Date: Wed, 5 Dec 2018 17:10:48 -0800 From: Kees Cook <keescook@...omium.org> To: Alexey Budankov <alexey.budankov@...ux.intel.com> Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Jann Horn <jannh@...gle.com>, Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Andi Kleen <ak@...ux.intel.com>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Mark Rutland <mark.rutland@....com>, Tvrtko Ursulin <tursulin@...ulin.net>, LKML <linux-kernel@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org> Subject: Re: [PATCH v4 1/2] Documentation/admin-guide: introduce perf-security.rst file On Tue, Nov 27, 2018 at 12:15 AM Alexey Budankov <alexey.budankov@...ux.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@...utronix.de> > Signed-off-by: Alexey Budankov <alexey.budankov@...ux.intel.com> Reviewed-by: Kees Cook <keescook@...omium.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) _ , _ , _ 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 _ and over data files generated by Perf tool user > +mode utility (Perf) _ , _ . The risk depends on the nature of data that > +perf_events performance monitoring units (PMU) _ collect and expose for > +performance analysis. Having that said perf_events/Perf performance monitoring > +is the subject for security access control management _ . > + > +perf_events/Perf access control > +------------------------------- > + > +To perform security checks, the Linux implementation splits processes into two > +categories _ : 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 _ (usually: effective UID, effective GID, and > +supplementary group list). > + > +Linux divides the privileges traditionally associated with superuser into > +distinct units, known as capabilities _ , 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 _ , 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 _ setting: > + > +-1: > + Impose no *scope* and *access* restrictions on using perf_events performance > + monitoring. Per-user per-cpu perf_event_mlock_kb _ 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 _ 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 > +------------ > + > +..  `<https://lwn.net/Articles/337493/>`_ > +..  `<http://man7.org/linux/man-pages/man2/perf_event_open.2.html>`_ > +..  `<http://web.eece.maine.edu/~vweaver/projects/perf_events/>`_ > +..  `<https://perf.wiki.kernel.org/index.php/Main_Page>`_ > +..  `<https://www.kernel.org/doc/html/latest/security/credentials.html>`_ > +..  `<http://man7.org/linux/man-pages/man7/capabilities.7.html>`_ > +..  `<http://man7.org/linux/man-pages/man2/ptrace.2.html>`_ > + -- Kees Cook
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