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

[Alexey Budankov]

Hi Ravi,

On 10.07.2020 16:31, Ravi Bangoria wrote:
> Hi Alexey,
> 
>> 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.

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