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(-) >>> >>
