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

[Thomas Gleixner]

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.

Thanks,

	tglx