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

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Date:   Wed, 3 Jun 2020 17:12:43 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Brendan Shanks <bshanks@...eweavers.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, x86@...nel.org, ebiederm@...ssion.com,
        andi@...much.email, Babu.Moger@....com
Subject: Re: [PATCH] x86/umip: Add emulation/spoofing for SLDT and STR
 instructions

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).

Thanks and BR,
Ricardo