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Subject: Re: [PATCH v2] perf_event_open.2: update the man page with
 CAP_PERFMON related information
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
References: <33c10554-c0ee-9e46-2946-67a9deac6752@linux.intel.com>
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Message-ID: <190971ba-31f1-ca41-60a9-38989fe82a64@gmail.com>
Date:   Tue, 27 Oct 2020 17:57:54 +0100
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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@linux.intel.com>

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.)

>  .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


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/