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

Alexey Budankov
2018-12-06T10:45:19Z

On 06.12.2018 4:10, Kees Cook wrote:
> 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>

Thanks!
-Alexey

>
> -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>`_
>> +
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