> On May 20, 2020, at 5:55 PM, Ricardo Neri > <ricardo.neri-calde...@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-calde...@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
