Re: [PATCH v4 1/2] Documentation/admin-guide: introduce perf-security.rst file

[Jonathan Corbet <corbet@lwn.net>]

On Tue, 27 Nov 2018 11:15:37 +0300
Alexey Budankov <alexey.budankov@linux.intel.com> wrote:

> +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.

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.

jon