Overview
========
For general security related questions of perf_event_open() syscall usage,
performance monitoring and observability operations by Perf see here:
https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
Enabling LSM based mandatory access control (MAC) to perf_event_open() syscall
==============================================================================
LSM hooks for mandatory access control for perf_event_open() syscall can be
used starting from Linux v5.3. Below are the steps to extend Fedora (v31) with
Targeted policy with perf_event_open() access control capabilities:
1. Download selinux-policy SRPM package (e.g. selinux-policy-3.14.4-48.fc31.src.rpm on FC31)
and install it so rpmbuild directory would exist in the current working directory:
# rpm -Uhv selinux-policy-3.14.4-48.fc31.src.rpm
2. Get into rpmbuild/SPECS directory and unpack the source code:
# rpmbuild -bp selinux-policy.spec
3. Place patch below at rpmbuild/BUILD/selinux-policy-b86eaaf4dbcf2d51dd4432df7185c0eaf3cbcc02
directory and apply it:
# patch -p1 < selinux-policy-perf-events-perfmon.patch
patching file policy/flask/access_vectors
patching file policy/flask/security_classes
# cat selinux-policy-perf-events-perfmon.patch
diff -Nura a/policy/flask/access_vectors b/policy/flask/access_vectors
--- a/policy/flask/access_vectors 2020-02-04 18:19:53.000000000 +0300
+++ b/policy/flask/access_vectors 2020-02-28 23:37:25.000000000 +0300
@@ -174,6 +174,7 @@
wake_alarm
block_suspend
audit_read
+ perfmon
}
#
@@ -1099,3 +1100,15 @@
class xdp_socket
inherits socket
+
+class perf_event
+{
+ open
+ cpu
+ kernel
+ tracepoint
+ read
+ write
+}
+
+
diff -Nura a/policy/flask/security_classes b/policy/flask/security_classes
--- a/policy/flask/security_classes 2020-02-04 18:19:53.000000000 +0300
+++ b/policy/flask/security_classes 2020-02-28 21:35:17.000000000 +0300
@@ -200,4 +200,6 @@
class xdp_socket
+class perf_event
+
# FLASK
4. Get into rpmbuild/SPECS directory and build policy packages from patched sources:
# rpmbuild --noclean --noprep -ba selinux-policy.spec
so you have this:
# ls -alh rpmbuild/RPMS/noarch/
total 33M
drwxr-xr-x. 2 root root 4.0K Mar 20 12:16 .
drwxr-xr-x. 3 root root 4.0K Mar 20 12:16 ..
-rw-r--r--. 1 root root 112K Mar 20 12:16 selinux-policy-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 1.2M Mar 20 12:17 selinux-policy-devel-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 2.3M Mar 20 12:17 selinux-policy-doc-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 12M Mar 20 12:17 selinux-policy-minimum-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 4.5M Mar 20 12:16 selinux-policy-mls-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 111K Mar 20 12:16 selinux-policy-sandbox-3.14.4-48.fc31.noarch.rpm
-rw-r--r--. 1 root root 14M Mar 20 12:17 selinux-policy-targeted-3.14.4-48.fc31.noarch.rpm
5. Install SELinux packages from Fedora repo, if not already done so, and
update with the patched rpms above:
# rpm -Uhv rpmbuild/RPMS/noarch/selinux-policy-*
6. Enable SELinux Permissive mode for Targeted policy, if not already done so:
# cat /etc/selinux/config
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - No SELinux policy is loaded.
SELINUX=permissive
# SELINUXTYPE= can take one of these three values:
# targeted - Targeted processes are protected,
# minimum - Modification of targeted policy. Only selected processes are protected.
# mls - Multi Level Security protection.
SELINUXTYPE=targeted
7. Enable filesystem SELinux labeling at the next reboot:
# touch /.autorelabel
8. Reboot machine and it will label filesystems and load Targeted policy into the kernel;
9. Login and check that dmesg output doesn't mention that perf_event class is unknown to SELinux subsystem;
10. Check that SELinux is enabled and in Permissive mode
# getenforce
Permissive
11. Turn SELinux into Enforcing mode:
# setenforce 1
# getenforce
Enforcing
Opening access to perf_event_open() syscall on Fedora with SELinux
==================================================================
Access to performance monitoring and observability operations by Perf
can be limited for superuser or CAP_PERFMON or CAP_SYS_ADMIN privileged
processes. MAC policy settings (e.g. SELinux) can be loaded into the kernel
and prevent unauthorized access to perf_event_open() syscall. In such case
Perf tool provides a message similar to the one below:
# perf stat
Error:
Access to performance monitoring and observability operations is limited.
