Patch "KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET" has been added to the 6.1-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET

to the 6.1-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-x86-fix-deadlock-for-kvm_xen_evtchn_reset.patch
and it can be found in the queue-6.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit f39621f148d4f59f362703d41af27c0a1c2ee37d
Author: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Date:   Wed Dec 28 05:33:41 2022 -0500

    KVM: x86: fix deadlock for KVM_XEN_EVTCHN_RESET
    
    [ Upstream commit a79b53aaaab53de017517bf9579b6106397a523c ]
    
    While KVM_XEN_EVTCHN_RESET is usually called with no vCPUs running,
    if that happened it could cause a deadlock.  This is due to
    kvm_xen_eventfd_reset() doing a synchronize_srcu() inside
    a kvm->lock critical section.
    
    To avoid this, first collect all the evtchnfd objects in an
    array and free all of them once the kvm->lock critical section
    is over and th SRCU grace period has expired.
    
    Reported-by: Michal Luczaj <mhal@xxxxxxx>
    Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
    Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index f3098c0e386a8..a58a426e6b1c0 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1757,18 +1757,42 @@ static int kvm_xen_eventfd_deassign(struct kvm *kvm, u32 port)
 
 static int kvm_xen_eventfd_reset(struct kvm *kvm)
 {
-	struct evtchnfd *evtchnfd;
+	struct evtchnfd *evtchnfd, **all_evtchnfds;
 	int i;
+	int n = 0;
 
 	mutex_lock(&kvm->lock);
+
+	/*
+	 * