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Message-ID: <20200710170911.GD7487@kernel.org>
Date: Fri, 10 Jul 2020 14:09:11 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
Peter Zijlstra <peterz@...radead.org>,
Alexei Starovoitov <ast@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
James Morris <jmorris@...ei.org>,
Namhyung Kim <namhyung@...nel.org>,
Serge Hallyn <serge@...lyn.com>, Jiri Olsa <jolsa@...hat.com>,
Song Liu <songliubraving@...com>,
Andi Kleen <ak@...ux.intel.com>,
Stephane Eranian <eranian@...gle.com>,
Igor Lubashev <ilubashe@...mai.com>,
Thomas Gleixner <tglx@...utronix.de>,
linux-kernel <linux-kernel@...r.kernel.org>,
"linux-security-module@...r.kernel.org"
<linux-security-module@...r.kernel.org>,
"selinux@...r.kernel.org" <selinux@...r.kernel.org>,
"intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
linux-man@...r.kernel.org
Subject: Re: [PATCH v8 00/12] Introduce CAP_PERFMON to secure system
performance monitoring and observability
Em Fri, Jul 10, 2020 at 05:30:50PM +0300, Alexey Budankov escreveu:
> On 10.07.2020 16:31, Ravi Bangoria wrote:
> >> Currently access to perf_events, i915_perf and other performance
> >> monitoring and observability subsystems of the kernel is open only for
> >> a privileged process [1] with CAP_SYS_ADMIN capability enabled in the
> >> process effective set [2].
> >> This patch set introduces CAP_PERFMON capability designed to secure
> >> system performance monitoring and observability operations so that
> >> CAP_PERFMON would assist CAP_SYS_ADMIN capability in its governing role
> >> for performance monitoring and observability subsystems of the kernel.
> > I'm seeing an issue with CAP_PERFMON when I try to record data for a
> > specific target. I don't know whether this is sort of a regression or
> > an expected behavior.
> Thanks for reporting and root causing this case. The behavior looks like
> kind of expected since currently CAP_PERFMON takes over the related part
> of CAP_SYS_ADMIN credentials only. Actually Perf security docs [1] say
> that access control is also subject to CAP_SYS_PTRACE credentials.
I think that stating that in the error message would be helpful, after
all, who reads docs? 8-)
I.e., this:
$ ./perf stat ls
Error:
Access to performance monitoring and observability operations is limited.
$
Could become:
$ ./perf stat ls
Error:
Access to performance monitoring and observability operations is limited.
Right now only CAP_PERFMON is granted, you may need CAP_SYS_PTRACE.
$
- Arnaldo
> CAP_PERFMON could be used to extend and substitute ptrace_may_access()
> check in perf_events subsystem to simplify user experience at least in
> this specific case.
>
> Alexei
>
> [1] https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html
>
> >
> > Without setting CAP_PERFMON:
> >
> > $ getcap ./perf
> > $ ./perf stat -a ls
> > Error:
> > Access to performance monitoring and observability operations is limited.
> > $ ./perf stat ls
> > Performance counter stats for 'ls':
> > 2.06 msec task-clock:u # 0.418 CPUs utilized
> > 0 context-switches:u # 0.000 K/sec
> > 0 cpu-migrations:u # 0.000 K/sec
> >
> > With CAP_PERFMON:
> >
> > $ getcap ./perf
> > ./perf = cap_perfmon+ep
> > $ ./perf stat -a ls
> > Performance counter stats for 'system wide':
> > 142.42 msec cpu-clock # 25.062 CPUs utilized
> > 182 context-switches # 0.001 M/sec
> > 48 cpu-migrations # 0.337 K/sec
> > $ ./perf stat ls
> > Error:
> > Access to performance monitoring and observability operations is limited.
> >
> > Am I missing something silly?
> >
> > Analysis:
> > ---------
> > A bit more analysis lead me to below kernel code fs/exec.c:
> >
> > begin_new_exec()
> > {
> > ...
> > if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP ||
> > !(uid_eq(current_euid(), current_uid()) &&
> > gid_eq(current_egid(), current_gid())))
> > set_dumpable(current->mm, suid_dumpable);
> > else
> > set_dumpable(current->mm, SUID_DUMP_USER);
> >
> > ...
> > commit_creds(bprm->cred);
> > }
> >
> > When I execute './perf stat ls', it's going into else condition and thus sets
> > dumpable flag as SUID_DUMP_USER. Then in commit_creds():
> >
> > int commit_creds(struct cred *new)
> > {
> > ...
> > /* dumpability changes */
> > if (...
> > !cred_cap_issubset(old, new)) {
> > if (task->mm)
> > set_dumpable(task->mm, suid_dumpable);
> > }
> >
> > !cred_cap_issubset(old, new) fails for perf without any capability and thus
> > it doesn't execute set_dumpable(). Whereas that condition passes for perf
> > with CAP_PERFMON and thus it overwrites old value (SUID_DUMP_USER) with
> > suid_dumpable in mm_flags. On an Ubuntu, suid_dumpable default value is
> > SUID_DUMP_ROOT. On Fedora, it's SUID_DUMP_DISABLE. (/proc/sys/fs/suid_dumpable).
> >
> > Now while opening an event:
> >
> > perf_event_open()
> > ptrace_may_access()
> > __ptrace_may_access() {
> > ...
> > if (mm &&
> > ((get_dumpable(mm) != SUID_DUMP_USER) &&
> > !ptrace_has_cap(cred, mm->user_ns, mode)))
> > return -EPERM;
> > }
> >
> > This if condition passes for perf with CAP_PERFMON and thus it returns -EPERM.
> > But it fails for perf without CAP_PERFMON and thus it goes ahead and returns
> > success. So opening an event fails when perf has CAP_PREFMON and tries to open
> > process specific event as normal user.
> >
> > Workarounds:
> > ------------
> > Based on above analysis, I found couple of workarounds (examples are on
> > Ubuntu 18.04.4 powerpc):
> >
> > Workaround1:
> > Setting SUID_DUMP_USER as default (in /proc/sys/fs/suid_dumpable) solves the
> > issue.
> >
> > # echo 1 > /proc/sys/fs/suid_dumpable
> > $ getcap ./perf
> > ./perf = cap_perfmon+ep
> > $ ./perf stat ls
> > Performance counter stats for 'ls':
> > 1.47 msec task-clock # 0.806 CPUs utilized
> > 0 context-switches # 0.000 K/sec
> > 0 cpu-migrations # 0.000 K/sec
> >
> > Workaround2:
> > Using CAP_SYS_PTRACE along with CAP_PERFMON solves the issue.
> >
> > $ cat /proc/sys/fs/suid_dumpable
> > 2
> > # setcap "cap_perfmon,cap_sys_ptrace=ep" ./perf
> > $ ./perf stat ls
> > Performance counter stats for 'ls':
> > 1.41 msec task-clock # 0.826 CPUs utilized
> > 0 context-switches # 0.000 K/sec
> > 0 cpu-migrations # 0.000 K/sec
> >
> > Workaround3:
> > Adding CAP_PERFMON to parent of perf (/bin/bash) also solves the issue.
> >
> > $ cat /proc/sys/fs/suid_dumpable
> > 2
> > # setcap "cap_perfmon=ep" /bin/bash
> > # setcap "cap_perfmon=ep" ./perf
> > $ bash
> > $ ./perf stat ls
> > Performance counter stats for 'ls':
> > 1.47 msec task-clock # 0.806 CPUs utilized
> > 0 context-switches # 0.000 K/sec
> > 0 cpu-migrations # 0.000 K/sec
> >
> > - Ravi
--
- Arnaldo
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