │ │ access avoiding to recompute. │
| │ │ │ ─ two AVCs exists: │
| │ │ │ ─ 1.kernel AVC caching decisions from Security Server │
| │ │ │ on behalf of kernel based object managers. │
| │ │ │ ─ 2.userspace AVC built into libselinux that caches │
| │ │ │ decisions when SELinux─aware applications use │
| │ │ │ avc_open(3) with avc_has_perm (3) or avc_has_perm_noaudit(3) │
| │ │ │ function calls saving kernel calls after first │
| │ │ │ decision has been made. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Bell─La Padula │BLP │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Common Criteria│CC │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Common │CIL │ │
| │ Intermediate │ │ │
| │ Language │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Discretionary │DAC │ │
| │ Access │ │ │
| │ Control │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ SELinux │ │ ─ consists of one or more processes associated │
| │ Domain │ │ to the type component of a Security Context. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Flux Advanced │FLASK │ ─ See Flux Research Group (http://www.cs.utah.edu/flux/) │
| │ Security Kernel │ │ μ─kernel Environment (Fluke) │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Linux Security │LSM │ ─ framework providing hooks into kernel components │
| │ Module │ │ (e.g. disk, net─services,...) used by │
| │ │ │ security modules (SELinux, ....) to perform │
| │ │ │ access control checks. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Mandatory Access│MAC │ ─ access control mechanism enforced by the system, │
| │ Control │ │ e.g. 'hard─wiring' the OS and applications or │
| │ │ │ via policies enforced by the administrator. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Multi─Category │MCS │ │
| │ Security │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Multi─Level │MLS │ ─ Based on Bell─La Padula model for │
| │ Security │ │ confidentiality in that (for example) a │
| │ │ │ process running at a 'Confidential' level │
| │ │ │ can read / write at their current level but │
| │ │ │ only read down levels or write up levels. │
| │ │ │ ─ "Today" it is more commonly used for │
| │ │ │ application separation utilising the │
| │ │ │ Multi─Category Security variant. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ SELinux Policy │ │ - Set of (thousands of) rules that define the type-enforcement rules │
| │ │ │ in the AV bitmap matrix/hashtable │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Object Manager │OM │ ─ Userspace&kernel components responsible │
| │ │ │ management (object labeling, creation, access, destruction) │
| │ │ │ of SELinux object under their control. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Security │SID │ │
| │ Identifier │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Simplified │SMACK │ │
| │ Mandatory │ │ │
| │ Access Control │ │ │
| │ Kernel │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Super─user │SUID │ │
| │ Identifier │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Type Enforcement│TE │ ─ set of rules declared in Policy describe │
| │ │ │ how the domain will interact with objects │
| │ │ │ ─ In practice: the AV bit─map used to check │
| │ │ │ where type1 is allowed actionN over type2 │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ User Identifier│UID │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ X(window) Access│XACE │ │
| │ Control │ │ │
| │ Extension │ │ │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Security Server │ │ A sub─system in the Linux kernel that makes access decisions │
| │ │ │ and computes security contexts based on Policy on behalf of │
| │ │ │ SELinux─aware applications and Object Managers. │
| │ │ │ The Security Server does not enforce a decision, it merely │
| │ │ │ states whether the operation is allowed or not according to the │
| │ │ │ Policy. It is the SELinux─aware application or Object │
| │ │ │ Manager responsibility to enforce the decision. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Security Context│ │ An SELinux Security Context is a variable length string that │
| │ │ │ consists of the following mandatory components │
| │ │ │ user:role:type and an optional [:range] component. │
| │ │ │ Generally abbreviated to 'context', and sometimes called a │
| │ │ │ 'label'. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Security │SID │ SIDs are unique opaque integer values mapped by the kernel │
| │ Identifier │ │ Security Server and userspace AVC that represent a Security Context. │
| │ │ │ The SIDs generated by the kernel Security Server are u32 │
| │ │ │ values that are passed via the Linux Security Module │
| │ │ │ hooks to/from the kernel Object Managers. │
| │─────────────────┼────────┼────────────────────────────────────────────────────────────────────────│
| │ Type Enforcement│ │ SELinux makes use of a specific style of type enforcement │
| │ │ │ (TE) to enforce Mandatory Access Control. This is where all │
| │ │ │ subjects and objects have a type identifier associated to them │
| │ │ │ that can then be used to enforce rules laid down by │
| │ │ │ Policy │
| └─────────────────┴────────┴────────────────────────────────────────────────────────────────────────┘
| ```
# SELinux Kernel Object Classes:
Reminder:
- For nearly every linux (kernel) object that must be protected there is a matching SELInux kernel class.
