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

Casey Schaufler
2019-12-05T16:49:22Z

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
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 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(-)
>