linux-kernel - Re: [PATCH v2 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls with non-zero "var

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Date:   Fri, 3 Dec 2021 23:48:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ajay Garg <ajaygargnsit@...il.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls
 with non-zero "var_cnt"

On Mon, Nov 01, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> > @@ -2331,6 +2331,11 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
> >  			ret = HV_STATUS_OPERATION_DENIED;
> >  			break;
> >  		}
> > +		if (unlikely(hc.var_cnt)) {
> > +			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
> > +			break;
> > +		}
> > +
> 
> Probably true for HVCALL_RESET_DEBUG_SESSION but I'm not sure about
> HVCALL_POST_DEBUG_DATA/HVCALL_RETRIEVE_DEBUG_DATA (note 'fallthrough'
> above) -- these are not described well in TLFS.

I'll drop the check for all the DEBUG hypercalls and add a note in the changelog
to call out that they're probably not supposed to use var_cnt, but that the TLFS
documentation isn't clear one way or the other.