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Subject: Re: umip: AMD Ryzen 3900X, pagefault after emulate SLDT/SIDT
 instruction
From:   Brendan Shanks <bshanks@codeweavers.com>
In-Reply-To: <20200519143815.cpsd2xfx2kl3khsq@wrt>
Date:   Tue, 19 May 2020 11:56:40 -0700
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.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>, linux-kernel@vger.kernel.org
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References: <20200519143815.cpsd2xfx2kl3khsq@wrt>
To:     Andreas Rammhold <andi@notmuch.email>
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> On May 19, 2020, at 7:38 AM, Andreas Rammhold <andi@notmuch.email> =
wrote:
>=20
> Hi,
>=20
> 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).
>=20
> 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).
>=20
> 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.
>=20
> Following that the process terminates with a page fault:
>> Unhandled exception: page fault on read access to 0xffffffff in =
32-bit code (0x0000000000437415).
>=20
> Assembly at that address:
>> 0x0000000000437415: sldt    0xffffffe8(%ebp)
>=20
> 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?
>=20
> 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 =
instruction, 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) =
Windows games that use SLDT though.
Can you share more information about the application you=E2=80=99re =
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=E2=80=99ll 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 don=E2=80=99t think =
there=E2=80=99s 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=3D514=E2=80=99 on the kernel command line.

Brendan Shanks
CodeWeavers