Re: [PATCH] KVM: x86: prevent setting unsupported XSAVE states

[Gleb Natapov <gleb@redhat.com>]

On Thu, Sep 05, 2013 at 02:21:54PM +0200, Paolo Bonzini wrote:
> A guest can still attempt to save and restore XSAVE states even if they
> have been masked in CPUID leaf 0Dh.  This usually is not visible to
> the guest, but is still wrong: "Any attempt to set a reserved bit (as
> determined by the contents of EAX and EDX after executing CPUID with
> EAX=0DH, ECX= 0H) in XCR0 for a given processor will result in a #GP
> exception".
> 
> The patch also performs the same checks as __kvm_set_xcr in KVM_SET_XSAVE.
> This catches migration from newer to older kernel/processor before the
> guest starts running.
> 
> Cc: kvm@vger.kernel.org
> Cc: Gleb Natapov <gleb@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/cpuid.c |  2 +-
>  arch/x86/kvm/x86.c   | 10 ++++++++--
>  arch/x86/kvm/x86.h   |  1 +
>  3 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index a20ecb5..d7c465d 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -182,7 +182,7 @@ static bool supported_xcr0_bit(unsigned bit)
>  {
>  	u64 mask = ((u64)1 << bit);
>  
> -	return mask & (XSTATE_FP | XSTATE_SSE | XSTATE_YMM) & host_xcr0;
> +	return mask & KVM_SUPPORTED_XCR0 & host_xcr0;
>  }
>  
>  #define F(x) bit(X86_FEATURE_##x)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3625798..801a882 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -586,6 +586,8 @@ int __kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
>  		return 1;
>  	if ((xcr0 & XSTATE_YMM) && !(xcr0 & XSTATE_SSE))
>  		return 1;
> +	if (xcr0 & ~KVM_SUPPORTED_XCR0)
> +		return 1;
Shouldn't we check guest's cpuid here?

>  	if (xcr0 & ~host_xcr0)
>  		return 1;
>  	kvm_put_guest_xcr0(vcpu);
> @@ -2980,10 +2982,14 @@ static int kvm_vcpu_ioctl_x86_set_xsave(struct kvm_vcpu *vcpu,
>  	u64 xstate_bv =
>  		*(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)];
>  
> -	if (cpu_has_xsave)
> +	if (cpu_has_xsave) {
> +		if (xstate_bv & ~KVM_SUPPORTED_XCR0)
> +			return -EINVAL;
> +		if (xstate_bv & ~host_xcr0)
> +			return -EINVAL;
And here?

>  		memcpy(&vcpu->arch.guest_fpu.state->xsave,
>  			guest_xsave->region, xstate_size);
> -	else {
> +	} else {
>  		if (xstate_bv & ~XSTATE_FPSSE)
>  			return -EINVAL;
>  		memcpy(&vcpu->arch.guest_fpu.state->fxsave,
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index e224f7a..587fb9e 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -122,6 +122,7 @@ int kvm_write_guest_virt_system(struct x86_emulate_ctxt *ctxt,
>  	gva_t addr, void *val, unsigned int bytes,
>  	struct x86_exception *exception);
>  
> +#define KVM_SUPPORTED_XCR0	(XSTATE_FP | XSTATE_SSE | XSTATE_YMM)
>  extern u64 host_xcr0;
>  
>  extern struct static_key kvm_no_apic_vcpu;
> -- 
> 1.8.3.1

--
			Gleb.