Re: [PATCH 2/4] kvm, vmx: move register clearing out of assembly path

[Sean Christopherson]

On Wed, Oct 24, 2018 at 10:28:57AM +0200, 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@xxxxxxxxx>
> Reviewed-by: Jan H. Schönherr <jschoenh@xxxxxxxxx>
> Reviewed-by: Konrad Jan Miller <kjm@xxxxxxxxx>
> ---
>  arch/x86/kvm/vmx.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 93562d5..9225099 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -10797,20 +10797,7 @@ 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"
> -		"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"
> @@ -10847,6 +10834,26 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  #endif
>  	      );
>  
> +	/* Don't let guest register values survive. */
> +	asm volatile (
> +		""
> +#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
> +		:: "a" (0), "b" (0), "S" (0), "D" (0)

Since clearing the GPRs exists to mitigate speculation junk, I think
we should keep the explicit XOR zeroing instead of deferring to the
compiler.  Explicit XORs will ensure the resulting assembly is the
same regardless of compiler, version, target arch, etc..., whereas the
compiler could theoretically use different zeroing methods[1], e.g. on
my system it generates "mov r32,r32" for EBX, ESI and EDI (loading
from EAX after EAX is zeroed).

And FWIW, I find the original code to be more readable since all GRPs
are zeroed with the same method.


[1] As an aside, I was expecting gcc to generate "xor r32,r32" with
    -mtune=sandybridge as sandybridge can do mov elimination on xors
    that explicitly zero a register but not on generic reg-to-reg mov,
    but I was unable to coerce gcc into using xor.

> +		: "cc"
> +#ifdef CONFIG_X86_64
> +		  , "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
> +#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
> -- 
> 2.7.4
>