- This class has a defined set of hardcoded actions that will be allowed/denied by a running policy.
- Type enforcement will determine which "source"-context has which list of allowed-actions over
a given class labeled with a "target"-context
- Next follows a summary of classes and actions extracted from
<https://github.com/SELinuxProject/refpolicy/blob/master/policy/fl ask/access_vectors>
```
┌ FILE-ALIKE RELATED OBJECTS ─────────────────────────────────────────────────────┐
│ common file │class dir │class file │class lnk_file │class chr_file │
│ │inherits file │inherits file │inherits file │inherits file │
│ │ │ │ │ │
│ ioctl │add_name │execute_no_trans │open │execute_no_trans │
│ read │remove_name │entrypoint │audit_access │entrypoint │
│ write │reparent │execmod │execmod │execmod │
│ create │search │open │ │open │
│ getattr │rmdir │audit_access │audit_access │
│ setattr │open │ │ │
│ lock │audit_access │
│ relabelfrom │execmod │
│ relabelto │ │
│ append │
│ map │class sock_file │class fifo_file │class fd │class blk_file │
│ unlink │inherits file │inherits file │ │inherits file │
│ link │ │ │use │ │
│ rename │open │open │ │open │
│ execute │audit_access │audit_access │audit_access │
│ swapon │execmod │execmod │execmod │
│ quotaon │ │ │ │
│ mounton │
└─────────────────────────────────────────────────────────────────────────────────┘
┌ NETWORK RELATED OBJECTS ─────────────────────────────────────────────┐
│ │
│ COMMON SOCKET │CLASS SOCKET │class rawip_socket │
│ │(inherits socket) │inherits socket │
│ # file inheri. │class netlink_socket │ │
│ │inherits socket │node_bind │
│ ioctl │class packet_socket │
│ read │inherits socket │class unix_stream_socket │
│ write │class key_socket │inherits socket │
│ create │inherits socket │ │
│ getattr │class unix_dgram_socket │connectto │
│ setattr │inherits socket │newconn │
│ lock │acceptfrom │
│ relabelfrom │class tcp_socket │class node │class netif │
│ relabelto │inherits socket │ │ │
│ append │ │tcp_recv │tcp_recv │
│ map │connectto │tcp_send │tcp_send │
│ │newconn │udp_recv │udp_recv │
│ # socket │acceptfrom │udp_send │udp_send │
│ bind │node_bind │rawip_recv │rawip_recv │
│ connect │name_connect │rawip_send │rawip_send │
│ listen │enforce_dest │dccp_recv │
│ accept │class udp_socket │dccp_recv │dccp_send │
│ getopt │inherits socket │dccp_send │ingress │
│ setopt │ │recvfrom │egress │
│ shutdown │node_bind │sendto │
│ recvfrom │ │
│ sendto │
│ recv_msg │
│ send_msg │
│ name_bind │
└──────────────────────────────────────────────────────────────────────┘
PROCESS RELATED CAPABILITY RELATED SYTEM OPERATIONS COMMON IPC
│ │ │ class system │
│ class process │ common cap │ ipc_info │create
│ │ │ syslog_read │destroy
│ fork │ chown │ syslog_mod │getattr
│ transition │ dac_override │ syslog_console │setattr
│ sigchld │ dac_read_search │ module_request │read
│ sigkill │ fowner │ module_load │write
│ sigstop │ fsetid │ halt │associate
│ signull │ kill │ reboot │unix_read
│ signal │ setgid │ status │unix_write
│ ptrace │ setuid │ start
│ getsched │ setpcap │ stop │COMMON X
│ setsched │ linux_immutable │ enable │ X_DEVICE
│ getsession │ net_bind_service │ disable │pointer,keyb.
│ getpgid │ net_broadcast │ reload │getattr
│ setpgid │ net_admin │setattr
│ getcap │ net_raw CLASS FS │use
│ setcap │ ipc_lock │ mount │read
│ share │ ipc_owner │ remount │write
│ getattr │ sys_module │ unmount │getfocus
│ setexec │ sys_rawio │ getattr │setfocus
│ setfscreate │ sys_chroot │ relabelfrom │bell
│ noatsecure │ sys_ptrace │ relabelto │force_cursor
│ siginh │ sys_pacct │ transition │freeze
│ setrlimit │ sys_admin │ associate │grab
│ rlimitinh │ sys_boot │ quotamod │manage
│ dyntransition │ sys_nice │ quotaget │list_property
│ setcurrent │ sys_resource │get_property
│ execmem │ sys_time │COMMON DDBB │set_property
│ execstack │ sys_tty_config │ │add
│ execheap │ mknod │create │remove
│ setkeycreate │ lease │drop │create
│ setsockcreate │ audit_write │getattr │destroy
│ getrlimit │ audit_control │setattr
│ setfcap │relabelfrom
│relabelto
┌ OTHERS ─────────────────────────────────────────────────────────────┐
│ security, capability, X─Windows, Netlink, D─BUS, nscd, IPSEC, │
│ dccp,memprotect db_database/db_table/db_column/..., (network) peer, │
│ tun_socket, binder infiniband , ... │
└─────────────────────────────────────────────────────────────────────┘
```
# SELINUX FS-LABELING
- When a new file-alike object is created its contexts
is copied (by default) from its parent's directory
by default.
- This behaviour can be modified with a type_transition rule
in the policy.
```
| $ sudo chcon --type var_t index.html ← Changes context *temporarely*
| └───┘ └───┘ (FS relabel will revert changes)
|
| $ sudo restorecon -v index.html
| $ sudo restorecon reset index.html ← similar to fixfiles(8) suited
| unconfined_u:object_r: var_t:s0 -> for individual file or dir. relabeling
| unconfined_u:object_r: httpd_sys_content_t*:s0
```
## OTHERS:
* `fixfiles(8)`: Relabels FS objects. By default relabel all mounted
FSs that support SELinux unless mounted with the context mount
option, automatically detemining the file sec.ctx specs to use for
the labeling.
* `genhomedircon(8)`: Script for generating correct file ctx specs
for user's home directories.
See also: Troubleshooting with restorecon
* NOTE: SELinux context for remote FS can be specified *at mount time*.
*Verify a file context against file_context(.local) ddbb (active policy) *
```
| # matchpathcon -V /www/html/index.html
| ^^
|
| In case of mismatch an error similar to next one will be displayed:
| /www/html/index.html has context unconfined_u:object_r:default_t:s0,
| should be system_u:object_r:httpd_sys_content_t:s0
```
# SELINUX login objects:
- SELinux users are not created with a command,
nor does they have its own login access to the server.
- SELinux users are*defined in the policy*loaded into
memory at boot time, and*there are only a few of these users*
- Standard Linux Users are mapped to SElinux users upon login
according to selinux-login-objects defined by policy and
customizable through semanage login.
```
| │Linux│N<·¹·>1│SELinux│1<·²·>N│roles│·³>│process │
| │User │ │user │ │ │ │domain (*_t)│
|
| ¹ To show N─to─1 relationship:
| # semanage login ─
| (Example CentOS 7 output)
| Login Name SELinux User MLS/MCS Range Service
|
| __default__ unconfined_u s0─s0:c0.c1023 *
| root unconfined_u s0─s0:c0.c1023 *
| system_u system_u s0─s0:c0.c1023 *
| └────┬────┘ └────┬─────┘ └────┬────┘
| linux 1 <········>1 SE user Multi Level/
| user /Category Sec.
|
| ² A user "enters a roll" when policy grants it.
| To show the 1-to-N relationship:
| # semanage user
| (Example CentOS 7 output)
| Labeling ...
| SELinux User Prefix ... SELinux Roles
|
| guest_u user ... guest_r
| root user ... staff_r sysadm_r system_r unconfined_r
| staff_u user ... staff_r sysadm_r system_r unconfined_r
| sysadm_u user ... sysadm_r
| system_u user ... system_r unconfined_r
| unconfined_u user ... system_r unconfined_r
| user_u user ... user_r
| xguest_u user ... xguest_r
|
| ³ A role "access a process domain" when policy grants it.
```
```
| semanage login [opts]
|
| map linux user to selinux user upon login by
| Adding/modifying/deleting(and listing) login object types
|
| semanage login [opts]
| --add -s "SEUSER" "linuxUser" [─r "MLC/MCS range"]
| --modify -s "SEUSER" [─r "MLC/MCS range"]
| --delete "linuxUser"
| --deleteall # (all = local/non─policy─defined)
| --extract # ← (for use within TX)
| --list [--locallist]
| show local
| (non─defined─by─policy)
| customizations
| OTHER OPTIONS:
| --noreload ← Do NOT reload policy after commit
| --store STORE ← Select alternate SELinux Policy Store
```
* Example: Restricting su/sudo
```
|[regularuser@localhost ~]$ id ─
|unconfined_u:unconfined_r:unconfined_t:s0
|
| regularuser@localhost ~]$ su ─ switcheduser
| Password: XXXX
| → [switcheduser@localhost ~]$
|
| #~ semanage login ─a ─s user_u regularuser
| └────────────────┬───────────┘
| Remap linux_user to selinux user
|
| [regularuser@localhost ~]$ id ─
| user_u:user_r:user_t:s0
|
| [regularuser@localhost ~]$ su ─ switcheduser
| Password: XXXX
| → su: Authentication failure
```
* Example: disable script execution
```
| By default, SELinux allows users mapped to the
| guest_t account to exec $HOME/ scripts
|
| #~ getsebool allow_guest_exec_content
| guest_exec_content ─ → hon
|
| [guestuser@localhost ~]$ ~/myscript.sh
| This is a test script
|
| #~ setsebool allow_guest_exec_content off
| [guestuser@localhost ~]$ ~/myscript.sh
| bash: myscript.sh: Permission denied
```
## SELINUX PROCESS CONTEXT-SWITCH
- Kernel takes care of automatic process-context-labeling once
a policy has been loaded.
- Next commands can be used to alter or display the process labeling:
- runcon(1): run command with given ctx (user, role and domain)
- secon(1): See ctx from a file|program|user-input
- newrole(1): creates new shell running with new sec.ctx.
Use must specify new role and/or type.
type is derived from role if not specified.
- run_init(9): Runs initrc script using the sec.ctx.
found in current policy's ctx/initrc_context
file. Ussually used to restart system
services in new intended domain
## PROCESS LABELING FLOW AT LOGIN
- Login process sets a default context (unconfined for targeted policy)
Policy defined Context transitions will change children process context
at runtime.
## PROCESS CONTEXT TRANSITION
- A default context is given upon login defined by the
policy login-objects (see `semanage login` notes )
- Context Transtion Flow
```
| app_1 → Execute /usr/bin/app_2 → app_2
|
| domain/type src_t app_exec_t target_t
| ↑ ↑
| Requirements 1)- policy must define 2)- policy must define
| exec permission to app_exec_t as entrypoint
| app_exec_t entry-point to target_t
| 3)- policy must allow
| transition from src_t
| to target_t
```
```
| Check 1): | Check 2): | Check 3):
| # sesearch | # sesearch | # sesearch
| -s src_t | -s target_t | -s src_t
| -t app_exec_t | -t target_t | -t target_t
| -c file | -c file | -c process ¹
| -p execute | -p entrypoint | -p transition ²
| -Ad | -Ad | -Ad
¹ (selinux-kernel)class
² action requite for class(See selinux object classes)
```
# SELINUX Booleans
* <https://linux.die.net/man/8/booleans>
- Allow to customize a given policy at runtime.
- Common to-many-apps actions that can be allowed/denied are "grouped"
into booleans
Ex:
- """ Do we allow the ftp server access to home directories? """
- """ Can httpd use mod_auth_ntlm_winbind ? """
- ...
```
| List Booleans Set: ---------------------------------------
| Take a look at files:
| $ cat /etc/selinux/targeted/modules/active/booleans.local
| allow_console_login login_console_enabled
| allow_execheap selinuxuser_execheap
| allow_execmod selinuxuser_execmod
| allow_execstack selinuxuser_execstack
| allow_mount_anyfile mount_anyfile
| allow_ssh_keysign ssh_keysign
| allow_staff_exec_content staff_exec_content
| user_ping selinuxuser_ping
| virt_sandbox_use_nfs virt_use_nfs
| ....
|
| List Modified Booleans Set: ------------------------------
| $ cat /etc/selinux/targeted/booleans.subs_dist
| # This file is auto─generated by libsemanage. Do not edit directly.
| httpd_read_user_content=1
| httpd_enable_homedirs=1
| Change Booleans ------------------------------------------
| # getsebool - # <· Show all booleans
|
| # setsebool \ # <· Set boolean
| ─P "mySELinBool" 0│1 -P: (optional) Persist reboot
|
| # togglesebool \ # <· toggle 1<>0 value
| ─P "mySELinBool"
```
- Example. Fix Apache DNS problem:
(REF: <https://stackoverflow.com/questions/23851452/cant-resolve-domain-names-in-php-under-apache/24019910#24019910>)
```
$ sudo setsebool -P nis_enabled 0
$ sudo setsebool -P httpd_can_network_connect 1
```
## SELINUX TROUBLESHOOTING
```
### Q: What an SELinux error means? <!-- { -->
It can means:
1. file-system labeling is wrong
```
| Fix examples:
| Ex 1:
| # sudo chcon --reference /var/www/html ..../index.html
| Ex 2:
| # restorecon -vR /var/www/html
| ^^^^^^^^^^
| uses info from
| /etc/selinux/targeted/contexts/files/file_contexts, ...
