Re: [PATCH 0/4] x86: enable User-Mode Instruction Prevention

[Ricardo Neri]

On Fri, 2016-11-11 at 23:51 +0300, Stas Sergeev wrote:
> 11.11.2016 07:14, Ricardo Neri ÐÐÑÐÑ:
> >> 10.11.2016 09:46, Ricardo Neri ÐÐÑÐÑ:
> >>> I took a closer look at the dosemu code. It appears that it does not
> >>> purposely utilize SGDT to obtain the descriptor table while in vm86. It
> >>> does use SGDT (in protected mode) to emulate certain functionality such
> >>> as the Virtual xxx Driver. In such a case, UMIP needs to be disabled.
> >>> However, this code seems to be disabled [1].
> >> Indeed.
> >> The code you've found, was copied from wine, because
> >> dosemu supports windows-3.1. But sgdt is in win32s part
> >> that is disabled in dosemu. It is however enabled in wine, or
> >> at least it was when I ported the VxD code from there. So you
> >> may want to ask wine devs if they still use sgdt and vm86.
> >> In dosemu, if we ever enable win32s support, we won't rely
> >> on sgdt. In fact, when some prot mode program under dosemu
> >> uses GDT selectors, in a fault handler we replace them with
> >> LDT selectors.
> > Actually, the SLDT instruction is also impacted by this feature. This
> We do not support programs that do SLDT.
> The "polite" programs use special DPMI API extension to get
> the selector that covers LDT. That allows us to manage an "ldt
> alias" - memory buffer where we emulate LDT by write-protecting it.
> If we ever support SLDT, we would very much like to trap it
> and provide the pointer to our alias. Some very old dos extenders
> for 286 may start to work with such change, that are currently
> unsupported.

I see.
> 
> > feature, will cause a GP fault if the instructions SGDT, SLDT, SIDT,
> > SMSW or STR are executed with CPL > 0. Would this be a problem for
> > dosemu?
> I am only a bit unsure about SMSW; the rest should be safe.
> Maybe some odd prog would use SMSW to check for FPU?
> Or to check for v86 mode by checking the PE bit?
> I am sure this is very uncommon, and if we find such prog, we
> can add an emulation of that instruction. I am pretty sure no one
> would get sufficiently hurt, but there will likely be 1-2 bug reports
> in our tracker, because if something is possible, then some DOS
> prog did that. :)

Fair enough.
> 
> >   The proposal now is to trap this GPU fault and give fake value
> > for these tables.
> If this fake value will be cooked up by the kernel without delivering
> the signal to dosemu process, then I don't see any problem at all.

Yes, the GP fault will be trapped in the kernel and not delivered to the
user space. All the user space will see is the fake value given by the
kernel.
> Of course you can provide the sane value for smsw.
> If that will go up to dosemu, then some coding may be needed
> on the user-space side.
> 
> > This is good news. This means that we could go ahead and give a fake
> > pointer to the GDT and the other impacted tables?
> Definitely.
> What these fake tables will look like, btw?
> Will they somehow resemble the real ones?
> Visible to user-space?
Since the intention is to hide these tables from the user space, I was
planning on giving 0x0 to all of it.

Thanks and BR,
Ricardo