Re: [PATCH] KVM: x86/hyperv: do not overwrite hc->ingpa for slow SIGNAL_EVENT hypercall

[Sean Christopherson <seanjc@google.com>]

On Fri, Sep 18, 2026, Paolo Bonzini wrote:
> When a guest makes a slow HVCALL_SIGNAL_EVENT hypercall with a connection ID
> that is valid in userspace but not registered in the kernel conn_to_evt,
> kvm_hvcall_signal_event() reads the connection ID and overwrites hc->ingpa.
> However, hc->param still signals that the hypercall was a slow one, and
> userspace will then treat the connection ID as an address.
> 
> Cc: stable@vger.kernel.org
> Fixes: bd38b32053eb ("KVM: hyper-v: Collect hypercall params into struct")
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>         originally reported by sashiko at
>         https://lore.kernel.org/kvm/20260918083322.C87F11F000FF@smtp.kernel.org/,
>         but the issue is preexisting and unrelated to the patch that was being
>         reviewed.
> 
>  arch/x86/kvm/hyperv.c | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index 9f5adcd26cba..e3a8e8236230 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -2501,6 +2501,7 @@ static int kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *h
>  {
>  	struct kvm_hv *hv = to_kvm_hv(vcpu->kvm);
>  	struct eventfd_ctx *eventfd;
> +	u64 conn_id;
>  	int ret;

This doesn't apply to any branch I can find, and there is some unnecessary variable
shadowing going on here as well.

The actual change looks good, but the diff is wonky.

>  	ret = kvm_hv_hypercall_check_params(vcpu, hc);
> @@ -2511,14 +2512,16 @@ static int kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *h
>  		int ret;
>  		gpa_t gpa = hc->ingpa;
>  
> -		if ((gpa & (__alignof__(hc->ingpa) - 1)) ||
> -		    offset_in_page(gpa) + sizeof(hc->ingpa) > PAGE_SIZE)
> +		if ((gpa & (__alignof__(conn_id) - 1)) ||
> +		    offset_in_page(gpa) + sizeof(conn_id) > PAGE_SIZE)
>  			return HV_STATUS_INVALID_ALIGNMENT;
>  
>  		ret = kvm_vcpu_read_guest(vcpu, gpa,
> -					  &hc->ingpa, sizeof(hc->ingpa));
> +					  &conn_id, sizeof(conn_id));
>  		if (ret < 0)
>  			return HV_STATUS_INVALID_ALIGNMENT;
> +	} else {
> +		conn_id = hc->ingpa;
>  	}
>  
>  	/*
> @@ -2526,15 +2529,15 @@ static int kvm_hvcall_signal_event(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *h
>  	 * have no use for it, and in all known usecases it is zero, so just
>  	 * report lookup failure if it isn't.
>  	 */
> -	if (hc->ingpa & 0xffff00000000ULL)
> +	if (conn_id & 0xffff00000000ULL)
>  		return HV_STATUS_INVALID_PORT_ID;
>  	/* remaining bits are reserved-zero */
> -	if (hc->ingpa & ~KVM_HYPERV_CONN_ID_MASK)
> +	if (conn_id & ~KVM_HYPERV_CONN_ID_MASK)
>  		return HV_STATUS_INVALID_HYPERCALL_INPUT;
>  
>  	/* the eventfd is protected by vcpu->kvm->srcu, but conn_to_evt isn't */
>  	rcu_read_lock();
> -	eventfd = idr_find(&hv->conn_to_evt, hc->ingpa);
> +	eventfd = idr_find(&hv->conn_to_evt, conn_id);
>  	rcu_read_unlock();
>  	if (!eventfd)
>  		return HV_STATUS_INVALID_PORT_ID;
> -- 
> 2.52.0
>