linux-kernel - Re: [PATCH v2] KVM: PPC: Book3S HV: relax check on H_SVM_INIT

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Message-ID: <20200521060858.GA5278@oc0525413822.ibm.com>
Date:   Wed, 20 May 2020 23:08:58 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Laurent Dufour <ldufour@...ux.ibm.com>
Cc:     kvm-ppc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, paulus@...ba.org, groug@...d.org,
        mpe@...erman.id.au, sukadev@...ux.ibm.com
Subject: Re: [PATCH v2] KVM: PPC: Book3S HV: relax check on H_SVM_INIT_ABORT

On Wed, May 20, 2020 at 07:43:08PM +0200, Laurent Dufour wrote:
> The commit 8c47b6ff29e3 ("KVM: PPC: Book3S HV: Check caller of H_SVM_*
> Hcalls") added checks of secure bit of SRR1 to filter out the Hcall
> reserved to the Ultravisor.
> 
> However, the Hcall H_SVM_INIT_ABORT is made by the Ultravisor passing the
> context of the VM calling UV_ESM. This allows the Hypervisor to return to
> the guest without going through the Ultravisor. Thus the Secure bit of SRR1
> is not set in that particular case.
> 
> In the case a regular VM is calling H_SVM_INIT_ABORT, this hcall will be
> filtered out in kvmppc_h_svm_init_abort() because kvm->arch.secure_guest is
> not set in that case.
> 
> Fixes: 8c47b6ff29e3 ("KVM: PPC: Book3S HV: Check caller of H_SVM_* Hcalls")
> Signed-off-by: Laurent Dufour <ldufour@...ux.ibm.com>


Reviewed-by: Ram Pai <linuxram@...ibm.com>

> ---
>  arch/powerpc/kvm/book3s_hv.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 93493f0cbfe8..6ad1a3b14300 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -1099,9 +1099,12 @@ int kvmppc_pseries_do_hcall(struct kvm_vcpu *vcpu)
>  			ret = kvmppc_h_svm_init_done(vcpu->kvm);
>  		break;
>  	case H_SVM_INIT_ABORT:
> -		ret = H_UNSUPPORTED;
> -		if (kvmppc_get_srr1(vcpu) & MSR_S)
> -			ret = kvmppc_h_svm_init_abort(vcpu->kvm);
> +		/*
> +		 * Even if that call is made by the Ultravisor, the SSR1 value
> +		 * is the guest context one, with the secure bit clear as it has
> +		 * not yet been secured. So we can't check it here.
> +		 */

Frankly speaking, the comment above when read in isolation; i.e without
the delete code above, feels out of place.  The reasoning for change is
anyway captured in the changelog.  So, I think, we should delete this
comment.

Also the comment above assumes the Ultravisor will call H_SVM_INIT_ABORT
with SRR1(S) bit not set; which may or may not be true.  Regardless of
who and how H_SVM_INIT_ABORT is called, we should just call
kvmppc_h_svm_init_abort() and let it deal with the complexities.


RP