Re: [PATCH v2 2/8] x86/enter: MACROS to set/clear IBRS

[Borislav Petkov]

On Fri, Jan 05, 2018 at 06:12:17PM -0800, Tim Chen wrote:

> Subject: Re: [PATCH v2 2/8] x86/enter: MACROS to set/clear IBRS

Your subject needs to have a verb and not scream:

Subject: [PATCH v2 2/8] x86/entry: Add macros to set/clear IBRS

> Create macros to control IBRS.  Use these macros to enable IBRS on kernel entry
> paths and disable IBRS on kernel exit paths.
> 
> The registers rax, rcx and rdx are touched when controlling IBRS
> so they need to be saved when they can't be clobbered.
> 
> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/entry/calling.h | 74 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 74 insertions(+)
> 
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index 45a63e0..09c870d 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -6,6 +6,8 @@
>  #include <asm/percpu.h>
>  #include <asm/asm-offsets.h>
>  #include <asm/processor-flags.h>
> +#include <asm/msr-index.h>
> +#include <asm/cpufeatures.h>
>  
>  /*
>  
> @@ -347,3 +349,75 @@ For 32-bit we have the following conventions - kernel is built with
>  .Lafter_call_\@:
>  #endif
>  .endm
> +
> +/*
> + * IBRS related macros
> + */
> +.macro PUSH_MSR_REGS
> +	pushq	%rax
> +	pushq	%rcx
> +	pushq	%rdx
> +.endm
> +
> +.macro POP_MSR_REGS
> +	popq	%rdx
> +	popq	%rcx
> +	popq	%rax
> +.endm
> +
> +.macro WRMSR_ASM msr_nr:req eax_val:req

WRMSR as a name is good enough.

Also, you need edx_val:req too in case we decide to reuse that macro
for something else later. Which I'm pretty sure we will, once it is out
there.

> +	movl	\msr_nr, %ecx
> +	movl	$0, %edx

... and then

	movl	\edx_val, %edx

> +	movl	\eax_val, %eax
> +.endm
> +
> +.macro ENABLE_IBRS
> +	ALTERNATIVE "jmp .Lskip_\@", "", X86_FEATURE_SPEC_CTRL
> +	PUSH_MSR_REGS
> +	WRMSR_ASM $MSR_IA32_SPEC_CTRL, $SPEC_CTRL_FEATURE_ENABLE_IBRS

This is overwriting the previous contents of the MSR. You need to read
it and OR-in its bits [63:2] with SPEC_CTRL_FEATURE_ENABLE_IBRS and
clear bit 0.

Unless the rest of this MSR is not going to be used for anything else.
Then you're fine.

-- 
Regards/Gruss,
    Boris.

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