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