1.. _perf_security: 2 3Perf events and tool security 4============================= 5 6Overview 7-------- 8 9Usage of Performance Counters for Linux (perf_events) [1]_ , [2]_ , [3]_ 10can impose a considerable risk of leaking sensitive data accessed by 11monitored processes. The data leakage is possible both in scenarios of 12direct usage of perf_events system call API [2]_ and over data files 13generated by Perf tool user mode utility (Perf) [3]_ , [4]_ . The risk 14depends on the nature of data that perf_events performance monitoring 15units (PMU) [2]_ and Perf collect and expose for performance analysis. 16Collected system and performance data may be split into several 17categories: 18 191. System hardware and software configuration data, for example: a CPU 20 model and its cache configuration, an amount of available memory and 21 its topology, used kernel and Perf versions, performance monitoring 22 setup including experiment time, events configuration, Perf command 23 line parameters, etc. 24 252. User and kernel module paths and their load addresses with sizes, 26 process and thread names with their PIDs and TIDs, timestamps for 27 captured hardware and software events. 28 293. Content of kernel software counters (e.g., for context switches, page 30 faults, CPU migrations), architectural hardware performance counters 31 (PMC) [8]_ and machine specific registers (MSR) [9]_ that provide 32 execution metrics for various monitored parts of the system (e.g., 33 memory controller (IMC), interconnect (QPI/UPI) or peripheral (PCIe) 34 uncore counters) without direct attribution to any execution context 35 state. 36 374. Content of architectural execution context registers (e.g., RIP, RSP, 38 RBP on x86_64), process user and kernel space memory addresses and 39 data, content of various architectural MSRs that capture data from 40 this category. 41 42Data that belong to the fourth category can potentially contain 43sensitive process data. If PMUs in some monitoring modes capture values 44of execution context registers or data from process memory then access 45to such monitoring modes requires to be ordered and secured properly. 46So, perf_events performance monitoring and observability operations are 47the subject for security access control management [5]_ . 48 49perf_events access control 50------------------------------- 51 52To perform security checks, the Linux implementation splits processes 53into two categories [6]_ : a) privileged processes (whose effective user 54ID is 0, referred to as superuser or root), and b) unprivileged 55processes (whose effective UID is nonzero). Privileged processes bypass 56all kernel security permission checks so perf_events performance 57monitoring is fully available to privileged processes without access, 58scope and resource restrictions. 59 60Unprivileged processes are subject to a full security permission check 61based on the process's credentials [5]_ (usually: effective UID, 62effective GID, and supplementary group list). 63 64Linux divides the privileges traditionally associated with superuser 65into distinct units, known as capabilities [6]_ , which can be 66independently enabled and disabled on per-thread basis for processes and 67files of unprivileged users. 68 69Unprivileged processes with enabled CAP_PERFMON capability are treated 70as privileged processes with respect to perf_events performance 71monitoring and observability operations, thus, bypass *scope* permissions 72checks in the kernel. CAP_PERFMON implements the principle of least 73privilege [13]_ (POSIX 1003.1e: 2.2.2.39) for performance monitoring and 74observability operations in the kernel and provides a secure approach to 75performance monitoring and observability in the system. 76 77For backward compatibility reasons the access to perf_events monitoring and 78observability operations is also open for CAP_SYS_ADMIN privileged 79processes but CAP_SYS_ADMIN usage for secure monitoring and observability 80use cases is discouraged with respect to the CAP_PERFMON capability. 81If system audit records [14]_ for a process using perf_events system call 82API contain denial records of acquiring both CAP_PERFMON and CAP_SYS_ADMIN 83capabilities then providing the process with CAP_PERFMON capability singly 84is recommended as the preferred secure approach to resolve double access 85denial logging related to usage of performance monitoring and observability. 86 87Prior Linux v5.9 unprivileged processes using perf_events system call 88are also subject for PTRACE_MODE_READ_REALCREDS ptrace access mode check 89[7]_ , whose outcome determines whether monitoring is permitted. 90So unprivileged processes provided with CAP_SYS_PTRACE capability are 91effectively permitted to pass the check. Starting from Linux v5.9 92CAP_SYS_PTRACE capability is not required and CAP_PERFMON is enough to 93be provided for processes to make performance monitoring and observability 94operations. 