Re: [PATCH v2] perf_event_open.2: update the man page with CAP_PERFMON related information

[Alexey Budankov]

On 27.10.2020 19:57, Michael Kerrisk (man-pages) wrote:
> Hello Alexey,
> 
> On 10/27/20 5:48 PM, Alexey Budankov wrote:
>>
>> Extend perf_event_open 2 man page with the information about
>> CAP_PERFMON capability designed to secure performance monitoring
>> and observability operation in a system according to the principle
>> of least privilege [1] (POSIX IEEE 1003.1e, 2.2.2.39).
>>
>> [1] https://sites.google.com/site/fullycapable/, posix_1003.1e-990310.pdf
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx>
> 
> Thanks for this. I've applied. I have a few questions/comments below.
> 
>> ---
>>  man2/perf_event_open.2 | 32 ++++++++++++++++++++++++++++++--
>>  1 file changed, 30 insertions(+), 2 deletions(-)
>>
>> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
>> index 4827a359d..9810bc554 100644
>> --- a/man2/perf_event_open.2
>> +++ b/man2/perf_event_open.2
>> @@ -97,6 +97,8 @@ when running on the specified CPU.
>>  .BR "pid == \-1" " and " "cpu >= 0"
>>  This measures all processes/threads on the specified CPU.
>>  This requires
>> +.B CAP_PERFMON
>> +(since Linux 5.8) or
>>  .B CAP_SYS_ADMIN
>>  capability or a
>>  .I /proc/sys/kernel/perf_event_paranoid
>> @@ -108,9 +110,11 @@ This setting is invalid and will return an error.
>>  When
>>  .I pid
>>  is greater than zero, permission to perform this system call
>> -is governed by a ptrace access mode
>> +is governed by
>> +.B CAP_PERFMON
>> +(since Linux 5.9) and a ptrace access mode
> 
> I want to check: did you really mean 5.9 here? (Everywhere else,
> 5.8 is mentioned, but perhaps this change came in the next kernel 
> version.)

Yes, it is not a typo. This thing was merged into v5.9.

Thanks,
Alexei

> 
>>  .B PTRACE_MODE_READ_REALCREDS
>> -check; see
>> +check on older Linux versions; see
>>  .BR ptrace (2).
>>  .PP
>>  The
>> @@ -2925,6 +2929,8 @@ to hold the result.
>>  This allows attaching a Berkeley Packet Filter (BPF)
>>  program to an existing kprobe tracepoint event.
>>  You need
>> +.B CAP_PERFMON
>> +(since Linux 5.8) or
>>  .B CAP_SYS_ADMIN
>>  privileges to use this ioctl.
>>  .IP
>> @@ -2967,6 +2973,8 @@ have multiple events attached to a tracepoint.
>>  Querying this value on one tracepoint event returns the id
>>  of all BPF programs in all events attached to the tracepoint.
>>  You need
>> +.B CAP_PERFMON
>> +(since Linux 5.8) or
>>  .B CAP_SYS_ADMIN
>>  privileges to use this ioctl.
>>  .IP
>> @@ -3175,6 +3183,8 @@ it was expecting.
>>  .TP
>>  .B EACCES
>>  Returned when the requested event requires
>> +.B CAP_PERFMON
>> +(since Linux 5.8) or
>>  .B CAP_SYS_ADMIN
>>  permissions (or a more permissive perf_event paranoid setting).
>>  Some common cases where an unprivileged process
>> @@ -3296,6 +3306,8 @@ setting is specified.
>>  It can also happen, as with
>>  .BR EACCES ,
>>  when the requested event requires
>> +.B CAP_PERFMON
>> +(since Linux 5.8) or
>>  .B CAP_SYS_ADMIN
>>  permissions (or a more permissive perf_event paranoid setting).
>>  This includes setting a breakpoint on a kernel address,
>> @@ -3326,6 +3338,22 @@ The official way of knowing if
>>  support is enabled is checking
>>  for the existence of the file
>>  .IR /proc/sys/kernel/perf_event_paranoid .
>> +.PP
>> +.B CAP_PERFMON
>> +capability (since Linux 5.8) provides secure approach to
>> +performance monitoring and observability operations in a system
>> +according to the principal of least privilege (POSIX IEEE 1003.1e).
>> +Accessing system performance monitoring and observability operations
>> +using
>> +.B CAP_PERFMON
>> +rather than the much more powerful
>> +.B CAP_SYS_ADMIN
>> +excludes chances to misuse credentials and makes operations more secure.
>> +.B CAP_SYS_ADMIN
>> +usage for secure system performance monitoring and observability
>> +is discouraged with respect to
>> +.B CAP_PERFMON
>> +capability.
> 
> Thank you for adding the above piece. That point of course
> really needs to be emphasized!
> 
> Thanks,
> 
> Michael
> 
>