linux-kernel - Re: [PATCH v2] x86/umip: Add emulation/spoofing for SLDT and STR instructions

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Date:   Mon, 8 Jun 2020 15:36:18 -0700
From:   Brendan Shanks <bshanks@...eweavers.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andreas Rammhold <andi@...much.email>,
        "Moger, Babu" <Babu.Moger@....com>
Subject: Re: [PATCH v2] x86/umip: Add emulation/spoofing for SLDT and STR
 instructions


> On Jun 8, 2020, at 2:53 PM, Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:
> 
> On Mon, Jun 08, 2020 at 11:14:54AM -0700, Brendan Shanks wrote:
>> Add emulation/spoofing of SLDT and STR for both 32- and 64-bit
>> processes.
>> 
>> Wine users have found a small number of Windows apps using SLDT that
>> were crashing when run on UMIP-enabled systems.
>> 
>> Reported-by: Andreas Rammhold <andi@...much.email>
>> Originally-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
>> Signed-off-by: Brendan Shanks <bshanks@...eweavers.com>
>> ---
>> 
>> v2: Return (GDT_ENTRY_LDT * 8) for SLDT when an LDT is set.
>> 
>> arch/x86/kernel/umip.c | 34 +++++++++++++++++++++++++---------
>> 1 file changed, 25 insertions(+), 9 deletions(-)
>> 
>> diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
>> index 8d5cbe1bbb3b..a85f0b0ec2b9 100644
>> --- a/arch/x86/kernel/umip.c
>> +++ b/arch/x86/kernel/umip.c
>> @@ -64,6 +64,8 @@
>> #define UMIP_DUMMY_GDT_BASE 0xfffffffffffe0000ULL
>> #define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL
>> 
>> +#define UMIP_DUMMY_TASK_REGISTER_SELECTOR 0x40
> 
> One more thing. How was this value selected? Would it be possible to use
> GDT_ENTRY_TSS*8? Linux already uses this value.

I used 0x40 because ‘sldt’ returned that value on every system I tested. GDT_ENTRY_TSS*8 also equals 0x40 (for 64-bit capable kernels), yes I can use that instead.


Thank you,

Brendan Shanks
CodeWeavers