perf-security: document collected perf_events/Perf data categories

Document and categorize system and performance data into groups that
can be captured by perf_events/Perf and explicitly indicate the group
that can contain process sensitive data.
Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/perf-security.rst

@@ -11,8 +11,34 @@ 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
+perf_events performance monitoring units (PMU) [2]_ and Perf collect and expose
+for performance analysis. Collected system and performance data may be split into
+several categories:
+
+1. System hardware and software configuration data, for example: a CPU model and
+   its cache configuration, an amount of available memory and its topology, used
+   kernel and Perf versions, performance monitoring setup including experiment
+   time, events configuration, Perf command line parameters, etc.
+
+2. User and kernel module paths and their load addresses with sizes, process and
+   thread names with their PIDs and TIDs, timestamps for captured hardware and
+   software events.
+
+3. Content of kernel software counters (e.g., for context switches, page faults,
+   CPU migrations), architectural hardware performance counters (PMC) [8]_ and
+   machine specific registers (MSR) [9]_ that provide execution metrics for
+   various monitored parts of the system (e.g., memory controller (IMC), interconnect
+   (QPI/UPI) or peripheral (PCIe) uncore counters) without direct attribution to any
+   execution context state.
+
+4. Content of architectural execution context registers (e.g., RIP, RSP, RBP on
+   x86_64), process user and kernel space memory addresses and data, content of
+   various architectural MSRs that capture data from this category.
+
+Data that belong to the fourth category can potentially contain sensitive process
+data. If PMUs in some monitoring modes capture values of execution context registers
+or data from process memory then access to such monitoring capabilities requires
+to be ordered and secured properly. So, perf_events/Perf performance monitoring
 is the subject for security access control management [5]_ .
 
 perf_events/Perf access control
@@ -134,6 +160,8 @@ Bibliography
 .. [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>`_
+.. [8] `<https://en.wikipedia.org/wiki/Hardware_performance_counter>`_
+.. [9] `<https://en.wikipedia.org/wiki/Model-specific_register>`_
 .. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_
 .. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_