e85a198e30
Implemented formatting of paragraphs to be not wider than 72 columns. Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
230 lines
10 KiB
ReStructuredText
230 lines
10 KiB
ReStructuredText
.. _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>`_
|
|
|