linux-kernel - Re: [PATCH v3 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls with non-zero "var

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Date:   Thu, 09 Dec 2021 10:55:50 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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 v3 7/8] KVM: x86: Reject fixeds-size Hyper-V hypercalls
 with non-zero "var_cnt"

Sean Christopherson <seanjc@...gle.com> writes:

> Reject Hyper-V hypercalls if the guest specifies a non-zero variable size
> header (var_cnt in KVM) for a hypercall that has a fixed header size.
> Per the TLFS:
>
>   It is illegal to specify a non-zero variable header size for a
>   hypercall that is not explicitly documented as accepting variable sized
>   input headers. In such a case the hypercall will result in a return
>   code of HV_STATUS_INVALID_HYPERCALL_INPUT.
>
> Note, at least some of the various DEBUG commands likely aren't allowed
> to use variable size headers, but the TLFS documentation doesn't clearly
> state what is/isn't allowed.  Omit them for now to avoid unnecessary
> breakage.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/hyperv.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index f33a5e890048..522ccd2f0db4 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -2250,14 +2250,14 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  
>  	switch (hc.code) {
>  	case HVCALL_NOTIFY_LONG_SPIN_WAIT:
> -		if (unlikely(hc.rep)) {
> +		if (unlikely(hc.rep || hc.var_cnt)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}
>  		kvm_vcpu_on_spin(vcpu, true);
>  		break;
>  	case HVCALL_SIGNAL_EVENT:
> -		if (unlikely(hc.rep)) {
> +		if (unlikely(hc.rep || hc.var_cnt)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}
> @@ -2267,7 +2267,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  		fallthrough;	/* maybe userspace knows this conn_id */
>  	case HVCALL_POST_MESSAGE:
>  		/* don't bother userspace if it has no way to handle it */
> -		if (unlikely(hc.rep || !to_hv_synic(vcpu)->active)) {
> +		if (unlikely(hc.rep || hc.var_cnt || !to_hv_synic(vcpu)->active)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}
> @@ -2280,14 +2280,14 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  				kvm_hv_hypercall_complete_userspace;
>  		return 0;
>  	case HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST:
> -		if (unlikely(!hc.rep_cnt || hc.rep_idx)) {
> +		if (unlikely(!hc.rep_cnt || hc.rep_idx || hc.var_cnt)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}
>  		ret = kvm_hv_flush_tlb(vcpu, &hc, false);
>  		break;
>  	case HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE:
> -		if (unlikely(hc.rep)) {
> +		if (unlikely(hc.rep || hc.var_cnt)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}
> @@ -2308,7 +2308,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>  		ret = kvm_hv_flush_tlb(vcpu, &hc, true);
>  		break;
>  	case HVCALL_SEND_IPI:
> -		if (unlikely(hc.rep)) {
> +		if (unlikely(hc.rep || hc.var_cnt)) {
>  			ret = HV_STATUS_INVALID_HYPERCALL_INPUT;
>  			break;
>  		}

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly