Documentation/admin-guide/perf-security.rst - linux - Git at Google

 .. _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]_ 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
 -------------------------------

 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 privileged users
 ---------------------------------

 Mechanisms of capabilities, privileged capability-dumb files [6]_ and
 file system ACLs [10]_ can be used to create a dedicated group of
 perf_events/Perf privileged users who are permitted to execute
 performance monitoring without scope limits. The following steps can be
 taken to create such a group of privileged Perf users.

 1. Create perf_users group of privileged Perf users, assign perf_users
    group to Perf tool executable and limit access to the executable for
    other users in the system who are not in the perf_users group:

 ::

    # groupadd perf_users
    # ls -alhF
    -rwxr-xr-x  2 root root  11M Oct 19 15:12 perf
    # chgrp perf_users perf
    # ls -alhF
    -rwxr-xr-x  2 root perf_users  11M Oct 19 15:12 perf
    # chmod o-rwx perf
    # ls -alhF
    -rwxr-x---  2 root perf_users  11M Oct 19 15:12 perf

 2. Assign the required capabilities to the Perf tool executable file and
    enable members of perf_users group with performance monitoring
    privileges [6]_ :

 ::

    # setcap "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
    # setcap -v "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
    perf: OK
    # getcap perf
    perf = cap_sys_ptrace,cap_sys_admin,cap_syslog+ep

 As a result, members of perf_users group are capable of conducting
 performance monitoring by using functionality of the configured Perf
 tool executable that, when executes, passes perf_events subsystem scope
 checks.

 This specific access control management is only available to superuser
 or root running processes with CAP_SETPCAP, CAP_SETFCAP [6]_
 capabilities.

 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.

 perf_events/Perf resource control
 ---------------------------------

 Open file descriptors
 +++++++++++++++++++++

 The perf_events system call API [2]_ allocates file descriptors for
 every configured PMU event. Open file descriptors are a per-process
 accountable resource governed by the RLIMIT_NOFILE [11]_ limit
 (ulimit -n), which is usually derived from the login shell process. When
 configuring Perf collection for a long list of events on a large server
 system, this limit can be easily hit preventing required monitoring
 configuration. RLIMIT_NOFILE limit can be increased on per-user basis
 modifying content of the limits.conf file [12]_ . Ordinarily, a Perf
 sampling session (perf record) requires an amount of open perf_event
 file descriptors that is not less than the number of monitored events
 multiplied by the number of monitored CPUs.

 Memory allocation
 +++++++++++++++++

 The amount of memory available to user processes for capturing
 performance monitoring data is governed by the perf_event_mlock_kb [2]_
 setting. This perf_event specific resource setting defines overall
 per-cpu limits of memory allowed for mapping by the user processes to
 execute performance monitoring. The setting essentially extends the
 RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped
 specifically for capturing monitored performance events and related data.

 For example, if a machine has eight cores and perf_event_mlock_kb limit
 is set to 516 KiB, then a user process is provided with 516 KiB * 8 =
 4128 KiB of memory above the RLIMIT_MEMLOCK limit (ulimit -l) for
 perf_event mmap buffers. In particular, this means that, if the user
 wants to start two or more performance monitoring processes, the user is
 required to manually distribute the available 4128 KiB between the
 monitoring processes, for example, using the --mmap-pages Perf record
 mode option. Otherwise, the first started performance monitoring process
 allocates all available 4128 KiB and the other processes will fail to
 proceed due to the lack of memory.

 RLIMIT_MEMLOCK and perf_event_mlock_kb resource constraints are ignored
 for processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf
 privileged users can be provided with memory above the constraints for
 perf_events/Perf performance monitoring purpose by providing the Perf
 executable 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>`_
 .. [8] `<https://en.wikipedia.org/wiki/Hardware_performance_counter>`_
 .. [9] `<https://en.wikipedia.org/wiki/Model-specific_register>`_
 .. [10] `<http://man7.org/linux/man-pages/man5/acl.5.html>`_
 .. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_
 .. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_
	.. _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]_ 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
	-------------------------------

	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 privileged users
	---------------------------------

	Mechanisms of capabilities, privileged capability-dumb files [6]_ and
	file system ACLs [10]_ can be used to create a dedicated group of
	perf_events/Perf privileged users who are permitted to execute
	performance monitoring without scope limits. The following steps can be
	taken to create such a group of privileged Perf users.

