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

evtchnfd 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, so extend the SRCU critical section
there.

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.

Co-developed-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index d7af402402484bb008f253213a86c186224a34c1..935f845d005cac2935e5556c73403f8c2044b264 100644 (file)
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1825,20 +1825,23 @@ 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
@@ -1846,20 +1849,21 @@ 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;
 }
 
 /*
@@ -2005,19 +2009,23 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
        gpa_t gpa;
        int idx;
 
+       /*
+        * evtchnfd is protected by kvm->srcu; the idr lookup instead
+        * is protected by RCU.
+        */
        idx = srcu_read_lock(&vcpu->kvm->srcu);
        gpa = kvm_mmu_gva_to_gpa_system(vcpu, param, NULL);
-       srcu_read_unlock(&vcpu->kvm->srcu, idx);
 
        if (!gpa || kvm_vcpu_read_guest(vcpu, gpa, &send, sizeof(send))) {
                *r = -EFAULT;
-               return true;
+               goto out_handled;
        }
 
-       /* The evtchn_ports idr is protected by vcpu->kvm->srcu */
+       rcu_read_lock();
        evtchnfd = idr_find(&vcpu->kvm->arch.xen.evtchn_ports, send.port);
+       rcu_read_unlock();
        if (!evtchnfd)
-               return false;
+               goto out_not_handled;
 
        if (evtchnfd->deliver.port.port) {
                int ret = kvm_xen_set_evtchn(&evtchnfd->deliver.port, vcpu->kvm);
@@ -2028,7 +2036,13 @@ static bool kvm_xen_hcall_evtchn_send(struct kvm_vcpu *vcpu, u64 param, u64 *r)
        }
 
        *r = 0;
+out_handled:
+       srcu_read_unlock(&vcpu->kvm->srcu, idx);
        return true;
+
+out_not_handled:
+       srcu_read_unlock(&vcpu->kvm->srcu, idx);
+       return false;
 }
 
 void kvm_xen_init_vcpu(struct kvm_vcpu *vcpu)