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] .

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 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.