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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 19 May 2020 17:54:53 -0700
Message-ID: <CALCETrXxYjrMoenqyxEoBmcitHY22tfpJQAzvS=iF_5+SEZa4w@mail.gmail.com>
Subject: Re: umip: AMD Ryzen 3900X, pagefault after emulate SLDT/SIDT instruction
To:     Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc:     Brendan Shanks <bshanks@codeweavers.com>,
        Andreas Rammhold <andi@notmuch.email>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Babu Moger <Babu.Moger@amd.com>,
        Kefeng Wang <wangkefeng.wang@huawei.com>,
        Jason Yan <yanaijie@huawei.com>,
        LKML <linux-kernel@vger.kernel.org>
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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> wr=
ote:
> > >
> > > 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 resu=
lts
> > > 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 ins=
truction 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 ins=
truction 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-b=
it code (0x0000000000437415).
> > >
> > > Assembly at that address:
> > >> 0x0000000000437415: sldt    0xffffffe8(%ebp)
> > >
> > > Running the same executable on the exact same kernel (and userland) b=
ut
> > > 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=3Dn.
> >
> > Hi Andreas,
> >
> > The problem is that the kernel does not emulate/spoof the SLDT instruct=
ion, only SGDT, SIDT, and SMSW.
> > SLDT and STR weren't thought to be commonly used, so emulation/spoofing=
 wasn=E2=80=99t added.
> > In the last few months I have seen reports of one or two (32-bit) Windo=
ws games that use SLDT though.
> > Can you share more information about the application you=E2=80=99re run=
ning?
> >
> > Maybe the best path is to add kernel emulation/spoofing for SLDT and ST=
R on 32 and 64-bit, just to cover all the cases. It should be a pretty simp=
le patch, I=E2=80=99ll start working on it.
>
> I have a patch for this already that I wrote for testing purposes:
>
> https://github.com/ricardon/tip/commit/1692889cb3f8accb523d44b682458e234b=
93be50
>
> 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