2020-06-08 18:17:31

by Brendan Shanks

Subject: [PATCH v2] x86/umip: Add emulation/spoofing for SLDT and STR instructions

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 <[email protected]>
Originally-by: Ricardo Neri <[email protected]>
Signed-off-by: Brendan Shanks <[email protected]>
---

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
+
/*
* The SGDT and SIDT instructions store the contents of the global descriptor
* table and interrupt table registers, respectively. The destination is a
@@ -244,16 +246,35 @@ 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 if (umip_inst == UMIP_INST_SLDT)
+ {
+#ifdef CONFIG_MODIFY_LDT_SYSCALL
+ down_read(&current->mm->context.ldt_usr_sem);
+ if (current->mm->context.ldt)
+ dummy_value = GDT_ENTRY_LDT * 8;
+ else
+ dummy_value = 0;
+ up_read(&current->mm->context.ldt_usr_sem);
+#else
+ dummy_value = 0;
+#endif
+ }

/*
- * Even though the CR0 register has 4 bytes, the number
+ * For these 3 instructions, the number
* of bytes to be copied in the result buffer is determined
* by whether the operand is a register or a memory location.
* If operand is a register, return as many bytes as the operand
* size. If operand is memory, return only the two least
- * siginificant bytes of CR0.
+ * siginificant bytes.
*/
if (X86_MODRM_MOD(insn->modrm.value) == 3)
*data_size = insn->opnd_bytes;
@@ -261,7 +282,6 @@ static int emulate_umip_insn(struct insn *insn, int umip_inst,
*data_size = 2;

memcpy(data, &dummy_value, *data_size);
- /* STR and SLDT are not emulated */
} else {
return -EINVAL;
}
@@ -383,10 +403,6 @@ bool fixup_umip_exception(struct pt_regs *regs)
umip_pr_warn(regs, "%s instruction cannot be used by applications.\n",
umip_insns[umip_inst]);

- /* Do not emulate (spoof) SLDT or STR. */
- if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT)
- return false;
-
umip_pr_warn(regs, "For now, expensive software emulation returns the result.\n");

if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
--
2.26.2

2020-06-08 21:55:15

by Ricardo Neri

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

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 <[email protected]>
> Originally-by: Ricardo Neri <[email protected]>
> Signed-off-by: Brendan Shanks <[email protected]>
> ---
>
> 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.

Thanks and BR,
Ricardo

2020-06-08 22:39:26

by Brendan Shanks

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

> On Jun 8, 2020, at 2:53 PM, Ricardo Neri <[email protected]> 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 <[email protected]>
>> Originally-by: Ricardo Neri <[email protected]>
>> Signed-off-by: Brendan Shanks <[email protected]>
>> ---
>>
>> 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