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

From: Alexey Budankov
Date: Tue Nov 27 2018 - 14:13:10 EST

On 27.11.2018 21:11, Jonathan Corbet wrote:
> On Tue, 27 Nov 2018 11:15:37 +0300
> Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> 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.


> jon