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

From: Stas Sergeev
Date: Thu Mar 09 2017 - 20:28:19 EST

09.03.2017 03:46, Ricardo Neri ÐÐÑÐÑ:
On Wed, 2017-03-08 at 17:08 +0300, Stas Sergeev 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.
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?
I suppose a umip=noemulation kernel parameter could be added in this
It doesn't need to be global: the app should be
able to change that on its own. Note that no app currently
requires this, so its just for the future, and in the
future the app can start using the new API for this,
if you provide one.

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.