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.
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
