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

From: Ricardo Neri
Date: Fri Mar 10 2017 - 19:13:30 EST

On Sat, 2017-03-11 at 02:58 +0300, Stas Sergeev wrote:
> 11.03.2017 02:47, Ricardo Neri ÐÐÑÐÑ:
> >>
> >>>> 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.
> >>> Would dosemu2 use 32-bit processes in order to keep segmentation? If it
> >>> could use 64-bit processes, emulation is not used in this case and the
> >>> SIGSEGV is delivered to user space.
> >> It does use the mix: 64bit process but some segments
> >> are 32bit for DOS code.
> > Do you mean that dosemu2 will start as a 64-bit process and will jump to
> > 32-bit code segments?
> Yes, so the offending insns are executed only in 32bit
> and 16bit segments, even if the process itself is 64bit.
> I guess you handle 16bit segments same as 32bit ones.

I have code to handle 16-bit and 32-bit address encodings differently.
Segmentation is used if !user_64bit_mode(regs). In such a case, the
emulation code will check the segment descriptor D flag and the
address-size overrides prefix to determine the address size and use
16-bit or 32-bit address encodings as applicable.

> > My emulation code should work in this case as it
> > will use segmentation in 32-bit code descriptors. Is there anything else
> > needed?
> If I understand you correctly, you are saying that SLDT
> executed in 64bit code segment, will inevitably segfault
> to userspace.

> If this is the case and it makes your code
> simpler, then its perfectly fine with me as dosemu does
> not do this and the 64bit DOS progs are not anticipated.

But if 32-bit or 16-bit code segments are used emulation will be used.

Thanks and BR,