doc/admin-guide: Document creation of CAP_PERFMON privileged shell
Document steps to create CAP_PERFMON privileged shell to unblock Perf tool usage in cases when capabilities can't be assigned to an executable due to limitations of used file system. Suggested-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Namhyung Kim <namhyung@kernel.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: linux-doc@vger.kernel.org Cc: linux-man@vger.kernel.org Cc: linux-security-module@vger.kernel.org Link: http://lore.kernel.org/lkml/0abda956-de6c-95b1-61e8-49e146501079@linux.intel.com Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>zero-sugar-mainline-defconfig
Alexey Budankov
Arnaldo Carvalho de Melo
parent
4cb3fb1cd9
commit
1dd88c195d
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@ -102,11 +102,11 @@ CAP_SYSLOG capability permits reading kernel space memory addresses from
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Privileged Perf users groups
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---------------------------------
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Mechanisms of capabilities, privileged capability-dumb files [6]_ and
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file system ACLs [10]_ can be used to create dedicated groups of
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privileged Perf users who are permitted to execute performance monitoring
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and observability without scope limits. The following steps can be
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taken to create such groups of privileged Perf users.
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Mechanisms of capabilities, privileged capability-dumb files [6]_,
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file system ACLs [10]_ and sudo [15]_ utility can be used to create
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dedicated groups of privileged Perf users who are permitted to execute
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performance monitoring and observability without limits. The following
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steps can be taken to create such groups of privileged Perf users.
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1. Create perf_users group of privileged Perf users, assign perf_users
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group to Perf tool executable and limit access to the executable for
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@ -136,7 +136,7 @@ taken to create such groups of privileged Perf users.
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# getcap perf
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perf = cap_sys_ptrace,cap_syslog,cap_perfmon+ep
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If the libcap installed doesn't yet support "cap_perfmon", use "38" instead,
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If the libcap [16]_ installed doesn't yet support "cap_perfmon", use "38" instead,
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i.e.:
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::
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@ -162,6 +162,60 @@ performance monitoring and observability by using functionality of the
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configured Perf tool executable that, when executes, passes perf_events
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subsystem scope checks.
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In case Perf tool executable can't be assigned required capabilities (e.g.
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file system is mounted with nosuid option or extended attributes are
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not supported by the file system) then creation of the capabilities
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privileged environment, naturally shell, is possible. The shell provides
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inherent processes with CAP_PERFMON and other required capabilities so that
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performance monitoring and observability operations are available in the
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environment without limits. Access to the environment can be open via sudo
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utility for members of perf_users group only. In order to create such
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environment:
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1. Create shell script that uses capsh utility [16]_ to assign CAP_PERFMON
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and other required capabilities into ambient capability set of the shell
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process, lock the process security bits after enabling SECBIT_NO_SETUID_FIXUP,
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SECBIT_NOROOT and SECBIT_NO_CAP_AMBIENT_RAISE bits and then change
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the process identity to sudo caller of the script who should essentially
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be a member of perf_users group:
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::
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# ls -alh /usr/local/bin/perf.shell
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-rwxr-xr-x. 1 root root 83 Oct 13 23:57 /usr/local/bin/perf.shell
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# cat /usr/local/bin/perf.shell
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exec /usr/sbin/capsh --iab=^cap_perfmon --secbits=239 --user=$SUDO_USER -- -l
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2. Extend sudo policy at /etc/sudoers file with a rule for perf_users group:
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::
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# grep perf_users /etc/sudoers
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%perf_users ALL=/usr/local/bin/perf.shell
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3. Check that members of perf_users group have access to the privileged
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shell and have CAP_PERFMON and other required capabilities enabled
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in permitted, effective and ambient capability sets of an inherent process:
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::
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$ id
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uid=1003(capsh_test) gid=1004(capsh_test) groups=1004(capsh_test),1000(perf_users) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
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$ sudo perf.shell
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[sudo] password for capsh_test:
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$ grep Cap /proc/self/status
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CapInh: 0000004000000000
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CapPrm: 0000004000000000
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CapEff: 0000004000000000
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CapBnd: 000000ffffffffff
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CapAmb: 0000004000000000
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$ capsh --decode=0000004000000000
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0x0000004000000000=cap_perfmon
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As a result, members of perf_users group have access to the privileged
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environment where they can use tools employing performance monitoring APIs
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governed by CAP_PERFMON Linux capability.
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This specific access control management is only available to superuser
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or root running processes with CAP_SETPCAP, CAP_SETFCAP [6]_
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capabilities.
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@ -267,3 +321,5 @@ Bibliography
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.. [12] `<http://man7.org/linux/man-pages/man5/limits.conf.5.html>`_
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.. [13] `<https://sites.google.com/site/fullycapable>`_
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.. [14] `<http://man7.org/linux/man-pages/man8/auditd.8.html>`_
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.. [15] `<https://man7.org/linux/man-pages/man8/sudo.8.html>`_
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.. [16] `<https://git.kernel.org/pub/scm/libs/libcap/libcap.git/>`_
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