Re: umip: AMD Ryzen 3900X, pagefault after emulate SLDT/SIDT instruction

[Andy Lutomirski]

On Tue, May 19, 2020 at 12:43 PM Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
>
> On Tue, May 19, 2020 at 11:56:40AM -0700, Brendan Shanks wrote:
> >
> > > On May 19, 2020, at 7:38 AM, Andreas Rammhold <andi@xxxxxxxxxxxxx> wrote:
> > >
> > > Hi,
> > >
> > > I've been running into a weird problem with UMIP on a current Ryzen
> > > 3900x with kernel 5.6.11 where a process receives a page fault after the
> > > kernel handled the SLDT (or SIDT) instruction (emulation).
> > >
> > > The program I am running is run through WINE in 32bit mode and tries to
> > > figure out if it is running in a VMWare machine by comparing the results
> > > of SLDT against well known constants (basically as shown in the
> > > [example] linked below).
> > >
> > > In dmesg I see the following log lines:
> > >> [99970.004756] umip: Program.exe[3080] ip:4373fb sp:32f3e0: SIDT instruction cannot be used by applications.
> > >> [99970.004757] umip: Program.exe[3080] ip:4373fb sp:32f3e0: For now, expensive software emulation returns the result.
> > >> [99970.004758] umip: Program.exe[3080] ip:437415 sp:32f3e0: SLDT instruction cannot be used by applications.
> > >
> > > Following that the process terminates with a page fault:
> > >> Unhandled exception: page fault on read access to 0xffffffff in 32-bit code (0x0000000000437415).
> > >
> > > Assembly at that address:
> > >> 0x0000000000437415: sldt    0xffffffe8(%ebp)
> > >
> > > Running the same executable on the exact same kernel (and userland) but
> > > on a Intel i7-8565U doesn't crash at this point. I am guessing the
> > > emulation is supposed to do something different on AMD CPUs?
>
> I am surprised you don't see it on the Intel processor. Maybe it does
> not have UMIP. Do you see umip when you do
>
> $ grep umip /proc/cpuinfo
>
> ?
> > >
> > > On the Ryzen the code executes successfully after setting CONFIG_X86_UMIP=n.
> >
> > Hi Andreas,
> >
> > The problem is that the kernel does not emulate/spoof the SLDT instruction, only SGDT, SIDT, and SMSW.
> > SLDT and STR weren't thought to be commonly used, so emulation/spoofing wasnât added.
> > In the last few months I have seen reports of one or two (32-bit) Windows games that use SLDT though.
> > Can you share more information about the application youâre running?
> >
> > Maybe the best path is to add kernel emulation/spoofing for SLDT and STR on 32 and 64-bit, just to cover all the cases. It should be a pretty simple patch, Iâll start working on it.
>
> I have a patch for this already that I wrote for testing purposes:
>
> https://github.com/ricardon/tip/commit/1692889cb3f8accb523d44b682458e234b93be50
>
> Perhaps it can be used as a starting point? Not sure what the spoofing
> value should be, though. Perhaps 0?

Possibly SLDT should return nonzero if there's an LDT.

--Andy