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Auditing SELinux Events

For SELinux there are two main types of audit event:

  1. AVC Audit Events - These are generated by the AVC subsystem as a result of access denials, or where specific events have requested an audit message (i.e. where an auditallow rule has been used in the policy).
  2. SELinux-aware Application Events - These are generated by the SELinux kernel services and SELinux-aware applications for events such as system errors, initialisation, policy load, changing boolean states, setting of enforcing / permissive mode, relabeling etc.

The audit and event messages are generally stored in one of the following logs (in F-27 anyway):

  1. The SELinux kernel boot events are logged in the /var/log/dmesg log.
  2. The system log /var/log/messages contains messages generated by SELinux before the audit daemon has been loaded.
  3. The audit log /var/log/audit/audit.log contains events that take place after the audit daemon has been loaded. The AVC audit messages of interest are described in the AVC Audit Events section with others described in the General SELinux Audit Events section. Fedora uses the audit framework auditd(8) as standard.

Notes:

  1. It is not mandatory for SELinux-aware applications to audit events or even log them in the audit log. The decision is made by the application designer.
  2. The format of audit messages do not need to conform to any format, however where possible applications should use the audit_log_user_avc_message(3) function with a suitably formatted message if using auditd(8). The type of audit events possible are defined in the include/libaudit.h and include/linux/audit.h files.
  3. Those libselinux library functions that output messages do so to stderr by default, however this can be changed by calling selinux_set_callback(3) and specifying an alternative log handler.

AVC Audit Events

The AVC Audit Message Keyword Descriptions table describes the general format of AVC audit messages in the audit.log when access has been denied or an audit event has been specifically requested. Other types of events are shown in the section that follows.

AVC Audit Message Keyword Descriptions:

type

  • For SELinux AVC events this can be:
    • type=AVC - for kernel events.
    • type=USER_AVC - for user-space object manager events.
  • Note that once the AVC event has been logged, another event with type=SYSCALL may follow that contains further information regarding the event.
  • The AVC event can always be tied to the relevant SYSCALL event as they have the same serial_number in the msg=audit(time:serial_number) field as shown in the following example:
    • type=AVC msg=audit(1243332701.744:101): avc: denied { getattr } for pid=2714 comm="ls" path="/usr/lib/locale/locale-archive" dev=dm-0 ino=353593 scontext=system_u:object_r:unlabeled_t:s0 tcontext=system_u:object_r:locale_t:s0 tclass=file
    • type=SYSCALL msg=audit(1243332701.744:101): arch=40000003 syscall=197 success=yes exit=0 a0=3 a1=553ac0 a2=552ff4 a3=bfc5eab0 items=0 ppid=2671 pid=2714 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="ls" exe="/bin/ls" subj=system_u:object_r:unlabeled_t:s0 key=(null)

msg

  • This will contain the audit keyword with a reference number (e.g. msg=audit(1243332701.744:101))

avc

  • This will be either denied when access has been denied or granted when an auditallow rule has been defined by the policy.
  • The entries that follow the avc= field depend on what type of event is being audited. Those shown below are generated by the kernel AVC audit function, however the user space AVC audit function will return fields relevant to the application being managed by their Object Manager.

pid and comm

  • If a task, then log the process id (pid) and the name of the executable file (comm).

capability

  • If a capability event then log the identifier.

path, name, dev and ino

  • If a File System event then log the relevant information. Note that the name field may not always be present.

laddr, lport, faddr and fport

  • If a Socket event then log the Source / Destination addresses and ports for IPv4 or IPv6 sockets (AF_INET).

path

  • If a File Socket event then log the path (AF_UNIX).

saddr, src, daddr, dest and netif

  • If a Network event then log the Source / Destination addresses and ports with the network interface for IPv4 or IPv6 networks (AF_INET).

sauid, hostname and addr

  • IPSec security association identifiers.

resid and restype

  • X-Windows resource ID and type.

scontext

  • The security context of the source or subject.

tcontext

  • The security context of the target or object.

tclass

  • The object class of the target or object.

permissive

  • Keyword introduced in Linux 4.17 to indicate whether the event was denied or granted due to global or per-domain permissive mode.

