linux-kernel - Re: [PATCH v1] perf_event_open.2: update the man page with CAP

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Date:   Mon, 24 Aug 2020 23:17:46 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:     linux-man@...r.kernel.org, James Morris <jmorris@...ei.org>,
        Serge Hallyn <serge@...lyn.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] perf_event_open.2: update the man page with
 CAP_PERFMON related information

Hi Michael,

On 23.08.2020 20:28, Michael Kerrisk (man-pages) wrote:
> Hello Alexey,
> 
> Could you look at the question below and update the patch.
> 
> On 2/17/20 9:18 AM, 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@...ux.intel.com>
>> ---
>>   man2/perf_event_open.2 | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>>
>> diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2
>> index 89d267c02..e9aab2ca1 100644
>> --- a/man2/perf_event_open.2
>> +++ b/man2/perf_event_open.2
>> @@ -98,6 +98,8 @@ 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
>> +or
>>   .B CAP_SYS_ADMIN
>>   capability or a
>>   .I /proc/sys/kernel/perf_event_paranoid
>> @@ -2920,6 +2922,8 @@ to hold the result.
>>   This allows attaching a Berkeley Packet Filter (BPF)
>>   program to an existing kprobe tracepoint event.
>>   You need
>> +.B CAP_PERFMON
>> +or
>>   .B CAP_SYS_ADMIN
>>   privileges to use this ioctl.
>>   .IP
>> @@ -2962,6 +2966,8 @@ 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
>> +or
>>   .B CAP_SYS_ADMIN
>>   privileges to use this ioctl.
>>   .IP
>> @@ -3170,6 +3176,8 @@ it was expecting.
>>   .TP
>>   .B EACCES
>>   Returned when the requested event requires
>> +.B CAP_PERFMON
>> +or
>>   .B CAP_SYS_ADMIN
>>   permissions (or a more permissive perf_event paranoid setting).
>>   Some common cases where an unprivileged process
>> @@ -3291,6 +3299,8 @@ setting is specified.
>>   It can also happen, as with
>>   .BR EACCES ,
>>   when the requested event requires
>> +.B CAP_PERFMON
>> +or
>>   .B CAP_SYS_ADMIN
>>   permissions (or a more permissive perf_event paranoid setting).
>>   This includes setting a breakpoint on a kernel address,
>> @@ -3321,6 +3331,23 @@ 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 X.Y) provides secure approach to
> 
> What's the version?

It's since Linux 5.8 .

> 
>> +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
>> +capability singly, without the rest of
>> +.B CAP_SYS_ADMIN
>> +credentials, excludes chances to misuse the credentials and makes
> 
> I think that wording like "using CAP_PERFMON rather than the much
> more powerful CAP_SYS_ADMIN..."

Sounds good to me like this, or similar:

"Accessing system performance monitoring and observability operations
 using CAP_PERFMON rather than the much more powerful CAP_SYS_ADMIN
 excludes chances to misuse credentials and makes operations more
 secure."

> 
>> +the operations more secure.
>> +.B CAP_SYS_ADMIN
>> +usage for secure system performance monitoring and observability
>> +is discouraged with respect to
>> +.B CAP_PERFMON
>> +capability.
>>   .SH BUGS
>>   The
>>   .B F_SETOWN_EX
> 
> Thanks,
> 
> Michael
> 

Thanks,
Alexei

P.S.
I am on vacations till 08/31.
Please expect delay in response.