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