Example Audit Events

This is an example denied message - note that there are two type=AVC calls, but only one corresponding type=SYSCALL entry.

type=AVC msg=audit(1242575005.122:101): avc: denied { rename } for
pid=2508 comm="canberra-gtk-pl"
name="c73a516004b572d8c845c74c49b2511d:runtime.tmp" dev=dm-0 ino=188999
scontext=test_u:staff_r:oddjob_mkhomedir_t:s0
tcontext=test_u:object_r:gnome_home_t:s0 tclass=lnk_file

type=AVC msg=audit(1242575005.122:101): avc: denied { unlink } for
pid=2508 comm="canberra-gtk-pl"
name="c73a516004b572d8c845c74c49b2511d:runtime" dev=dm-0 ino=188578
scontext=test_u:staff_r:oddjob_mkhomedir_t:s0
tcontext=system_u:object_r:gnome_home_t:s0 tclass=lnk_file

type=SYSCALL msg=audit(1242575005.122:101): arch=40000003 syscall=38
success=yes exit=0 a0=82d2760 a1=82d2850 a2=da6660 a3=82cb550 items=0
ppid=2179 pid=2508 auid=500 uid=500 gid=500 euid=500 suid=500 fsuid=500
egid=500 sgid=500 fsgid=500 tty=(none) ses=1 comm="canberra-gtk-pl"
exe="/usr/bin/canberra-gtk-play"
subj=test_u:staff_r:oddjob_mkhomedir_t:s0 key=(null)

These are example X-Windows object manager audit messages:

type=USER_AVC msg=audit(1267534171.023:18): user pid=1169 uid=0
auid=4294967295 ses=4294967295
subj=system_u:unconfined_r:unconfined_t msg='avc: denied { getfocus }
for request=X11:GetInputFocus comm=X-setest xdevice="Virtual core
keyboard" scontext=unconfined_u:unconfined_r:x_select_paste_t
tcontext=system_u:unconfined_r:unconfined_t tclass=x_keyboard :
exe="/usr/bin/Xorg" sauid=0 hostname=? addr=? terminal=?'

type=USER_AVC msg=audit(1267534395.930:19): user pid=1169 uid=0
auid=4294967295 ses=4294967295
subj=system_u:unconfined_r:unconfined_t msg='avc: denied { read } for
request=SELinux:SELinuxGetClientContext comm=X-setest resid=3c00001
restype=<unknown>
scontext=unconfined_u:unconfined_r:x_select_paste_t
tcontext=unconfined_u:unconfined_r:unconfined_t tclass=x_resource :
exe="/usr/bin/Xorg" sauid=0 hostname=? addr=? terminal=?'

This is an example granted audit message:

type=AVC msg=audit(1239116352.727:311): avc: granted { transition } for
pid=7687 comm="bash" path="/usr/move_file/move_file_c" dev=dm-0
ino=402139 scontext=unconfined_u:unconfined_r:unconfined_t
tcontext=unconfined_u:unconfined_r:move_file_t tclass=process

type=SYSCALL msg=audit(1239116352.727:311): arch=40000003 syscall=11
success=yes exit=0 a0=8a6ea98 a1=8a56fa8 a2=8a578e8 a3=0 items=0
ppid=2660 pid=7687 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0
sgid=0 fsgid=0 tty=(none) ses=1 comm="move_file_c"
exe="/usr/move_file/move_file_c"
subj=unconfined_u:unconfined_r:move_file_t key=(null)

General SELinux Audit Events

This section shows a selection of non-AVC SELinux-aware services audit events taken from the audit.log. For a list of valid type= entries, the following include files should be consulted: include/libaudit.h and include/linux/audit.h.

Note that there can be what appears to be multiple events being generated for the same event. For example the kernel security server will generate a MAC_POLICY_LOAD event to indicate that the policy has been reloaded, but then each userspace object manager could then generate a USER_MAC_POLICY_LOAD event to indicate that it had also processed the event.

Policy reload - MAC_POLICY_LOAD, USER_MAC_POLICY_LOAD - These events were generated when the policy was reloaded.

type=MAC_POLICY_LOAD msg=audit(1336662937.117:394): policy loaded
auid=0 ses=2

type=SYSCALL msg=audit(1336662937.117:394): arch=c000003e syscall=1
success=yes exit=4345108 a0=4 a1=7f0a0c547000 a2=424d14 a3=7fffe3450f20
items=0 ppid=3845 pid=3848 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0
egid=0 sgid=0 fsgid=0 tty=pts2 ses=2 comm="load_policy"
exe="/sbin/load_policy"
subj=unconfined_u:unconfined_r:load_policy_t:s0-s0:c0.c1023
key=(null)

type=USER_MAC_POLICY_LOAD msg=audit(1336662938.535:395): pid=0 uid=0
auid=4294967295 ses=4294967295
subj=system_u:system_r:xserver_t:s0-s0:c0.c1023 msg='avc: received
policyload notice (seqno=2) : exe="/usr/bin/Xorg" sauid=0 hostname=?
addr=? terminal=?'

