Documentation/admin-guide: introduce perf-security.rst file

Implement initial version of perf-security.rst documentation file
covering security concerns of perf_event_paranoid settings.
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/perf-security.rst

+.. _perf_security:
+
+Perf Events and tool security
+=============================
+
+Overview
+--------
+
+Usage of Performance Counters for Linux (perf_events) [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
+perf_events 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
+perf_events performance monitoring units (PMU) [2]_ collect and expose for
+performance analysis. Having that said perf_events/Perf performance monitoring
+is the subject for security access control management [5]_ .
+
+perf_events/Perf access control
+-------------------------------
+
+To perform security checks, the 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
+perf_events performance monitoring is fully available to privileged processes
+without access, scope and resource restrictions.
+
+Unprivileged processes are subject to a full security permission check based on
+the process's credentials [5]_ (usually: effective UID, effective GID, and
+supplementary group list).
+
+Linux divides the privileges traditionally associated with superuser into
+distinct units, known as capabilities [6]_ , which can be independently enabled
+and disabled on per-thread basis for processes and files of unprivileged users.
+
+Unprivileged processes with enabled CAP_SYS_ADMIN capability are treated as
+privileged processes with respect to perf_events performance monitoring and
+bypass *scope* permissions checks in the kernel.
+
+Unprivileged processes using perf_events system call API is also subject for
+PTRACE_MODE_READ_REALCREDS ptrace access mode check [7]_ , whose outcome
+determines whether monitoring is permitted. So unprivileged processes provided
+with CAP_SYS_PTRACE capability are effectively permitted to pass the check.
+
+Other capabilities being granted to unprivileged processes can effectively
+enable capturing of additional data required for later performance analysis of
+monitored processes or a system. For example, CAP_SYSLOG capability permits
+reading kernel space memory addresses from /proc/kallsyms file.
+
+perf_events/Perf unprivileged users
+-----------------------------------
+
+perf_events/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 perf_events 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 perf_events 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.
+
+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] `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_
+