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

Andy Lutomirski
2017-03-08T16:06:23Z

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.

>
> 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);

op could indicate that we want to so SET, OR, AND, or READ. word
would be 0 for now. It could be a prctl, too.

Things in the mask could include:

COMPAT_MASK0_X86_64_VSYSCALL [1]
COMPAT_MASK0_X86_UMIP_FIXUP

I'm sure I could think of more along these lines.

Then DOSEMU (and future WINE versions, too) could just mask off
X86_UMIP_FIXUP and do their own emulation

[1] For those of you thinking about this and realizing that VSYSCALL
readability is inherently global and not per-task, I know how to fix
that for essentially no cost :)

--Andy