Subject: Re: [PATCH 1/2] KVM: PPC: Book3S HV: check caller of H_SVM_* Hcalls

From: Greg Kurz <[email-masked]>
Date: Fri, 20 Mar 2020 13:22:48 +0100

On Fri, 20 Mar 2020 11:26:42 +0100
Laurent Dufour <[email-masked]> wrote:

> The Hcall named H_SVM_* are reserved to the Ultravisor. However, nothing
> prevent a malicious VM or SVM to call them. This could lead to weird result
> and should be filtered out.
> 
> Checking the Secure bit of the calling MSR ensure that the call is coming
> from either the Ultravisor or a SVM. But any system call made from a SVM
> are going through the Ultravisor, and the Ultravisor should filter out
> these malicious call. This way, only the Ultravisor is able to make such a
> Hcall.

"Ultravisor should filter" ? And what if it doesn't (eg. because of a bug) ?

Shouldn't we also check the HV bit of the calling MSR as well to
disambiguate SVM and UV ?

> 
> Cc: Bharata B Rao <[email-masked]>
> Cc: Paul Mackerras <[email-masked]>
> Cc: Benjamin Herrenschmidt <[email-masked]>
> Cc: Michael Ellerman <[email-masked]>
> Signed-off-by: Laurent Dufour <[email-masked]>
> ---
>  arch/powerpc/kvm/book3s_hv.c | 32 +++++++++++++++++++++-----------
>  1 file changed, 21 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 33be4d93248a..43773182a737 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1074,25 +1074,35 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
>  					 kvmppc_get_gpr(vcpu, 6));
>  		break;
>  	case H_SVM_PAGE_IN:
> -		ret = kvmppc_h_svm_page_in(vcpu->kvm,
> -					   kvmppc_get_gpr(vcpu, 4),
> -					   kvmppc_get_gpr(vcpu, 5),
> -					   kvmppc_get_gpr(vcpu, 6));
> +		ret = H_UNSUPPORTED;
> +		if (kvmppc_get_srr1(vcpu) & MSR_S)
> +			ret = kvmppc_h_svm_page_in(vcpu->kvm,
> +						   kvmppc_get_gpr(vcpu, 4),
> +						   kvmppc_get_gpr(vcpu, 5),
> +						   kvmppc_get_gpr(vcpu, 6));

If calling kvmppc_h_svm_page_in() produces a "weird result" when
the MSR_S bit isn't set, then I think it should do the checking
itself, ie. pass vcpu.

This would also prevent adding that many lines in kvmppc_pseries_do_hcall()
which is a big enough function already. The checking could be done in a
helper in book3s_hv_uvmem.c and used by all UV specific hcalls.

>  		break;
>  	case H_SVM_PAGE_OUT:
> -		ret = kvmppc_h_svm_page_out(vcpu->kvm,
> -					    kvmppc_get_gpr(vcpu, 4),
> -					    kvmppc_get_gpr(vcpu, 5),
> -					    kvmppc_get_gpr(vcpu, 6));
> +		ret = H_UNSUPPORTED;
> +		if (kvmppc_get_srr1(vcpu) & MSR_S)
> +			ret = kvmppc_h_svm_page_out(vcpu->kvm,
> +						    kvmppc_get_gpr(vcpu, 4),
> +						    kvmppc_get_gpr(vcpu, 5),
> +						    kvmppc_get_gpr(vcpu, 6));
>  		break;
>  	case H_SVM_INIT_START:
> -		ret = kvmppc_h_svm_init_start(vcpu->kvm);
> +		ret = H_UNSUPPORTED;
> +		if (kvmppc_get_srr1(vcpu) & MSR_S)
> +			ret = kvmppc_h_svm_init_start(vcpu->kvm);
>  		break;
>  	case H_SVM_INIT_DONE:
> -		ret = kvmppc_h_svm_init_done(vcpu->kvm);
> +		ret = H_UNSUPPORTED;
> +		if (kvmppc_get_srr1(vcpu) & MSR_S)
> +			ret = kvmppc_h_svm_init_done(vcpu->kvm);
>  		break;
>  	case H_SVM_INIT_ABORT:
> -		ret = kvmppc_h_svm_init_abort(vcpu->kvm);
> +		ret = H_UNSUPPORTED;
> +		if (kvmppc_get_srr1(vcpu) & MSR_S)
> +			ret = kvmppc_h_svm_init_abort(vcpu->kvm);
>  		break;
>  
>  	default: