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

Andy Lutomirski
2017-03-08T16:46:20Z

On Wed, Mar 8, 2017 at 8:29 AM, Stas Sergeev <stsp@list.ru> wrote:
> 08.03.2017 19:06, Andy Lutomirski пишет:
>>
>> On Wed, Mar 8, 2017 at 6:08 AM, Stas Sergeev <stsp@list.ru> wrote:
>>>
>>> 08.03.2017 03:32, Ricardo Neri пишет:
>>>>
>>>> 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.

Not sure. Ricardo?

>
>>> 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.

That's what I meant. Turning off FIXUP_UMIP would leave UMIP on but
turn off the fixup, so you'd get a SIGSEGV indicating #GP (or a vm86
GP exit).

--Andy