Re: [PATCH v8 00/12] Introduce CAP_PERFMON to secure system performance monitoring and observability

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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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