95 96Other capabilities being granted to unprivileged processes can 97effectively enable capturing of additional data required for later 98performance analysis of monitored processes or a system. For example, 99CAP_SYSLOG capability permits reading kernel space memory addresses from 100/proc/kallsyms file. 101 102Privileged Perf users groups 103--------------------------------- 104 105Mechanisms of capabilities, privileged capability-dumb files [6]_, 106file system ACLs [10]_ and sudo [15]_ utility can be used to create 107dedicated groups of privileged Perf users who are permitted to execute 108performance monitoring and observability without limits. The following 109steps can be taken to create such groups of privileged Perf users. 110 1111. Create perf_users group of privileged Perf users, assign perf_users 112 group to Perf tool executable and limit access to the executable for 113 other users in the system who are not in the perf_users group: 114 115:: 116 117 # groupadd perf_users 118 # ls -alhF 119 -rwxr-xr-x 2 root root 11M Oct 19 15:12 perf 120 # chgrp perf_users perf 121 # ls -alhF 122 -rwxr-xr-x 2 root perf_users 11M Oct 19 15:12 perf 123 # chmod o-rwx perf 124 # ls -alhF 125 -rwxr-x--- 2 root perf_users 11M Oct 19 15:12 perf 126 1272. Assign the required capabilities to the Perf tool executable file and 128 enable members of perf_users group with monitoring and observability 129 privileges [6]_ : 130 131:: 132 133 # setcap "cap_perfmon,cap_sys_ptrace,cap_syslog=ep" perf 134 # setcap -v "cap_perfmon,cap_sys_ptrace,cap_syslog=ep" perf 135 perf: OK 136 # getcap perf 137 perf = cap_sys_ptrace,cap_syslog,cap_perfmon+ep 138 139If the libcap [16]_ installed doesn't yet support "cap_perfmon", use "38" instead, 140i.e.: 141 142:: 143 144 # setcap "38,cap_ipc_lock,cap_sys_ptrace,cap_syslog=ep" perf 145 146Note that you may need to have 'cap_ipc_lock' in the mix for tools such as 147'perf top', alternatively use 'perf top -m N', to reduce the memory that 148it uses for the perf ring buffer, see the memory allocation section below. 149 150Using a libcap without support for CAP_PERFMON will make cap_get_flag(caps, 38, 151CAP_EFFECTIVE, &val) fail, which will lead the default event to be 'cycles:u', 152so as a workaround explicitly ask for the 'cycles' event, i.e.: 153 154:: 155 156 # perf top -e cycles 157 158To get kernel and user samples with a perf binary with just CAP_PERFMON. 159 160As a result, members of perf_users group are capable of conducting 161performance monitoring and observability by using functionality of the 162configured Perf tool executable that, when executes, passes perf_events 163subsystem scope checks. 164 165In case Perf tool executable can't be assigned required capabilities (e.g. 166file system is mounted with nosuid option or extended attributes are 167not supported by the file system) then creation of the capabilities 168privileged environment, naturally shell, is possible. The shell provides 169inherent processes with CAP_PERFMON and other required capabilities so that 170performance monitoring and observability operations are available in the 171environment without limits. Access to the environment can be open via sudo 172utility for members of perf_users group only. In order to create such 173environment: 174 1751. Create shell script that uses capsh utility [16]_ to assign CAP_PERFMON 176 and other required capabilities into ambient capability set of the shell 177 process, lock the process security bits after enabling SECBIT_NO_SETUID_FIXUP, 178 SECBIT_NOROOT and SECBIT_NO_CAP_AMBIENT_RAISE bits and then change 179 the process identity to sudo caller of the script who should essentially 180 be a member of perf_users group: 181 182:: 183 184 # ls -alh /usr/local/bin/perf.shell 185 -rwxr-xr-x. 1 root root 83 Oct 13 23:57 /usr/local/bin/perf.shell 186 # cat /usr/local/bin/perf.shell 187 exec /usr/sbin/capsh --iab=^cap_perfmon --secbits=239 --user=$SUDO_USER -- -l 188 1892. Extend sudo policy at /etc/sudoers file with a rule for perf_users group: 190 191:: 192 193 # grep perf_users /etc/sudoers 194 %perf_users ALL=/usr/local/bin/perf.shell 195 1963. Check that members of perf_users group have access to the privileged 197 shell and have CAP_PERFMON and other required capabilities enabled 198 in permitted, effective and ambient capability sets of an inherent process: 199 200:: 201 202 $ id 203 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 204 $ sudo perf.shell 205 [sudo] password for capsh_test: 206 $ grep Cap /proc/self/status 207 CapInh: 0000004000000000 208 CapPrm: 0000004000000000 209 CapEff: 0000004000000000 210 CapBnd: 000000ffffffffff 211 CapAmb: 0000004000000000 212 $ capsh --decode=0000004000000000 213 0x0000004000000000=cap_perfmon 214 215As a result, members of perf_users group have access to the privileged 216environment where they can use tools employing performance monitoring APIs 217governed by CAP_PERFMON Linux capability. 218 219This specific access control management is only available to superuser 220or root running processes with CAP_SETPCAP, CAP_SETFCAP [6]_ 221capabilities. 