Enforced MAC policy settings (SELinux) can limit access to performance
monitoring and observability operations. Inspect system audit records for
more perf_event access control information and adjusting the policy.
Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open
access to performance monitoring and observability operations for users
without CAP_PERFMON or CAP_SYS_ADMIN Linux capability.
perf_event_paranoid setting is -1:
-1: Allow use of (almost) all events by all users
Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK
>= 0: Disallow raw and ftrace function tracepoint access
>= 1: Disallow CPU event access
>= 2: Disallow kernel profiling
To make the adjusted perf_event_paranoid setting permanent preserve it
in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)
To make sure that access is limited by MAC policy settings inspect system
audit records using journalctl command or /var/log/audit/audit.log so the
output would contain AVC denied records related to perf_event:
# journalctl --reverse --no-pager | grep perf_event
python3[1318099]: SELinux is preventing perf from open access on the perf_event labeled unconfined_t.
If you believe that perf should be allowed open access on perf_event labeled unconfined_t by default.
setroubleshoot[1318099]: SELinux is preventing perf from open access on the perf_event labeled unconfined_t. For complete SELinux messages run: sealert -l 4595ce5b-e58f-462c-9d86-3bc2074935de
audit[1318098]: AVC avc: denied { open } for pid=1318098 comm="perf" scontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 tcontext=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 tclass=perf_event permissive=0
In order to open access to perf_event_open() syscall MAC policy settings can
require to be extended. On SELinux system this can be done by loading a special
policy module extending base policy settings. Perf related policy module can
be generated using the system audit records about blocking perf_event access.
Run the command below to generate my-perf.te policy extension file with
perf_event related rules:
# ausearch -c 'perf' --raw | audit2allow -M my-perf && cat my-perf.te
module my-perf 1.0;
require {
type unconfined_t;
class perf_event { cpu kernel open read tracepoint write };
}
#============= unconfined_t ==============
allow unconfined_t self:perf_event { cpu kernel open read tracepoint write };
Now compile, pack and load my-perf.pp extension module into the kernel:
# checkmodule -M -m -o my-perf.mod my-perf.te
# semodule_package -o my-perf.pp -m my-perf.mod
# semodule -X 300 -i my-perf.pp
After all those taken steps above access to perf_event_open() syscall should
now be allowed by the policy settings. Check access running Perf like this:
# perf stat
^C
Performance counter stats for 'system wide':
36,387.41 msec cpu-clock # 7.999 CPUs utilized
2,629 context-switches # 0.072 K/sec
57 cpu-migrations # 0.002 K/sec
1 page-faults # 0.000 K/sec
263,721,559 cycles # 0.007 GHz
175,746,713 instructions # 0.67 insn per cycle
19,628,798 branches # 0.539 M/sec
1,259,201 branch-misses # 6.42% of all branches
4.549061439 seconds time elapsed
The generated perf-event.pp related policy extension module can be removed
from the kernel using this command:
# semodule -X 300 -r my-perf
Alternatively the module can be temporarily disabled and enabled back using
these two commands:
# semodule -d my-perf
# semodule -e my-perf
If something went wrong
=======================
To turn SELinux into Permissive mode:
# setenforce 0
To fully disable SELinux during kernel boot [3] set kernel command line parameter selinux=0
To remove SELinux labeling from local filesystems:
# find / -mount -print0 | xargs -0 setfattr -h -x security.selinux
To fully turn SELinux off a machine set SELINUX=disabled at /etc/selinux/config file and reboot;
Links
=====
[1] https://download-ib01.fedoraproject.org/pub/fedora/linux/updates/31/Everything/SRPMS/Packages/s/selinux-policy-3.14.4-49.fc31.src.rpm
[2] https://docs.fedoraproject.org/en-US/Fedora/11/html/Security-Enhanced_Linux/sect-Security-Enhanced_Linux-Working_with_SELinux-Enabling_and_Disabling_SELinux.html
[3] https://danwalsh.livejournal.com/10972.html
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