Re: [PATCH v11 0/5] Enabling Ring 3 MONITOR/MWAIT feature for Knights Landing

[Pavel Machek]

On Mon 2017-01-09 09:59:42, Thomas Gleixner wrote:
> On Tue, 3 Jan 2017, Pavel Machek wrote:
> 
> > On Tue 2016-12-20 14:48:41, Grzegorz Andrejczuk wrote:
> > > Following patches enable the use of the feature that allows
> > > the Intel Xeon Phi x200 devices to use MONITOR/MWAIT instructions
> > > outside ring 0. It allows userspace applications to use
> > > more efficient synchronization operations, which improves performance
> > > and energy efficiency.
> > 
> > What kind of security holes does it bring us?
> > 
> > rdseed can be used for two processes to communicate when they should
> > not (serious problem for android). Can this be used for something similar?
> 
> No. The monitor address must be in the virtual address space of the
> caller, so it's like any other 'read' operation.

Hmm. Unlike any other 'read' operation, it allows userland to modulate
CPU frequency with great precision; probably allowing userland to
transmit at radio bands, and allowing sound production without
syscalls (singing capacitors).

But userland can probably do that already, with help of
high-resolution timers, so nothing new theree... that I can think off.

Lets wait for the presentation at 39c3 :-).
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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