222 223Unprivileged users 224----------------------------------- 225 226perf_events *scope* and *access* control for unprivileged processes 227is governed by perf_event_paranoid [2]_ setting: 228 229-1: 230 Impose no *scope* and *access* restrictions on using perf_events 231 performance monitoring. Per-user per-cpu perf_event_mlock_kb [2]_ 232 locking limit is ignored when allocating memory buffers for storing 233 performance data. This is the least secure mode since allowed 234 monitored *scope* is maximized and no perf_events specific limits 235 are imposed on *resources* allocated for performance monitoring. 236 237>=0: 238 *scope* includes per-process and system wide performance monitoring 239 but excludes raw tracepoints and ftrace function tracepoints 240 monitoring. CPU and system events happened when executing either in 241 user or in kernel space can be monitored and captured for later 242 analysis. Per-user per-cpu perf_event_mlock_kb locking limit is 243 imposed but ignored for unprivileged processes with CAP_IPC_LOCK 244 [6]_ capability. 245 246>=1: 247 *scope* includes per-process performance monitoring only and 248 excludes system wide performance monitoring. CPU and system events 249 happened when executing either in user or in kernel space can be 250 monitored and captured for later analysis. Per-user per-cpu 251 perf_event_mlock_kb locking limit is imposed but ignored for 252 unprivileged processes with CAP_IPC_LOCK capability. 253 254>=2: 255 *scope* includes per-process performance monitoring only. CPU and 256 system events happened when executing in user space only can be 257 monitored and captured for later analysis. Per-user per-cpu 258 perf_event_mlock_kb locking limit is imposed but ignored for 259 unprivileged processes with CAP_IPC_LOCK capability. 260 261Resource control 262--------------------------------- 263 264Open file descriptors 265+++++++++++++++++++++ 266 267The perf_events system call API [2]_ allocates file descriptors for 268every configured PMU event. Open file descriptors are a per-process 269accountable resource governed by the RLIMIT_NOFILE [11]_ limit 270(ulimit -n), which is usually derived from the login shell process. When 271configuring Perf collection for a long list of events on a large server 272system, this limit can be easily hit preventing required monitoring 273configuration. RLIMIT_NOFILE limit can be increased on per-user basis 274modifying content of the limits.conf file [12]_ . Ordinarily, a Perf 275sampling session (perf record) requires an amount of open perf_event 276file descriptors that is not less than the number of monitored events 277multiplied by the number of monitored CPUs. 278 279Memory allocation 280+++++++++++++++++ 281 282The amount of memory available to user processes for capturing 283performance monitoring data is governed by the perf_event_mlock_kb [2]_ 284setting. This perf_event specific resource setting defines overall 285per-cpu limits of memory allowed for mapping by the user processes to 286execute performance monitoring. The setting essentially extends the 287RLIMIT_MEMLOCK [11]_ limit, but only for memory regions mapped 288specifically for capturing monitored performance events and related data. 289 290For example, if a machine has eight cores and perf_event_mlock_kb limit 291is set to 516 KiB, then a user process is provided with 516 KiB * 8 = 2924128 KiB of memory above the RLIMIT_MEMLOCK limit (ulimit -l) for 293perf_event mmap buffers. In particular, this means that, if the user 294wants to start two or more performance monitoring processes, the user is 295required to manually distribute the available 4128 KiB between the 296monitoring processes, for example, using the --mmap-pages Perf record 297mode option. Otherwise, the first started performance monitoring process 298allocates all available 4128 KiB and the other processes will fail to 299proceed due to the lack of memory. 300 301RLIMIT_MEMLOCK and perf_event_mlock_kb resource constraints are ignored 302for processes with the CAP_IPC_LOCK capability. Thus, perf_events/Perf 303privileged users can be provided with memory above the constraints for 304perf_events/Perf performance monitoring purpose by providing the Perf 305executable with CAP_IPC_LOCK capability. 306 307Bibliography 308------------ 309 310.. [1] `<https://lwn.net/Articles/337493/>`_ 311.. [2] `<http://man7.org/linux/man-pages/man2/perf_event_open.2.html>`_ 312.. [3] `<http://web.eece.maine.edu/~vweaver/projects/perf_events/>`_ 313.. [4] `<https://perf.wiki.kernel.org/index.php/Main_Page>`_ 314.. [5] `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_ 315.. [6] `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_ 316.. [7] `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_ 317.. [8] `<https://en.wikipedia.org/wiki/Hardware_performance_counter>`_ 318.. [9] `<https://en.wikipedia.org/wiki/Model-specific_register>`_ 319.. [10] `<http://man7.org/linux/man-pages/man5/acl.5.html>`_ 320.. [11] `<http://man7.org/linux/man-pages/man2/getrlimit.2.html>`_ 321.. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_ 322.. [13] `<https://sites.google.com/site/fullycapable>`_ 323.. [14] `<http://man7.org/linux/man-pages/man8/auditd.8.html>`_ 324.. [15] `<https://man7.org/linux/man-pages/man8/sudo.8.html>`_ 325.. [16] `<https://git.kernel.org/pub/scm/libs/libcap/libcap.git/>`_ 326