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. > 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. 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(-) >> > >