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
