[PATCH v1 0/3] Introduce CAP_SYS_PERFMON capability for secure Perf users groups

Thu Dec 5 17:33:15 UTC 2019

On 12/5/2019 9:05 AM, Alexey Budankov wrote:
> Hello Casey,
>  
> On 05.12.2019 19:49, Casey Schaufler wrote:
>> On 12/5/2019 8:15 AM, Alexey Budankov wrote:
>>> Currently access to perf_events functionality [1] beyond the scope permitted
>>> by perf_event_paranoid [1] kernel setting is allowed to a privileged process
>>> [2] with CAP_SYS_ADMIN capability enabled in the process effective set [3].
>>>
>>> This patch set introduces CAP_SYS_PERFMON capability devoted to secure performance
>>> monitoring activity so that CAP_SYS_PERFMON would assist CAP_SYS_ADMIN in its
>>> governing role for perf_events based performance monitoring of a system.
>>>
>>> CAP_SYS_PERFMON aims to harden system security and integrity when monitoring
>>> performance using perf_events subsystem by processes and Perf privileged users
>>> [2], thus decreasing attack surface that is available to CAP_SYS_ADMIN
>>> privileged processes [3].
>> Are there use cases where you would need CAP_SYS_PERFMON where you
>> would not also need CAP_SYS_ADMIN? If you separate a new capability
> Actually, there are. Perf tool that has record, stat and top modes could run with
> CAP_SYS_PERFMON capability as mentioned below and provide system wide performance
> data. Currently for that to work the tool needs to be granted with CAP_SYS_ADMIN.

The question isn't whether the tool could use the capability, it's whether
the tool would also need CAP_SYS_ADMIN to be useful. Are there existing
tools that could stop using CAP_SYS_ADMIN in favor of CAP_SYS_PERFMON?
My bet is that any tool that does performance monitoring is going to need
CAP_SYS_ADMIN for other reasons.

>
>> from CAP_SYS_ADMIN but always have to use CAP_SYS_ADMIN in conjunction
>> with the new capability it is all rather pointless.
>>
>> The scope you've defined for this CAP_SYS_PERFMON is very small.
>> Is there a larger set of privilege checks that might be applicable
>> for it?
> CAP_SYS_PERFMON could be applied broadly, though, this patch set enables record
> and stat mode use cases for system wide performance monitoring in kernel and
> user modes.

The granularity of capabilities is something we have to watch
very carefully. Sure, CAP_SYS_ADMIN covers a lot of things, but
if we broke it up "properly" we'd have hundreds of capabilities.
If you want control that finely we have SELinux.

