[PATCH v1 2/2]: Documentation/admin-guide: introduce perf-security.rst file

[Alexey Budankov]

Implement initial version of perf-security.rst documentation file 
initially covering security concerns related to PCL/Perf performance 
monitoring in multiuser environments.

Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
---
 Documentation/admin-guide/perf-security.rst | 83 +++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/Documentation/admin-guide/perf-security.rst b/Documentation/admin-guide/perf-security.rst
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+.. _perf_security:
+
+PCL/Perf security
+=================
+
+Overview
+--------
+
+Usage of Performance Counters for Linux (PCL) [1]_ , [2]_ , [3]_ can impose a
+considerable risk of leaking sensitive data accessed by monitored processes.
+The data leakage is possible both in scenarios of direct usage of PCL system
+call API [2]_ and over data files generated by Perf tool user mode utility
+(Perf) [3]_ , [4]_ . The risk depends on the nature of data that PCL performance
+monitoring units (PMU) [2]_ collect and expose for performance analysis.
+Having that said PCL/Perf performance monitoring is the subject for security
+access control management [5]_ .
+
+PCL/Perf access control
+-----------------------
+
+For the purpose of performing security checks Linux implementation splits
+processes into two categories [6]_ : a) privileged processes (whose effective
+user ID is 0, referred to as superuser or root), and b) unprivileged processes
+(whose effective UID is nonzero). Privileged processes bypass all kernel
+security permission checks so PCL performance monitoring is fully available to
+privileged processes without *access*, *scope* and *resource* restrictions.
+Unprivileged processes are subject to full security permission check based
+on the process's credentials [5]_ (usually: effective UID, effective GID,
+and supplementary group list).
+
+PCL/Perf unprivileged users
+---------------------------
+
+PCL/Perf *scope* and *access* control for unprivileged processes is governed by
+perf_event_paranoid [2]_ setting:
+
+**-1**:
+     Impose no *scope* and *access* restrictions on using PCL performance
+     monitoring. Per-user per-cpu perf_event_mlock_kb [2]_ locking limit is
+     ignored when allocating memory buffers for storing performance data.
+     This is the least secure mode since allowed monitored *scope* is
+     maximized and no PCL specific limits are imposed on *resources*
+     allocated for performance monitoring.
+
+**>=0**:
+     *scope* includes per-process and system wide performance monitoring
+     but excludes raw tracepoints and ftrace function tracepoints monitoring.
+     CPU and system events happened when executing either in user or
+     in kernel space can be monitored and captured for later analysis.
+     Per-user per-cpu perf_event_mlock_kb locking limit is imposed but
+     ignored for unprivileged processes with CAP_IPC_LOCK [6]_ capability.
+
+**>=1**:
+     *scope* includes per-process performance monitoring only and excludes
+     system wide performance monitoring. CPU and system events happened when
+     executing either in user or in kernel space can be monitored and
+     captured for later analysis. Per-user per-cpu perf_event_mlock_kb
+     locking limit is imposed but ignored for unprivileged processes with
+     CAP_IPC_LOCK capability.
+
+**>=2**:
+     *scope* includes per-process performance monitoring only. CPU and system
+     events happened when executing in user space only can be monitored and
+     captured for later analysis. Per-user per-cpu perf_event_mlock_kb
+     locking limit is imposed but ignored for unprivileged processes with
+     CAP_IPC_LOCK capability.
+
+**>=3**:
+     Restrict *access* to PCL performance monitoring for unprivileged processes.
+     This is the default on Debian and Android [7]_ , [8]_ .
+
+Bibliography
+------------
+
+.. [1] `<https://lwn.net/Articles/337493/>`_
+.. [2] `<http://man7.org/linux/man-pages/man2/perf_event_open.2.html>`_
+.. [3] `<http://web.eece.maine.edu/~vweaver/projects/perf_events/>`_
+.. [4] `<https://perf.wiki.kernel.org/index.php/Main_Page>`_
+.. [5] `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_
+.. [6] `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_
+.. [7] `<https://lkml.org/lkml/2016/1/11/587>`_
+.. [8] `<https://android-review.googlesource.com/#/c/234743/>`_
+