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 .. _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 modes requires to be ordered and secured properly.
 So, perf_events performance monitoring and observability operations are
 the subject for security access control management [5]_ .
 
 perf_events 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_PERFMON capability are treated
 as privileged processes with respect to perf_events performance
 monitoring and observability operations, thus, bypass *scope* permissions
 checks in the kernel. CAP_PERFMON implements the principle of least
 privilege [13]_ (POSIX 1003.1e: 2.2.2.39) for performance monitoring and
 observability operations in the kernel and provides a secure approach to
 performance monitoring and observability in the system.
 
 For backward compatibility reasons the access to perf_events monitoring and
 observability operations is also open for CAP_SYS_ADMIN privileged
 processes but CAP_SYS_ADMIN usage for secure monitoring and observability
 use cases is discouraged with respect to the CAP_PERFMON capability.
 If system audit records [14]_ for a process using perf_events system call
 API contain denial records of acquiring both CAP_PERFMON and CAP_SYS_ADMIN
 capabilities then providing the process with CAP_PERFMON capability singly
 is recommended as the preferred secure approach to resolve double access
 denial logging related to usage of performance monitoring and observability.
 
 Prior Linux v5.9 unprivileged processes using perf_events system call
 are 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. Starting from Linux v5.9
 CAP_SYS_PTRACE capability is not required and CAP_PERFMON is enough to
 be provided for processes to make performance monitoring and observability
 operations.
 
 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.
 
 Privileged Perf users groups
 ---------------------------------
 
 Mechanisms of capabilities, privileged capability-dumb files [6]_,
 file system ACLs [10]_ and sudo [15]_ utility can be used to create
 dedicated groups of privileged Perf users who are permitted to execute
 performance monitoring and observability without limits. The following
 steps can be taken to create such groups 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 monitoring and observability
    privileges [6]_ :
 
 ::
 
    # setcap "cap_perfmon,cap_sys_ptrace,cap_syslog=ep" perf
    # setcap -v "cap_perfmon,cap_sys_ptrace,cap_syslog=ep" perf
    perf: OK
    # getcap perf
    perf = cap_sys_ptrace,cap_syslog,cap_perfmon+ep
 
 If the libcap [16]_ installed doesn't yet support "cap_perfmon", use "38" instead,
 i.e.:
 
 ::
 
    # setcap "38,cap_ipc_lock,cap_sys_ptrace,cap_syslog=ep" perf
 
 Note that you may need to have 'cap_ipc_lock' in the mix for tools such as
 'perf top', alternatively use 'perf top -m N', to reduce the memory that
 it uses for the perf ring buffer, see the memory allocation section below.
 
 Using a libcap without support for CAP_PERFMON will make cap_get_flag(caps, 38,
 CAP_EFFECTIVE, &val) fail, which will lead the default event to be 'cycles:u',
 so as a workaround explicitly ask for the 'cycles' event, i.e.:
 
 ::
 
   # perf top -e cycles
 
 To get kernel and user samples with a perf binary with just CAP_PERFMON.
 
 As a result, members of perf_users group are capable of conducting
 performance monitoring and observability by using functionality of the
 configured Perf tool executable that, when executes, passes perf_events
 subsystem scope checks.
 
 In case Perf tool executable can't be assigned required capabilities (e.g.
 file system is mounted with nosuid option or extended attributes are
 not supported by the file system) then creation of the capabilities
 privileged environment, naturally shell, is possible. The shell provides
 inherent processes with CAP_PERFMON and other required capabilities so that
 performance monitoring and observability operations are available in the
 environment without limits. Access to the environment can be open via sudo
 utility for members of perf_users group only. In order to create such
 environment:
 
 1. Create shell script that uses capsh utility [16]_ to assign CAP_PERFMON
    and other required capabilities into ambient capability set of the shell
    process, lock the process security bits after enabling SECBIT_NO_SETUID_FIXUP,
    SECBIT_NOROOT and SECBIT_NO_CAP_AMBIENT_RAISE bits and then change
    the process identity to sudo caller of the script who should essentially
    be a member of perf_users group:
 
 ::
 
    # ls -alh /usr/local/bin/perf.shell
    -rwxr-xr-x. 1 root root 83 Oct 13 23:57 /usr/local/bin/perf.shell
    # cat /usr/local/bin/perf.shell
    exec /usr/sbin/capsh --iab=^cap_perfmon --secbits=239 --user=$SUDO_USER -- -l
 
 2. Extend sudo policy at /etc/sudoers file with a rule for perf_users group:
 
 ::
 
    # grep perf_users /etc/sudoers
    %perf_users    ALL=/usr/local/bin/perf.shell
 
 3. Check that members of perf_users group have access to the privileged
    shell and have CAP_PERFMON and other required capabilities enabled
    in permitted, effective and ambient capability sets of an inherent process:
 
 ::
 
   $ id
   uid=1003(capsh_test) gid=1004(capsh_test) groups=1004(capsh_test),1000(perf_users) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
   $ sudo perf.shell
   [sudo] password for capsh_test:
   $ grep Cap /proc/self/status
   CapInh:        0000004000000000
   CapPrm:        0000004000000000
   CapEff:        0000004000000000
   CapBnd:        000000ffffffffff
   CapAmb:        0000004000000000
   $ capsh --decode=0000004000000000
   0x0000004000000000=cap_perfmon
 
 As a result, members of perf_users group have access to the privileged
 environment where they can use tools employing performance monitoring APIs
 governed by CAP_PERFMON Linux capability.
 
 This specific access control management is only available to superuser
 or root running processes with CAP_SETPCAP, CAP_SETFCAP [6]_
 capabilities.
 
 Unprivileged users
 -----------------------------------
 
 perf_events *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.
 
 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>`_
 .. [13] `<https://sites.google.com/site/fullycapable>`_
 .. [14] `<http://man7.org/linux/man-pages/man8/auditd.8.html>`_
 .. [15] `<https://man7.org/linux/man-pages/man8/sudo.8.html>`_
 .. [16] `<https://git.kernel.org/pub/scm/libs/libcap/libcap.git/>`_

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