>
> Thanks,
> Alexey
>
>>  
>>
>>> CAP_SYS_PERFMON aims to take over CAP_SYS_ADMIN credentials related to
>>> performance monitoring functionality of perf_events and balance amount of
>>> CAP_SYS_ADMIN credentials in accordance with the recommendations provided in
>>> the man page for CAP_SYS_ADMIN [3]: "Note: this capability is overloaded;
>>> see Notes to kernel developers, below."
>>>
>>> For backward compatibility reasons performance monitoring functionality of 
>>> perf_events subsystem remains available under CAP_SYS_ADMIN but its usage for
>>> secure performance monitoring use cases is discouraged with respect to the
>>> introduced CAP_SYS_PERFMON capability.
>>>
>>> In the suggested implementation CAP_SYS_PERFMON enables Perf privileged users
>>> [2] to conduct secure performance monitoring using perf_events in the scope
>>> of available online CPUs when executing code in kernel and user modes.
>>>
>>> Possible alternative solution to this capabilities balancing, system security
>>> hardening task could be to use the existing CAP_SYS_PTRACE capability to govern
>>> perf_events' performance monitoring functionality, since process debugging is
>>> similar to performance monitoring with respect to providing insights into
>>> process memory and execution details. However CAP_SYS_PTRACE still provides
>>> users with more credentials than are required for secure performance monitoring
>>> using perf_events subsystem and this excess is avoided by using the dedicated
>>> CAP_SYS_PERFMON capability.
>>>
>>> libcap library utilities [4], [5] and Perf tool can be used to apply
>>> CAP_SYS_PERFMON capability for secure performance monitoring beyond the scope
>>> permitted by system wide perf_event_paranoid kernel setting and below are the
>>> steps to evaluate the advancement suggested by the patch set:
>>>
>>>   - patch, build and boot the kernel
>>>   - patch, build Perf tool e.g. to /home/user/perf
>>>   ...
>>>   # git clone git://git.kernel.org/pub/scm/libs/libcap/libcap.git libcap
>>>   # pushd libcap
>>>   # patch libcap/include/uapi/linux/capabilities.h with [PATCH 1/3]
>>>   # make
>>>   # pushd progs
>>>   # ./setcap "cap_sys_perfmon,cap_sys_ptrace,cap_syslog=ep" /home/user/perf
>>>   # ./setcap -v "cap_sys_perfmon,cap_sys_ptrace,cap_syslog=ep" /home/user/perf
>>>   /home/user/perf: OK
>>>   # ./getcap /home/user/perf
>>>   /home/user/perf = cap_sys_ptrace,cap_syslog,cap_sys_perfmon+ep
>>>   # echo 2 > /proc/sys/kernel/perf_event_paranoid
>>>   # cat /proc/sys/kernel/perf_event_paranoid 
>>>   2
>>>   ...
>>>   $ /home/user/perf top
>>>     ... works as expected ...
>>>   $ cat /proc/`pidof perf`/status
>>>   Name:	perf
>>>   Umask:	0002
>>>   State:	S (sleeping)
>>>   Tgid:	2958
>>>   Ngid:	0
>>>   Pid:	2958
>>>   PPid:	9847
>>>   TracerPid:	0
>>>   Uid:	500	500	500	500
>>>   Gid:	500	500	500	500
>>>   FDSize:	256
>>>   ...
>>>   CapInh:	0000000000000000
>>>   CapPrm:	0000004400080000
>>>   CapEff:	0000004400080000 => 01000100 00000000 00001000 00000000 00000000
>>>                                      cap_sys_perfmon,cap_sys_ptrace,cap_syslog
>>>   CapBnd:	0000007fffffffff
>>>   CapAmb:	0000000000000000
>>>   NoNewPrivs:	0
>>>   Seccomp:	0
>>>   Speculation_Store_Bypass:	thread vulnerable
>>>   Cpus_allowed:	ff
>>>   Cpus_allowed_list:	0-7
>>>   ...
>>>
>>> Usage of cap_sys_perfmon effectively avoids unused credentials excess:
>>> - with cap_sys_admin:
>>>   CapEff:	0000007fffffffff => 01111111 11111111 11111111 11111111 11111111
>>> - with cap_sys_perfmon:
>>>   CapEff:	0000004400080000 => 01000100 00000000 00001000 00000000 00000000
>>>                                     38   34               19
>>>                            sys_perfmon   syslog           sys_ptrace
>>>
>>> The patch set is for tip perf/core repository:
>>>   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip perf/core
>>>   tip sha1: ceb9e77324fa661b1001a0ae66f061b5fcb4e4e6
>>>
>>> [1] http://man7.org/linux/man-pages/man2/perf_event_open.2.html
>>> [2] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>>> [3] http://man7.org/linux/man-pages/man7/capabilities.7.html
>>> [4] http://man7.org/linux/man-pages/man8/setcap.8.html
>>> [5] https://git.kernel.org/pub/scm/libs/libcap/libcap.git
>>> [6] https://sites.google.com/site/fullycapable/, posix_1003.1e-990310.pdf
>>>
>>> ---
>>> Alexey Budankov (3):
>>>   capabilities: introduce CAP_SYS_PERFMON to kernel and user space
>>>   perf/core: apply CAP_SYS_PERFMON to CPUs and kernel monitoring
>>>   perf tool: extend Perf tool with CAP_SYS_PERFMON support
>>>
>>>  include/linux/perf_event.h          |  6 ++++--
>>>  include/uapi/linux/capability.h     | 10 +++++++++-
>>>  security/selinux/include/classmap.h |  4 ++--
>>>  tools/perf/design.txt               |  3 ++-
>>>  tools/perf/util/cap.h               |  4 ++++
>>>  tools/perf/util/evsel.c             | 10 +++++-----
>>>  tools/perf/util/util.c              | 15 +++++++++++++--
>>>  7 files changed, 39 insertions(+), 13 deletions(-)
>>>
>>