[PATCH v3 3/3] perf docs: introduce security.txt file to document related issues
From: Alexey Budankov
Date: Thu Apr 30 2020 - 03:16:42 EST
Publish instructions on how to apply LSM hooks for access control
to perf_event_open() syscall on Fedora distro with Targeted SELinux
policy and then manage access to the syscall.
Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
---
tools/perf/Documentation/security.txt | 237 ++++++++++++++++++++++++++
1 file changed, 237 insertions(+)
create mode 100644 tools/perf/Documentation/security.txt
diff --git a/tools/perf/Documentation/security.txt b/tools/perf/Documentation/security.txt
new file mode 100644
index 000000000000..4fe3b8b1958f
--- /dev/null
+++ b/tools/perf/Documentation/security.txt
@@ -0,0 +1,237 @@
+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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