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linux-next/Documentation/admin-guide/perf-security.rst
Alexey Budankov e152c7b7bf perf-security: elaborate on perf_events/Perf privileged users 
Elaborate on possible perf_event/Perf privileged users groups
and document steps about creating such groups.

Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
2019-02-17 16:04:56 -07:00

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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 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>`_
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