9d87bbae2d
Extend perf-security.rst file with perf_events/Perf resource control section describing RLIMIT_NOFILE and perf_event_mlock_kb settings for performance monitoring user processes. Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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140 lines
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ReStructuredText
.. _perf_security:
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Perf Events and tool security
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=============================
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Overview
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--------
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Usage of Performance Counters for Linux (perf_events) [1]_ , [2]_ , [3]_ can
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impose a considerable risk of leaking sensitive data accessed by monitored
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processes. The data leakage is possible both in scenarios of direct usage of
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perf_events system call API [2]_ and over data files generated by Perf tool user
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mode utility (Perf) [3]_ , [4]_ . The risk depends on the nature of data that
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perf_events performance monitoring units (PMU) [2]_ collect and expose for
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performance analysis. Having that said perf_events/Perf performance monitoring
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is the subject for security access control management [5]_ .
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perf_events/Perf access control
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-------------------------------
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To perform security checks, the Linux implementation splits processes into two
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categories [6]_ : a) privileged processes (whose effective user ID is 0, referred
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to as superuser or root), and b) unprivileged processes (whose effective UID is
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nonzero). Privileged processes bypass all kernel security permission checks so
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perf_events performance monitoring is fully available to privileged processes
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without access, scope and resource restrictions.
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Unprivileged processes are subject to a full security permission check based on
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the process's credentials [5]_ (usually: effective UID, effective GID, and
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supplementary group list).
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Linux divides the privileges traditionally associated with superuser into
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distinct units, known as capabilities [6]_ , which can be independently enabled
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and disabled on per-thread basis for processes and files of unprivileged users.
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Unprivileged processes with enabled CAP_SYS_ADMIN capability are treated as
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privileged processes with respect to perf_events performance monitoring and
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bypass *scope* permissions checks in the kernel.
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Unprivileged processes using perf_events system call API is also subject for
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PTRACE_MODE_READ_REALCREDS ptrace access mode check [7]_ , whose outcome
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determines whether monitoring is permitted. So unprivileged processes provided
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with CAP_SYS_PTRACE capability are effectively permitted to pass the check.
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Other capabilities being granted to unprivileged processes can effectively
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enable capturing of additional data required for later performance analysis of
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monitored processes or a system. For example, CAP_SYSLOG capability permits
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reading kernel space memory addresses from /proc/kallsyms file.
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perf_events/Perf unprivileged users
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-----------------------------------
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perf_events/Perf *scope* and *access* control for unprivileged processes is
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governed by perf_event_paranoid [2]_ setting:
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-1:
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Impose no *scope* and *access* restrictions on using perf_events performance
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monitoring. Per-user per-cpu perf_event_mlock_kb [2]_ locking limit is
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ignored when allocating memory buffers for storing performance data.
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This is the least secure mode since allowed monitored *scope* is
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maximized and no perf_events specific limits are imposed on *resources*
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allocated for performance monitoring.
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>=0:
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*scope* includes per-process and system wide performance monitoring
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but excludes raw tracepoints and ftrace function tracepoints monitoring.
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CPU and system events happened when executing either in user or
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in kernel space can be monitored and captured for later analysis.
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Per-user per-cpu perf_event_mlock_kb locking limit is imposed but
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ignored for unprivileged processes with CAP_IPC_LOCK [6]_ capability.
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>=1:
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*scope* includes per-process performance monitoring only and excludes
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system wide performance monitoring. CPU and system events happened when
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executing either in user or in kernel space can be monitored and
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captured for later analysis. Per-user per-cpu perf_event_mlock_kb
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locking limit is imposed but ignored for unprivileged processes with
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CAP_IPC_LOCK capability.
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>=2:
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*scope* includes per-process performance monitoring only. CPU and system
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events happened when executing in user space only can be monitored and
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captured for later analysis. Per-user per-cpu perf_event_mlock_kb
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locking limit is imposed but ignored for unprivileged processes with
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CAP_IPC_LOCK capability.
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perf_events/Perf resource control
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---------------------------------
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Open file descriptors
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+++++++++++++++++++++
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The perf_events system call API [2]_ allocates file descriptors for every configured
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PMU event. Open file descriptors are a per-process accountable resource governed
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by the RLIMIT_NOFILE [11]_ limit (ulimit -n), which is usually derived from the login
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shell process. When configuring Perf collection for a long list of events on a
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large server system, this limit can be easily hit preventing required monitoring
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configuration. RLIMIT_NOFILE limit can be increased on per-user basis modifying
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content of the limits.conf file [12]_ . Ordinarily, a Perf sampling session
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(perf record) requires an amount of open perf_event file descriptors that is not
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less than the number of monitored events multiplied by the number of monitored CPUs.
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Memory allocation
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+++++++++++++++++
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The amount of memory available to user processes for capturing performance monitoring
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data is governed by the perf_event_mlock_kb [2]_ setting. This perf_event specific
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resource setting defines overall per-cpu limits of memory allowed for mapping
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by the user processes to execute performance monitoring. The setting essentially
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extends the RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped specifically
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for capturing monitored performance events and related data.
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For example, if a machine has eight cores and perf_event_mlock_kb limit is set
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to 516 KiB, then a user process is provided with 516 KiB * 8 = 4128 KiB of memory
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above the RLIMIT_MEMLOCK limit (ulimit -l) for perf_event mmap buffers. In particular,
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this means that, if the user wants to start two or more performance monitoring
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processes, the user is required to manually distribute the available 4128 KiB between the
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monitoring processes, for example, using the --mmap-pages Perf record mode option.
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Otherwise, the first started performance monitoring process allocates all available
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4128 KiB and the other processes will fail to proceed due to the lack of memory.
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RLIMIT_MEMLOCK and perf_event_mlock_kb resource constraints are ignored for
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processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf privileged users
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can be provided with memory above the constraints for perf_events/Perf performance
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monitoring purpose by providing the Perf executable with CAP_IPC_LOCK capability.
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Bibliography
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------------
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.. [1] `<https://lwn.net/Articles/337493/>`_
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.. [2] `<http://man7.org/linux/man-pages/man2/perf_event_open.2.html>`_
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.. [3] `<http://web.eece.maine.edu/~vweaver/projects/perf_events/>`_
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.. [4] `<https://perf.wiki.kernel.org/index.php/Main_Page>`_
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.. [5] `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_
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.. [6] `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_
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.. [7] `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_
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.. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_
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.. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_
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