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76e7fd84 AB |
1 | .. _perf_security: |
2 | ||
3 | Perf Events and tool security | |
4 | ============================= | |
5 | ||
6 | Overview | |
7 | -------- | |
8 | ||
9 | Usage of Performance Counters for Linux (perf_events) [1]_ , [2]_ , [3]_ can | |
10 | impose a considerable risk of leaking sensitive data accessed by monitored | |
11 | processes. The data leakage is possible both in scenarios of direct usage of | |
12 | perf_events system call API [2]_ and over data files generated by Perf tool user | |
13 | mode utility (Perf) [3]_ , [4]_ . The risk depends on the nature of data that | |
14 | perf_events performance monitoring units (PMU) [2]_ collect and expose for | |
15 | performance analysis. Having that said perf_events/Perf performance monitoring | |
16 | is the subject for security access control management [5]_ . | |
17 | ||
18 | perf_events/Perf access control | |
19 | ------------------------------- | |
20 | ||
21 | To perform security checks, the Linux implementation splits processes into two | |
22 | categories [6]_ : a) privileged processes (whose effective user ID is 0, referred | |
23 | to as superuser or root), and b) unprivileged processes (whose effective UID is | |
24 | nonzero). Privileged processes bypass all kernel security permission checks so | |
25 | perf_events performance monitoring is fully available to privileged processes | |
26 | without access, scope and resource restrictions. | |
27 | ||
28 | Unprivileged processes are subject to a full security permission check based on | |
29 | the process's credentials [5]_ (usually: effective UID, effective GID, and | |
30 | supplementary group list). | |
31 | ||
32 | Linux divides the privileges traditionally associated with superuser into | |
33 | distinct units, known as capabilities [6]_ , which can be independently enabled | |
34 | and disabled on per-thread basis for processes and files of unprivileged users. | |
35 | ||
36 | Unprivileged processes with enabled CAP_SYS_ADMIN capability are treated as | |
37 | privileged processes with respect to perf_events performance monitoring and | |
38 | bypass *scope* permissions checks in the kernel. | |
39 | ||
40 | Unprivileged processes using perf_events system call API is also subject for | |
41 | PTRACE_MODE_READ_REALCREDS ptrace access mode check [7]_ , whose outcome | |
42 | determines whether monitoring is permitted. So unprivileged processes provided | |
43 | with CAP_SYS_PTRACE capability are effectively permitted to pass the check. | |
44 | ||
45 | Other capabilities being granted to unprivileged processes can effectively | |
46 | enable capturing of additional data required for later performance analysis of | |
47 | monitored processes or a system. For example, CAP_SYSLOG capability permits | |
48 | reading kernel space memory addresses from /proc/kallsyms file. | |
49 | ||
50 | perf_events/Perf unprivileged users | |
51 | ----------------------------------- | |
52 | ||
53 | perf_events/Perf *scope* and *access* control for unprivileged processes is | |
54 | governed by perf_event_paranoid [2]_ setting: | |
55 | ||
56 | -1: | |
57 | Impose no *scope* and *access* restrictions on using perf_events performance | |
58 | monitoring. Per-user per-cpu perf_event_mlock_kb [2]_ locking limit is | |
59 | ignored when allocating memory buffers for storing performance data. | |
60 | This is the least secure mode since allowed monitored *scope* is | |
61 | maximized and no perf_events specific limits are imposed on *resources* | |
62 | allocated for performance monitoring. | |
63 | ||
64 | >=0: | |
65 | *scope* includes per-process and system wide performance monitoring | |
66 | but excludes raw tracepoints and ftrace function tracepoints monitoring. | |
67 | CPU and system events happened when executing either in user or | |
68 | in kernel space can be monitored and captured for later analysis. | |
69 | Per-user per-cpu perf_event_mlock_kb locking limit is imposed but | |
70 | ignored for unprivileged processes with CAP_IPC_LOCK [6]_ capability. | |
71 | ||
72 | >=1: | |
73 | *scope* includes per-process performance monitoring only and excludes | |
74 | system wide performance monitoring. CPU and system events happened when | |
75 | executing either in user or in kernel space can be monitored and | |
76 | captured for later analysis. Per-user per-cpu perf_event_mlock_kb | |
77 | locking limit is imposed but ignored for unprivileged processes with | |
78 | CAP_IPC_LOCK capability. | |
79 | ||
80 | >=2: | |
81 | *scope* includes per-process performance monitoring only. CPU and system | |
82 | events happened when executing in user space only can be monitored and | |
83 | captured for later analysis. Per-user per-cpu perf_event_mlock_kb | |
84 | locking limit is imposed but ignored for unprivileged processes with | |
85 | CAP_IPC_LOCK capability. | |
86 | ||
87 | Bibliography | |
88 | ------------ | |
89 | ||
90 | .. [1] `<https://lwn.net/Articles/337493/>`_ | |
91 | .. [2] `<http://man7.org/linux/man-pages/man2/perf_event_open.2.html>`_ | |
92 | .. [3] `<http://web.eece.maine.edu/~vweaver/projects/perf_events/>`_ | |
93 | .. [4] `<https://perf.wiki.kernel.org/index.php/Main_Page>`_ | |
94 | .. [5] `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_ | |
95 | .. [6] `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_ | |
96 | .. [7] `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_ | |
97 |