Re: [PATCH v7 24/26] x86: Enable User-Mode Instruction Prevention

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On Tue, Jul 25, 2017 at 05:44:08PM -0700, Ricardo Neri wrote:
> On Fri, 2017-06-09 at 18:10 +0200, Borislav Petkov wrote:
> > On Fri, May 05, 2017 at 11:17:22AM -0700, Ricardo Neri wrote:
> > > User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
> > > bit in %cr4.
> > > 
> > > It makes sense to enable UMIP at some point while booting, before user
> > > spaces come up. Like SMAP and SMEP, is not critical to have it enabled
> > > very early during boot. This is because UMIP is relevant only when there is
> > > a userspace to be protected from. Given the similarities in relevance, it
> > > makes sense to enable UMIP along with SMAP and SMEP.
> > > 
> > > UMIP is enabled by default. It can be disabled by adding clearcpuid=514
> > > to the kernel parameters.


> So would this become a y when more machines have UMIP?

I guess. Stuff which proves reliable and widespread gets automatically
enabled with time, in most cases. IMHO, of course.

> Why would static_cpu_has() reply wrong if alternatives are not in place?
> Because it uses the boot CPU data? When it calls _static_cpu_has() it
> would do something equivalent to

Nevermind - I forgot that static_cpu_has() now drops to dynamic check
before alternatives application.


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