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

[Andy Lutomirski <luto@amacapital.net>]

> On May 20, 2020, at 5:55 PM, Ricardo Neri <ricardo.neri-calderon@linux.intel.com> wrote:
> 
> On Tue, May 19, 2020 at 05:54:53PM -0700, Andy Lutomirski wrote:
>>> On Tue, May 19, 2020 at 12:43 PM Ricardo Neri
>>> <ricardo.neri-calderon@linux.intel.com> 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@notmuch.email> 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 wasn’t 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 you’re 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, I’ll 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.
> 
> I guess the value should be in the same hole of the x86_64 memory map,
> right? Currently sgdt and sidt return 0xfffffffffffe0000 and
> 0xffffffffffff0000, respectively.

As far as I’m concerned, it makes no difference whether it’s a hole in the memory map.  But sure.

> 
> Thanks and BR,
> Ricardo