Commit 70eae030 authored by Paolo Bonzini's avatar Paolo Bonzini
Browse files

KVM: x86/xen: Fix SRCU/RCU usage in readers of evtchn_ports 



The evtchnfd structure itself must be protected by either kvm->lock or
SRCU. Use the former in kvm_xen_eventfd_update(), since the lock is
being taken anyway; kvm_xen_hcall_evtchn_send() instead is a reader and
does not need kvm->lock, and is called in SRCU critical section from the
kvm_x86_handle_exit function.

It is also important to use rcu_read_{lock,unlock}() in
kvm_xen_hcall_evtchn_send(), because idr_remove() will *not*
use synchronize_srcu() to wait for readers to complete.

Remove a superfluous if (kvm) check before calling synchronize_srcu()
in kvm_xen_eventfd_deassign() where kvm has been dereferenced already.

Co-developed-by: default avatarMichal Luczaj <mhal@rbox.co>
Signed-off-by: default avatarMichal Luczaj <mhal@rbox.co>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarDavid Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20221226120320.1125390-3-dwmw2@infradead.org>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 92c58965

arch/x86/kvm/xen.c

+18 −11
Original line number Diff line number Diff line
@@ -1808,20 +1808,23 @@ static int kvm_xen_eventfd_update(struct kvm *kvm, 
{

	u32 port = data->u.evtchn.send_port;

	struct evtchnfd *evtchnfd;

	int ret;



	if (!port || port >= max_evtchn_port(kvm))

		return -EINVAL;



	/* Protect writes to evtchnfd as well as the idr lookup.  */

	mutex_lock(&kvm->lock);

	evtchnfd = idr_find(&kvm->arch.xen.evtchn_ports, port);

	mutex_unlock(&kvm->lock);



	ret = -ENOENT;

	if (!evtchnfd)

		return -ENOENT;

		goto out_unlock;



	/* For an UPDATE, nothing may change except the priority/vcpu */

	ret = -EINVAL;

	if (evtchnfd->type != data->u.evtchn.type)

		return -EINVAL;

		goto out_unlock;



	/*

	 * Port cannot change, and if it's zero that was an eventfd
@@ -1829,20 +1832,21 @@ static int kvm_xen_eventfd_update(struct kvm *kvm, 
	 */

	if (!evtchnfd->deliver.port.port ||

	    evtchnfd->deliver.port.port != data->u.evtchn.deliver.port.port)

		return -EINVAL;

		goto out_unlock;



	/* We only support 2 level event channels for now */

	if (data->u.evtchn.deliver.port.priority != KVM_IRQ_ROUTING_XEN_EVTCHN_PRIO_2LEVEL)

		return -EINVAL;

		goto out_unlock;



	mutex_lock(&kvm->lock);

	evtchnfd->deliver.port.priority = data->u.evtchn.deliver.port.priority;

	if (evtchnfd->deliver.port.vcpu_id != data->u.evtchn.deliver.port.vcpu) {

		evtchnfd->deliver.port.vcpu_id = data->u.evtchn.deliver.port.vcpu;

		evtchnfd->deliver.port.vcpu_idx = -1;

	}

	ret = 0;

out_unlock:

	mutex_unlock(&kvm->lock);

	return 0;

	return ret;

}



/*
@@ -1935,7 +1939,6 @@ static int kvm_xen_eventfd_deassign(struct kvm *kvm, u32 port) 
	if (!evtchnfd)

		return -ENOENT;



	if (kvm)

	synchronize_srcu(&kvm->srcu);

	if (!evtchnfd->deliver.port.port)

		eventfd_ctx_put(evtchnfd->deliver.eventfd.ctx);
@@ -1989,14 +1992,18 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r) 


	/* Sanity check: this structure is the same for 32-bit and 64-bit */

	BUILD_BUG_ON(sizeof(send) != 4);



	if (kvm_read_guest_virt(vcpu, param, &send, sizeof(send), &e)) {

		*r = -EFAULT;

		return true;

	}



	/* The evtchn_ports idr is protected by vcpu->kvm->srcu */

	/*

	 * evtchnfd is protected by kvm->srcu; the idr lookup instead

	 * is protected by RCU.

	 */

	rcu_read_lock();

	evtchnfd = idr_find(&vcpu->kvm->arch.xen.evtchn_ports, send.port);

	rcu_read_unlock();

	if (!evtchnfd)

		return false;