Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

[Stas Sergeev]

08.03.2017 19:06, Andy Lutomirski ÐÐÑÐÑ:
> On Wed, Mar 8, 2017 at 6:08 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 08.03.2017 03:32, Ricardo Neri ÐÐÑÐÑ:
> > > These are the instructions covered by UMIP:
> > > * SGDT - Store Global Descriptor Table
> > > * SIDT - Store Interrupt Descriptor Table
> > > * SLDT - Store Local Descriptor Table
> > > * SMSW - Store Machine Status Word
> > > * STR - Store Task Register
> > >
> > > This patchset initially treated tasks running in virtual-8086 mode as a
> > > special case. However, I received clarification that DOSEMU[8] does not
> > > support applications that use these instructions.
> Can you remind me what was special about it?  It looks like you still
> emulate them in v8086 mode.
Indeed, sorry, I meant prot mode here. :)
So I wonder what was cited to be special about v86.

> > Yes, this is the case.
> > But at least in the past there was an attempt to
> > support SLDT as it is used by an ancient pharlap
> > DOS extender (currently unsupported by dosemu1/2).
> > So how difficult would it be to add an optional
> > possibility of delivering such SIGSEGV to userspace
> > so that the kernel's dummy emulation can be overridden?
> > It doesn't need to be a matter of this particular
> > patch set, i.e. this proposal should not trigger a
> > v7 resend of all 21 patches. :) But it would be useful
> > for the future development of dosemu2.
> What I'd actually like to see is a totally separate patchset that adds
> an inheritable (but reset on exec) per-task mask of legacy
> compatibility features to disable.  Maybe:
>
> sys_adjust_compat_mask(int op, int word, u32 mask);
No no, since I meant prot mode, this is not what I need.
I would never need to disable UMIP as to allow the
prot mode apps to do SLDT. Instead it would be good
to have an ability to provide a replacement for the dummy
emulation that is currently being proposed for kernel.
All is needed for this, is just to deliver a SIGSEGV.