Change enforcement mode - MAC_STATUS - This was generated when the SELinux enforcement mode was changed:

type=MAC_STATUS msg=audit(1336836093.835:406): enforcing=1
old_enforcing=0 auid=0 ses=2

type=SYSCALL msg=audit(1336836093.835:406): arch=c000003e syscall=1
success=yes exit=1 a0=3 a1=7fffe743f9e0 a2=1 a3=0 items=0 ppid=2047
pid=5591 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0
tty=pts0 ses=2 comm="setenforce" exe="/usr/sbin/setenforce"
subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)

Change boolean value - MAC_CONFIG_CHANGE - This event was generated when setsebool(8) was run to change a boolean. Note that the boolean name plus new and old values are shown in the MAC_CONFIG_CHANGE type event with the SYSCALL event showing what process executed the change.

type=MAC_CONFIG_CHANGE msg=audit(1336665376.629:423):
bool=domain_paste_after_confirm_allowed val=0 old_val=1 auid=0
ses=2

type=SYSCALL msg=audit(1336665376.629:423): arch=c000003e syscall=1
success=yes exit=2 a0=3 a1=7fff42803200 a2=2 a3=7fff42803f80 items=0
ppid=2015 pid=4664 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0
sgid=0 fsgid=0 tty=pts0 ses=2 comm="setsebool" exe="/usr/sbin/setsebool"
subj=unconfined_u:unconfined_r:setsebool_t:s0-s0:c0.c1023 key=(null)

NetLabel - MAC_UNLBL_STCADD - Generated when adding a static non-mapped label. There are many other NetLabel events possible, such as: MAC_MAP_DEL, MAC_CIPSOV4_DEL, ...

type=MAC_UNLBL_STCADD msg=audit(1336664587.640:413): netlabel: auid=0
ses=2 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
netif=lo src=127.0.0.1
sec_obj=system_u:object_r:unconfined_t:s0-s0:c0,c100 res=1

type=SYSCALL msg=audit(1336664587.640:413): arch=c000003e syscall=46
success=yes exit=96 a0=3 a1=7fffde77f160 a2=0 a3=666e6f636e753a72
items=0 ppid=2015 pid=4316 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0
egid=0 sgid=0 fsgid=0 tty=pts0 ses=2 comm="netlabelctl"
exe="/sbin/netlabelctl"
subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)

Labeled IPSec - MAC_IPSEC_EVENT - Generated when running setkey(8) to load IPSec configuration:

type=MAC_IPSEC_EVENT msg=audit(1336664781.473:414): op=SAD-add auid=0
ses=2 subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
sec_alg=1 sec_doi=1
sec_obj=system_u:system_r:postgresql_t:s0-s0:c0,c200 src=127.0.0.1
dst=127.0.0.1 spi=592(0x250) res=1

type=SYSCALL msg=audit(1336664781.473:414): arch=c000003e syscall=44
success=yes exit=176 a0=4 a1=7fff079d5100 a2=b0 a3=0 items=0 ppid=2015
pid=4356 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0
tty=pts0 ses=2 comm="setkey" exe="/sbin/setkey"
subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 key=(null)

SELinux kernel errors - SELINUX_ERR - These example events were generated by the kernel security server. These were generated by the kernel security server because anon_webapp_t has been give privileges that are greater than that given to the process that started the new thread (this is not allowed).

type=SELINUX_ERR msg=audit(1311948547.151:138):
op=security_compute_av reason=bounds
scontext=system_u:system_r:anon_webapp_t:s0-s0:c0,c100,c200
tcontext=system_u:object_r:security_t:s0 tclass=dir
perms=ioctl,read,lock

type=SELINUX_ERR msg=audit(1311948547.151:138):
op=security_compute_av reason=bounds
scontext=system_u:system_r:anon_webapp_t:s0-s0:c0,c100,c200
tcontext=system_u:object_r:security_t:s0 tclass=file
perms=ioctl,read,write,getattr,lock,append,open

These were generated by the kernel security server when an SELinux-aware application was trying to use setcon(3) to create a new thread. To fix this a typebounds statement is required in the policy.

type=SELINUX_ERR msg=audit(1311947138.440:126):
op=security_bounded_transition result=denied
oldcontext=system_u:system_r:httpd_t:s0-s0:c0.c300
newcontext=system_u:system_r:anon_webapp_t:s0-s0:c0,c100,c200

type=SYSCALL msg=audit(1311947138.440:126): arch=c000003e syscall=1
success=no exit=-1 a0=b a1=7f1954000a10 a2=33 a3=6e65727275632f72
items=0 ppid=3295 pid=3473 auid=4294967295 uid=48 gid=48 euid=48 suid=48
fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4294967295 comm="httpd"
exe="/usr/sbin/httpd" subj=system_u:system_r:httpd_t:s0-s0:c0.c300
key=(null)

The security_compute_sid event is generated by the security server when an invalid context is encountered. In this scenario a specified domain transition results in an invalid context wheel.id:wheel.role:mount.subj:s0 because the wheel.role role is not authorised to associate with the mount.subj type. To fix this a RBAC role types rule is required in the policy.

