linux-kernel - [PATCH] KVM: x86/xen: Fix SRCU/RCU usage in readers of evtchn

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Message-Id: <20221224085304.1629042-1-pbonzini@redhat.com>
Date:   Sat, 24 Dec 2022 03:53:04 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     paul@....org, seanjc@...gle.com, dwmw2@...radead.org,
        Michal Luczaj <mhal@...x.co>
Subject: [PATCH] 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@...x.co>
Signed-off-by: Michal Luczaj <mhal@...x.co>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/xen.c | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index d7af40240248..935f845d005c 100644
--- 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)
-- 
2.31.1