Re: [PATCH v4 1/2] Documentation/admin-guide: introduce perf-security.rst file

Kees Cook
2018-12-06T01:11:06Z

On Tue, Nov 27, 2018 at 12:15 AM Alexey Budankov
<alexey.budankov@linux.intel.com> wrote:
>
>
> Implement initial version of perf-security.rst documentation file
> covering security concerns of perf_event_paranoid settings.
>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> Changes in v4:
> - added docs for perf_event related capabilities
> Changes in v3:
> - toning down of the markup for "scope, access and resource"
> - adding definite article for "Linux implementation"
> Changes in v2:
> - reverted patches order in the set to avoid CI issue
> - replaced old PCL referencing by PE (Perf Events)
> - skipped >=3 setting documentation at the moment
> ---
> Documentation/admin-guide/perf-security.rst | 97 +++++++++++++++++++++
> 1 file changed, 97 insertions(+)
> create mode 100644 Documentation/admin-guide/perf-security.rst
>
> diff --git a/Documentation/admin-guide/perf-security.rst b/Documentation/admin-guide/perf-security.rst
> new file mode 100644
> index 000000000000..f73ebfe9bfe2
> --- /dev/null
> +++ b/Documentation/admin-guide/perf-security.rst
> @@ -0,0 +1,97 @@
> +.. _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]_ 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.
> +
> +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>`_
> +

--
Kees Cook