Re: [PATCH 0/4] x86: enable User-Mode Instruction Prevention

[Ricardo Neri]

On Tue, 2016-11-08 at 17:52 +0100, Thomas Gleixner wrote:
> On Tue, 8 Nov 2016, Andy Lutomirski wrote:
> > On Tue, Nov 8, 2016 at 5:16 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > On Mon, Nov 07, 2016 at 10:12:09PM -0800, Ricardo Neri wrote:
> > >> There is a caveat, however. Certain applications running in virtual-8086
> > >> mode, such as DOSEMU[1] and Wine[2], want to utilize the SGDT, SIDT and
> > >> SLDT instructions for legitimate reasons. In order to keep such
> > >> applications working, UMIP must be disabled/enabled when entering/exiting
> > >> virtual-8086 mode.
> > >
> > > Would it not be better to emulate these instructions for them? What way
> > > we can verify they're not malicious.
> > 
> > Forget malice -- if they are really needed for some silly vm86-using
> > program, let's trap them and emulate them so they return dummy values.
> 
> handle_vm86_fault() already does instruction emulation, so adding the few
> bits there is the right thing to do. Then we just can enable UMIP
> unconditionally and be done with it.

Ah. I didn't think about that. It make sense to me. I will rework this
series with this approach.
> 
> Thanks,
> 
> 	tglx