	1. Create perf_users group of privileged Perf users, assign perf_users
	group to Perf tool executable and limit access to the executable for
	other users in the system who are not in the perf_users group:

	::

	# groupadd perf_users
	# ls -alhF
	-rwxr-xr-x 2 root root 11M Oct 19 15:12 perf
	# chgrp perf_users perf
	# ls -alhF
	-rwxr-xr-x 2 root perf_users 11M Oct 19 15:12 perf
	# chmod o-rwx perf
	# ls -alhF
	-rwxr-x--- 2 root perf_users 11M Oct 19 15:12 perf

	2. Assign the required capabilities to the Perf tool executable file and
	enable members of perf_users group with performance monitoring
	privileges [6]_ :

	::

	# setcap "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
	# setcap -v "cap_sys_admin,cap_sys_ptrace,cap_syslog=ep" perf
	perf: OK
	# getcap perf
	perf = cap_sys_ptrace,cap_sys_admin,cap_syslog+ep

	As a result, members of perf_users group are capable of conducting
	performance monitoring by using functionality of the configured Perf
	tool executable that, when executes, passes perf_events subsystem scope
	checks.

	This specific access control management is only available to superuser
	or root running processes with CAP_SETPCAP, CAP_SETFCAP [6]_
	capabilities.

	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.

	perf_events/Perf resource control
	---------------------------------

	Open file descriptors
	+++++++++++++++++++++

	The perf_events system call API [2]_ allocates file descriptors for
	every configured PMU event. Open file descriptors are a per-process
	accountable resource governed by the RLIMIT_NOFILE [11]_ limit
	(ulimit -n), which is usually derived from the login shell process. When
	configuring Perf collection for a long list of events on a large server
	system, this limit can be easily hit preventing required monitoring
	configuration. RLIMIT_NOFILE limit can be increased on per-user basis
	modifying content of the limits.conf file [12]_ . Ordinarily, a Perf
	sampling session (perf record) requires an amount of open perf_event
	file descriptors that is not less than the number of monitored events
	multiplied by the number of monitored CPUs.

	Memory allocation
	+++++++++++++++++

	The amount of memory available to user processes for capturing
	performance monitoring data is governed by the perf_event_mlock_kb [2]_
	setting. This perf_event specific resource setting defines overall
	per-cpu limits of memory allowed for mapping by the user processes to
	execute performance monitoring. The setting essentially extends the
	RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped
	specifically for capturing monitored performance events and related data.

	For example, if a machine has eight cores and perf_event_mlock_kb limit
	is set to 516 KiB, then a user process is provided with 516 KiB * 8 =
	4128 KiB of memory above the RLIMIT_MEMLOCK limit (ulimit -l) for
	perf_event mmap buffers. In particular, this means that, if the user
	wants to start two or more performance monitoring processes, the user is
	required to manually distribute the available 4128 KiB between the
	monitoring processes, for example, using the --mmap-pages Perf record
	mode option. Otherwise, the first started performance monitoring process
	allocates all available 4128 KiB and the other processes will fail to
	proceed due to the lack of memory.

	RLIMIT_MEMLOCK and perf_event_mlock_kb resource constraints are ignored
	for processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf
	privileged users can be provided with memory above the constraints for
	perf_events/Perf performance monitoring purpose by providing the Perf
	executable 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>`_
	.. [8] `<https://en.wikipedia.org/wiki/Hardware_performance_counter>`_
	.. [9] `<https://en.wikipedia.org/wiki/Model-specific_register>`_
	.. [10] `<http://man7.org/linux/man-pages/man5/acl.5.html>`_
	.. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_
	.. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_