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

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Message-Id: <44E6353E-FFC1-4414-A5D7-2EF249A090BE@codeweavers.com>
Date:   Mon, 8 Jun 2020 11:13:44 -0700
From:   Brendan Shanks <bshanks@...eweavers.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        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] x86/umip: Add emulation/spoofing for SLDT and STR
 instructions


> On Jun 5, 2020, at 3:42 PM, Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:
> 
> On Fri, Jun 05, 2020 at 11:58:13AM -0700, Brendan Shanks wrote:
>> 
>>> On Jun 3, 2020, at 9:39 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>> 
>>> On Wed, Jun 3, 2020 at 5:12 PM Ricardo Neri
>>> <ricardo.neri-calderon@...ux.intel.com <mailto:ricardo.neri-calderon@...ux.intel.com>> wrote:
>>>> 
>>>> On Tue, Jun 02, 2020 at 11:42:12AM -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>
>>>>> ---
>>>>> arch/x86/kernel/umip.c | 23 ++++++++++++++---------
>>>>> 1 file changed, 14 insertions(+), 9 deletions(-)
>>>>> 
>>>>> diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
>>>>> index 8d5cbe1bbb3b..59dfceac5cc0 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
>>>>> +
>>>>> /*
>>>>> * The SGDT and SIDT instructions store the contents of the global descriptor
>>>>> * table and interrupt table registers, respectively. The destination is a
>>>>> @@ -244,16 +246,24 @@ static int emulate_umip_insn(struct insn *insn, int umip_inst,
>>>>>             *data_size += UMIP_GDT_IDT_LIMIT_SIZE;
>>>>>             memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
>>>>> 
>>>>> -     } else if (umip_inst == UMIP_INST_SMSW) {
>>>>> -             unsigned long dummy_value = CR0_STATE;
>>>>> +     } else if (umip_inst == UMIP_INST_SMSW || umip_inst == UMIP_INST_SLDT ||
>>>>> +                umip_inst == UMIP_INST_STR) {
>>>>> +             unsigned long dummy_value;
>>>>> +
>>>>> +             if (umip_inst == UMIP_INST_SMSW)
>>>>> +                     dummy_value = CR0_STATE;
>>>>> +             else if (umip_inst == UMIP_INST_STR)
>>>>> +                     dummy_value = UMIP_DUMMY_TASK_REGISTER_SELECTOR;
>>>>> +             else
>>>>> +                     dummy_value = 0;
>>>> 
>>>> Perhaps you can return a non-zero value for SLDT if it has an LDT, as
>>>> Andy had suggested. Maybe this can be implemented by looking at
>>>> current->mm->context.ldt
>>>> 
>>>> I guess the non-zero value can be (GDT_ENTRY_LDT*8).
>>> 
>>> You could probably even get away with always returning a nonzero
>>> value.  After all, an empty LDT is quite similar to no LDT.
>> 
>> 
>> Is something like this what you both had in mind?
> 
>> I don’t have any software handy to test the LDT-present case though.
> 
> Perhaps you can insert a test in the kernel selftest. Something like
> this (based on Andreas' test program):
> 
> --- a/tools/testing/selftests/x86/ldt_gdt.c
> +++ b/tools/testing/selftests/x86/ldt_gdt.c
> @@ -220,12 +220,23 @@ static void install_invalid(const struct user_desc *desc, bool oldmode)
> 	}
> }
> 
> +unsigned long test(void)
> +{
> +	 unsigned char ldtr[5] = "\xef\xbe\xad\xde";
> +	 unsigned long ldt = 0;
> +	 asm("sldt %0\n" : "=m" (ldtr));
> +	 ldt = *((unsigned long *)&ldtr[0]);
> +	 printf ("LDT base: 0x%lx\n", ldt);
> +	 return (ldt);
> +}
> +
> static int safe_modify_ldt(int func, struct user_desc *ptr,
>                           unsigned long bytecount)
> {
> 	int ret = syscall(SYS_modify_ldt, 0x11, ptr, bytecount);
> 	if (ret < -1)
> 	errno = -ret;
> +	test();
> 	return ret;
> }

Thanks for the tip, I gave that a try and got the same results under UMIP and a non-UMIP system. I’ll send this next version of the patch out.

Brendan Shanks
CodeWeavers