On 27.11.2018 21:11, Jonathan Corbet wrote:

On Tue, 27 Nov 2018 11:15:37 +0300
Alexey Budankov [off-list ref] wrote:

quoted

+To perform security checks, the Linux implementation splits processes into two
+categories [6]_ : a) privileged processes (whose effective user ID is 0, referred
+to as superuser or root), and b) unprivileged processes (whose effective UID is
+nonzero). Privileged processes bypass all kernel security permission checks so
+perf_events performance monitoring is fully available to privileged processes
+without access, scope and resource restrictions.
+
+Unprivileged processes are subject to a full security permission check based on
+the process's credentials [5]_ (usually: effective UID, effective GID, and
+supplementary group list).
+
+Linux divides the privileges traditionally associated with superuser into
+distinct units, known as capabilities [6]_ , which can be independently enabled
+and disabled on per-thread basis for processes and files of unprivileged users.
+
+Unprivileged processes with enabled CAP_SYS_ADMIN capability are treated as
+privileged processes with respect to perf_events performance monitoring and
+bypass *scope* permissions checks in the kernel.
+
+Unprivileged processes using perf_events system call API is also subject for
+PTRACE_MODE_READ_REALCREDS ptrace access mode check [7]_ , whose outcome
+determines whether monitoring is permitted. So unprivileged processes provided
+with CAP_SYS_PTRACE capability are effectively permitted to pass the check.

It's good to have more information here.  I could certainly quibble
further with things - a process with CAP_SYS_ADMIN is not "unprivileged"!
- but I don't want to stand in the way of this any further.  I *would*
still like to see an ack from the perf world, though.

Thanks for meaningful comments! Looking forward to ack from perf world.

With regard to Kees's comment on merging the two patches; I would probably
add the new document to index.rst in the same patch, but it doesn't matter
that much.  Not worth redoing the patch just for that.

Thanks,
Alexey

jon