linux-kernel - Re: [PATCH v3] x86/umip: Add emulation/spoofing for SLDT and STR instructions

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Message-ID: <20200609003812.GA26268@ranerica-svr.sc.intel.com>
Date:   Mon, 8 Jun 2020 17:38:12 -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 v3] x86/umip: Add emulation/spoofing for SLDT and STR
 instructions

On Mon, Jun 08, 2020 at 03:44:24PM -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>
> ---
> 
> v3: Use (GDT_ENTRY_TSS * 8) for task register selector instead of
> harcoding 0x40.
> 
>  arch/x86/kernel/umip.c | 32 +++++++++++++++++++++++---------
>  1 file changed, 23 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> index 8d5cbe1bbb3b..166c579b0273 100644
> --- a/arch/x86/kernel/umip.c
> +++ b/arch/x86/kernel/umip.c
> @@ -244,16 +244,35 @@ 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 = GDT_ENTRY_TSS * 8;
> +		else if (umip_inst == UMIP_INST_SLDT)
> +		{

This brace should go in the previous line. Also, if you use braces in
the last part of the conditional you should probably use them in the
previous ones. I guess in this case it woudln't improve readability.
Instead, you can probably have a switch instead of the three ifs. That
probably does improve readability and solves the dilemma of needing to
put braces in all the one-line conditionals.

BTW, you should also delete the comment at the top of the file saying
that str and sldt will not be emulated:

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 166c579b0273..0984a55eb8c0 100644
--- a/arch/x86/kernel/umip.c
+++ b/arch/x86/kernel/umip.c
@@ -45,9 +45,6 @@
  * value that, lies close to the top of the kernel memory. The limit for the GDT
  * and the IDT are set to zero.
  *
- * Given that SLDT and STR are not commonly used in programs that run on WineHQ
- * or DOSEMU2, they are not emulated.
- *
  * The instruction smsw is emulated to return the value that the register CR0
  * has at boot time as set in the head_32.
  *

Thanks and BR,
Ricardo