diff options
Diffstat (limited to 'Documentation/admin-guide/perf-security.rst')
| -rw-r--r-- | Documentation/admin-guide/perf-security.rst | 253 |
1 files changed, 193 insertions, 60 deletions
diff --git a/Documentation/admin-guide/perf-security.rst b/Documentation/admin-guide/perf-security.rst index f73ebfe9bfe2..72effa7c23b9 100644 --- a/Documentation/admin-guide/perf-security.rst +++ b/Documentation/admin-guide/perf-security.rst @@ -6,83 +6,211 @@ 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]_ collect and expose for -performance analysis. Having that said perf_events/Perf performance monitoring -is the subject for security access control management [5]_ . +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. +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: +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. + 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. + 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. + *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. + *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 ------------ @@ -94,4 +222,9 @@ Bibliography .. [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>`_ |