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

[Ricardo Neri]

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?

Thanks and BR,
Ricardo