SELINUX_ERR op=security_compute_sid
invalid_context="wheel.id:wheel.role:mount.subj:s0"
scontext=wheel.id:wheel.role:user.subj:s0
tcontext=sys.id:sys.role:mount.exec:s0 tclass=process

SYSCALL arch=c000003e syscall=59 success=no exit=-13 a0=55cbda501fe0
a1=55cb da502220 a2=55cbda49b210 a3=8 items=0 ppid=1303 pid=99865
auid=1000 uid=1000 gid=1000 euid=1000 suid=1000 fsuid=1000 egid=1000
sgid=1000 fsgid=1000 tty=pts1 ses=1 comm="bash" exe="/usr/bin/bash"
subj=wheel.id:wheel.role:user.subj:s0 key=(null)

Role changes - USER_ROLE_CHANGE - Used newrole(1) to set a new role that was not valid.

type=USER_ROLE_CHANGE msg=audit(1336837198.928:429): pid=0 uid=0
auid=0 ses=2
subj=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
msg='newrole:
old-context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
new-context=?: exe="/usr/bin/newrole" hostname=? addr=?
terminal=/dev/pts/0 res=failed'

Capability Audit Exemptions

In the general case a rejected capability check will result in an audit event. There are however some instances in the kernel where denied capability checks are not audited, which could lead to differences in behavior between enforcing and permissive mode.

List of exemptions (no guarantee for completeness)(locations are based on kernel v6.5 unless otherwise specified):

  • fs/proc/base.c#L1110, fs/proc/base.c#L1129

    If not granted CAP_SYS_RESOURCE the OOM kill score adjustment min value is not set.

  • fs/overlayfs/inode.c#L429, fs/xattr.c#L1298

    If not granted CAP_SYS_ADMIN in its namespace extended attributes in the trusted namespace are not listed.

  • fs/xfs/xfs_fsmap.c#L894

    If not granted CAP_SYS_ADMIN the XFS data device's bnobt is queried instead of rmapbt.

  • fs/xfs/xfs_ioctl.c#L1199, fs/xfs/xfs_iops.c#L709

    If not granted CAP_FOWNER XFS quota checks on transactions are performed.

  • io_uring/io_uring.c#L3887

    If not granted CAP_IPC_LOCK io_uring operations are accounted against the user's RLIMIT_MEMLOCK limit.

  • kernel/capability.c#L519

    If not granted CAP_SYS_PTRACE tracing an unsafe (e.g. no_new_privs set or shared, see fs/exec.c:check_unsafe_exec()) task or a coredump of a non-user process is not permitted.

  • kernel/ksyms_common.c#L37

    If not granted CAP_SYSLOG kallsyms information are not shown, except if kernel profiling is enabled and is explicitly not set to paranoid.

  • kernel/ptrace.c#L282

    If not granted CAP_SYS_PTRACE in its namespace several fields in the PID directory entry stat files are not populated (startcode, endcode, startstack, kstkesp, kstkeip, wchan, start_data, end_data, start_brk, arg_start, arg_end, env_start, env_end and exit_code).

  • kernel/seccomp.c#L662

    If not granted CAP_SYS_ADMIN in its namespace preparing a seccomp filter running without no_new_privs is not permitted.

  • lib/vsprintf.c#L881

    If not granted CAP_SYSLOG restricted pointers are not included in strings formatted via %pK.

  • net/vmw_vsock/af_vsock.c#L779

    If not granted CAP_NET_ADMIN in its namespace new VSOCK sockets are not marked as trusted.

  • net/sysctl_net.c#L48

    If not granted CAP_NET_ADMIN in its namespace the inodes of /proc/sys/net have more restricted DAC permissions.

  • security/commoncap.c#L1405

    If not granted CAP_SYS_ADMIN allocation of a new virtual mapping are restricted in size to reserve memory for sysadmin.

  • security/integrity/ima/ima_policy.c#L607

    If not granted CAP_SETUID rules regarding foreign UIDs are not matched.

  • security/integrity/ima/ima_policy.c#L618

    If not granted CAP_SETGID rules regarding foreign GIDs are not matched.

  • security/landlock/syscalls.c#L413

    If not granted CAP_SYS_ADMIN in its namespace enforcing a Landlock ruleset running without no_new_privs is not permitted.

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