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

From: Alexey Budankov <alexey.budankov@linux.intel.com>
To: Jonathan Corbet <corbet@lwn.net>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andi Kleen <ak@linux.intel.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Tvrtko Ursulin <tursulin@ursulin.net>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	"kernel-hardening@lists.openwall.com" 
	<kernel-hardening@lists.openwall.com>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v4 1/2] Documentation/admin-guide: introduce perf-security.rst file
Date: Tue, 27 Nov 2018 22:13:01 +0300	[thread overview]
Message-ID: <5e1e9c08-dff5-956d-4c2b-8ee090c387c3@linux.intel.com> (raw)
In-Reply-To: <20181127111148.75746f02@lwn.net>

On 27.11.2018 21:11, Jonathan Corbet wrote:
> 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.

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
> 