Messages in this thread | | | From | Ravi Bangoria <> | Subject | Re: [PATCH v8 00/12] Introduce CAP_PERFMON to secure system performance monitoring and observability | Date | Fri, 10 Jul 2020 19:01:21 +0530 |
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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
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