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

[Brendan Shanks]

> 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?
> 
> 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.

Alternately, I did work on a Wine patch to emulate the UMIP instructions in user-space, but it adds a lot of code and I donât think thereâs much appetite for it in upstream Wine (especially since the kernel emulation is sufficient for almost all cases).
https://www.winehq.org/pipermail/wine-devel/2020-February/160027.html

In the meantime, an easy way to disable UMIP without rebuilding the kernel is to pass 'clearcpuid=514â on the kernel command line.

Brendan Shanks
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