Em Mon, Jul 13, 2020 at 12:48:25PM +0300, Alexey Budankov escreveu: > > On 10.07.2020 20:09, Arnaldo Carvalho de Melo wrote: > > 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-) > At least those who write it :D ... Everybody should read it, sure :-) > > 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. > > $ > > It would better provide reference to perf security docs in the tool output. So add a 3rd line: $ ./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. Please read the 'Perf events and tool security' document: https://www.kernel.org/doc/html/latest/admin-guide/perf-security.html > Looks like extending ptrace_may_access() check for perf_events with CAP_PERFMON You mean the following? diff --git a/kernel/events/core.c b/kernel/events/core.c index 856d98c36f56..a2397f724c10 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -11595,7 +11595,7 @@ SYSCALL_DEFINE5(perf_event_open, * perf_event_exit_task() that could imply). */ err = -EACCES; - if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) + if (!perfmon_capable() && !ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) goto err_cred; } > makes monitoring simpler and even more secure to use since Perf tool need > not to start/stop/single-step and read/write registers and memory and so on > like a debugger or strace-like tool. What do you think? I tend to agree, Peter? > Alexei > > > > > - 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
