Re: [PATCH v2 3/4] perf tool: make Perf tool aware of SELinux access control

[Alexey Budankov]

On 23.04.2020 16:27, Arnaldo Carvalho de Melo wrote:
> Em Wed, Apr 22, 2020 at 05:45:34PM +0300, Alexey Budankov escreveu:
>>
>> Implement SELinux sysfs check to see if the system is in enforcing
>> mode and print warning message with pointers to check audit logs.
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
>> ---
>>  tools/perf/util/cloexec.c |  4 ++--
>>  tools/perf/util/evsel.c   | 40 +++++++++++++++++++++++----------------
>>  2 files changed, 26 insertions(+), 18 deletions(-)
>>
>> diff --git a/tools/perf/util/cloexec.c b/tools/perf/util/cloexec.c
>> index a12872f2856a..9c8ec816261b 100644
>> --- a/tools/perf/util/cloexec.c
>> +++ b/tools/perf/util/cloexec.c
>> @@ -65,7 +65,7 @@ static int perf_flag_probe(void)
>>  		return 1;
>>  	}
>>  
>> -	WARN_ONCE(err != EINVAL && err != EBUSY,
>> +	WARN_ONCE(err != EINVAL && err != EBUSY && err != EACCES,
>>  		  "perf_event_open(..., PERF_FLAG_FD_CLOEXEC) failed with unexpected error %d (%s)\n",
>>  		  err, str_error_r(err, sbuf, sizeof(sbuf)));
>>  
>> @@ -83,7 +83,7 @@ static int perf_flag_probe(void)
>>  	if (fd >= 0)
>>  		close(fd);
>>  
>> -	if (WARN_ONCE(fd < 0 && err != EBUSY,
>> +	if (WARN_ONCE(fd < 0 && err != EBUSY && err != EACCES,
>>  		      "perf_event_open(..., 0) failed unexpectedly with error %d (%s)\n",
>>  		      err, str_error_r(err, sbuf, sizeof(sbuf))))
>>  		return -1;
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index 9fa92649adb4..82492ca12405 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -2514,32 +2514,40 @@ int perf_evsel__open_strerror(struct evsel *evsel, struct target *target,
>>  			      int err, char *msg, size_t size)
>>  {
>>  	char sbuf[STRERR_BUFSIZE];
>> -	int printed = 0;
>> +	int printed = 0, enforced = 0;
>>  
>>  	switch (err) {
>>  	case EPERM:
>>  	case EACCES:
>> +		printed += scnprintf(msg + printed, size - printed,
>> +			"Access to performance monitoring and observability operations is limited.\n");
>> +
>> +		if (!sysfs__read_int("fs/selinux/enforce", &enforced)) {
>> +			if (enforced) {
>> +				printed += scnprintf(msg + printed, size - printed,
>> +					"Enforced MAC policy settings (SELinux) can limit access to performance\n"
>> +					"monitoring and observability operations. Inspect system audit records for\n"
>> +					"more perf_event access control information and adjusting the policy.\n");
>> +			}
>> +		}
>> +
>>  		if (err == EPERM)
>> -			printed = scnprintf(msg, size,
>> -				"No permission to enable %s event.\n\n",
>> +			printed += scnprintf(msg, size,
>> +				"No permission to enable %s event.\n",
>>  				perf_evsel__name(evsel));
> 
> This removal of a newline doesn't seem necessary to this patch.

There will be break in the middle of the message then, but ok.

>   
>>  		return scnprintf(msg + printed, size - printed,
>> -		 "You may not have permission to collect %sstats.\n\n"
>> -		 "Consider tweaking /proc/sys/kernel/perf_event_paranoid,\n"
>> -		 "which controls use of the performance events system by\n"
>> -		 "unprivileged users (without CAP_PERFMON or CAP_SYS_ADMIN).\n\n"
>> -		 "The current value is %d:\n\n"
>> +		 "Consider adjusting /proc/sys/kernel/perf_event_paranoid setting to open\n"
>> +		 "access to performance monitoring and observability operations for users\n"
>> +		 "without CAP_PERFMON capability. perf_event_paranoid setting is %d:\n"
> 
> Here we need as well to check if the kernel/libcap supports CAP_PERFMON
> to provide a better error message.

I will change change "CAP_PERFMON" to "CAP_PERFMON or CAP_SYS_ADMIN" in the new message.

> 
>>  		 "  -1: Allow use of (almost) all events by all users\n"
>>  		 "      Ignore mlock limit after perf_event_mlock_kb without CAP_IPC_LOCK\n"
>> -		 ">= 0: Disallow ftrace function tracepoint by users without CAP_PERFMON or CAP_SYS_ADMIN\n"
>> -		 "      Disallow raw tracepoint access by users without CAP_SYS_PERFMON or CAP_SYS_ADMIN\n"
>> -		 ">= 1: Disallow CPU event access by users without CAP_PERFMON or CAP_SYS_ADMIN\n"
>> -		 ">= 2: Disallow kernel profiling by users without CAP_PERFMON or CAP_SYS_ADMIN\n\n"
>> -		 "To make this setting permanent, edit /etc/sysctl.conf too, e.g.:\n\n"
>> -		 "	kernel.perf_event_paranoid = -1\n" ,
>> -				 target->system_wide ? "system-wide " : "",
>> -				 perf_event_paranoid());
>> +		 ">= 0: Disallow raw and ftrace function tracepoint access\n"
>> +		 ">= 1: Disallow CPU event access\n"
>> +		 ">= 2: Disallow kernel profiling\n"
>> +		 "To make the adjusted perf_event_paranoid setting permanent preserve it\n"
>> +		 "in /etc/sysctl.conf (e.g. kernel.perf_event_paranoid = <setting>)",
>> +		 perf_event_paranoid());
>>  	case ENOENT:
>>  		return scnprintf(msg, size, "The %s event is not supported.",
>>  				 perf_evsel__name(evsel));

Thanks,
Alexey