Re: [Patch v7 10/18] x86/speculation: Turn on or off STIBP according to a task's TIF_STIBP

[Thomas Gleixner]

On Tue, 20 Nov 2018, Tim Chen wrote:
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 74bef48..48fcd46 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -406,6 +406,8 @@ static __always_inline void spec_ctrl_update_msr(unsigned long tifn)
>  	if (static_cpu_has(X86_FEATURE_SSBD))
>  		msr |= ssbd_tif_to_spec_ctrl(tifn);
>  
> +	msr |= stibp_tif_to_spec_ctrl(tifn);

This is wrong. If STIBP is disabled, but the TIF flag is set to control
IBPB and this path is entered to handle a SSBD change, then this will set
STIBP as well.

> +
>  	wrmsrl(MSR_IA32_SPEC_CTRL, msr);
>  }
>  
> @@ -418,7 +420,17 @@ static __always_inline void spec_ctrl_update_msr(unsigned long tifn)
>  static __always_inline void __speculation_ctrl_update(unsigned long tifp,
>  						      unsigned long tifn)
>  {
> -	bool updmsr = false;
> +	bool updmsr;
> +
> +	/*
> +	 * Need STIBP defense against Spectre v2 attack
> +	 * if SMT is in use and enhanced IBRS is unsupported.
> +	 */
> +	if (static_cpu_has(X86_FEATURE_STIBP) && cpu_use_smt_and_hotplug &&
> +	    !static_cpu_has(X86_FEATURE_USE_IBRS_ENHANCED))
> +		updmsr = !!((tifp ^ tifn) & _TIF_SPEC_INDIR_BRANCH);

At the end of the series this has then two alternative patch bits and two
static keys. This can be done with a single static key which is controlled
from the bug code.

I'll send out a cleaned up version of that stuff later today.

Thanks,

	tglx