| to determine what a file or dir's ctx should be
| Ex 3:
| # semanage fcontext -a -e /var/www/ /my/alternative/www/
| ^^^^^^^^
| WARN: we are just defining what the context is,
| we are not writing what the
| extended attributes are. It just means:
| "On relable it must look like this"
| # restorecon -vR /my/alternative/www
| ^^^^^^^^^^
| must be run to actually proceed with relabeling
```
2. Policy needs to be tweaked
```
- booleans
- policy modules
```
3. A bug in the policy
```
| App vendors must supply policy modules for SELinux systems.
| You must submit a ticket to the app vendor following steps:
|
| STEP 1 ------------------------------------------------------
| # setenforce 0 <· change to "permissive" mode and run
| the application through all its paces
| to log all SELinux denials
| STEP 2 ------------------------------------------------------
| From /var/log/messages Copy and Paste the proposed solution:
| # grep httpd /var/log/audit/audit.log │ \
| audit2allow -MO quirrellocal
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| generate new policy file
| (See also audit2why(1)
| STEP 3 ------------------------------------------------------
| # semodule - squirrellocal.pp ← Import the new module
| STEP 4 ------------------------------------------------------
| # setenforce 1 ← Re─enable enforcement
```
4. You have been, or are being, broken into!!
```
| - HOUSTON: We have a problem
```
<!-- } -->
### SELinux BUG FIXING GENERAL PROCEDURE
```
| STEP 01: Check /var/log/messages for messages like
| ...
| ... setroubleshoot: SELinux is preventing /usr/bin/httpd \
| from getattr access on the directory /home/fred. \
| For complet SELinux messages, run \
| sealert -l 37acc7d8-e955-4333-123a-1d027dbcea72
|
| STEP 02: Run the indicated command
| ~# sealert -l 37acc7d8-e955-4333-123a-1d027dbcea72
| → SELinux is preventing /usr/sbin/httpd from search access \
| on the directory /home/fred
| → ...
| → Do
| → setsebool -P httpd_read_user_content 1
| → ...
| → setsebool -P httpd_enable_homedirs 1
```
### AUDITING SELINUX ERROR MESSAGES
```
| #~ ausearch -m avc -c httpd
| └──────┘ └──┘
| standard filter by
| linux AVC related
| audit messaged
| framework
| tool
| (auditd daemon
| must be running)
| → ...
| → time → Thu Aug 21 16:42:17 2014
| → type=AVC msg=audit(1408603337.115:914): avc: denied { getattr } for \
| pid=10204 comm="httpd" path="/www/html/index.html" dev="dm─0" ino=8445484 \
| scontext=system_u:system_r:httpd_t:s0 \
| tcontext=unconfined_u:object_r:default_t:s0 tclass=file
| └ translates to:
| type=AVC: ...: avc: The message comes from the AVC log and it's an AVC event
| denied { getattr }: The permission that was attempted and the result it got.
| In this case the get attribute operation was denied.
| pid=10204 process id of the process that attempted the access.
| comm="httpd" shows the process command for the pid
| path: resource trying to be accessed.
| dev : device
| ino : inode
| scontext: source security context of the process.
| tcontext: target security context of the resource.
| tclass: target resource class.
|
| sealert tool
| #~ cat /var/log/messages │ grep "SELinux is preventing"
| → ...
| → ... setroubleshoot: SELinux is preventing /usr/bin/su from using the setuid capability \
| For complete SELinux messages. run sealert -l e9e6c6d8-f217-414c-a14e-4bccb70cfbce
| #~ sealert -l e9e6c6d8-f217-414c-a14e-4bccb70cfbce
| → SELinux is preventing /usr/bin/su from using the setuid capability.
| → ...
| → Raw Audit Messages
| → type=AVC msg=audit(1408931985.387:850): avc: denied { setuid } for pid=5855 \
| comm="sudo" capability=7 scontext=user_u:user_r:user_t:s0 \
| tcontext=user_u:user_r:user_t:s0 tclass=capability
| → type=SYSCALL msg=audit(1408931985.387:850): arch=x86_64 syscall=setresuid success=no \
| exit=EPERM a0=ffffffff a1=1 a2=ffffffff a3=7fae591b92e0 items=0 ppid=5739 pid=5855 \
| auid=1008 uid=0 gid=1008 euid=0 suid=0 fsuid=0 egid=0 sgid=1008 fsgid=0 tty=pts2 ses=22 \
| comm=sudo exe=/usr/bin/sudo subj=user_u:user_r:user_t:s0 key=(null)
| →
| → Hash: su,user_t,user_t,capability,setuid
| →
```
[[security.selinux}]]