On 06.12.2018 4:10, Kees Cook wrote: > On Tue, Nov 27, 2018 at 12:15 AM Alexey Budankov > <alexey.budankov@xxxxxxxxxxxxxxx> wrote: >> >> >> Implement initial version of perf-security.rst documentation file >> covering security concerns of perf_event_paranoid settings. >> >> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> >> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> 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>`_ >> + > > >
