linux-kernel - Re: [PATCH v2 2/3] kvm, vmx: move register clearing out of assembly path

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Date:   Mon, 29 Oct 2018 10:26:08 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Julian Stecklina <jsteckli@...zon.de>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Julian Stecklina <js@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] kvm, vmx: move register clearing out of assembly
 path

On Mon, Oct 29, 2018 at 04:40:43PM +0100, Julian Stecklina wrote:
> Split the security related register clearing out of the large inline
> assembly VM entry path. This results in two slightly less complicated
> inline assembly statements, where it is clearer what each one does.
> 
> Signed-off-by: Julian Stecklina <jsteckli@...zon.de>
> Reviewed-by: Jan H. Schönherr <jschoenh@...zon.de>
> Reviewed-by: Konrad Jan Miller <kjm@...zon.de>
> Reviewed-by: Jim Mattson <jmattson@...gle.com>
> ---
>  arch/x86/kvm/vmx.c | 45 ++++++++++++++++++++++++++++-----------------
>  1 file changed, 28 insertions(+), 17 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index a6e5a5c..29a2ee7 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -11281,24 +11281,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  		"mov %%r13, %c[r13](%0) \n\t"
>  		"mov %%r14, %c[r14](%0) \n\t"
>  		"mov %%r15, %c[r15](%0) \n\t"
> -		/*
> -		* Clear host registers marked as clobbered to prevent
> -		* speculative use.
> -		*/
> -		"xor %%r8d,  %%r8d \n\t"
> -		"xor %%r9d,  %%r9d \n\t"
> -		"xor %%r10d, %%r10d \n\t"
> -		"xor %%r11d, %%r11d \n\t"
> -		"xor %%r12d, %%r12d \n\t"
> -		"xor %%r13d, %%r13d \n\t"
> -		"xor %%r14d, %%r14d \n\t"
> -		"xor %%r15d, %%r15d \n\t"
>  #endif
> -
> -		"xor %%eax, %%eax \n\t"
> -		"xor %%ebx, %%ebx \n\t"
> -		"xor %%esi, %%esi \n\t"
> -		"xor %%edi, %%edi \n\t"
>  		"pop  %%" _ASM_BP "; pop  %%" _ASM_DX " \n\t"
>  		".pushsection .rodata \n\t"
>  		".global vmx_return \n\t"
> @@ -11336,6 +11319,34 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  	      );
>  
>  	/*
> +	 * Don't let guest register values survive. Registers that cannot
> +	 * contain guest values anymore are not touched.

I think it's a good idea to explicitly call out that clearing the GPRs
is done to prevent speculative use.  Simply stating that we don't want
to let guest register values survive doesn't explain *why*.

What about:

	/*
	 * Explicitly clear (in addition to marking them as clobbered) all GPRs
	 * that have not been loaded with host state to prevent speculatively
	 * using the guest's values.
	 */

> +	 */
> +	asm volatile (
> +		"xor %%eax, %%eax \n\t"
> +		"xor %%ebx, %%ebx \n\t"
> +		"xor %%esi, %%esi \n\t"
> +		"xor %%edi, %%edi \n\t"
> +#ifdef CONFIG_X86_64
> +		"xor %%r8d,  %%r8d \n\t"
> +		"xor %%r9d,  %%r9d \n\t"
> +		"xor %%r10d, %%r10d \n\t"
> +		"xor %%r11d, %%r11d \n\t"
> +		"xor %%r12d, %%r12d \n\t"
> +		"xor %%r13d, %%r13d \n\t"
> +		"xor %%r14d, %%r14d \n\t"
> +		"xor %%r15d, %%r15d \n\t"
> +#endif
> +		::: "cc"
> +#ifdef CONFIG_X86_64
> +		 , "rax", "rbx", "rsi", "rdi"
> +		 , "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
> +#else
> +		 , "eax", "ebx", "esi", "edi"
> +#endif
> +		);
> +
> +	/*
>  	 * We do not use IBRS in the kernel. If this vCPU has used the
>  	 * SPEC_CTRL MSR it may have left it on; save the value and
>  	 * turn it off. This is much more efficient than blindly adding
> -- 
> 2.7.4
>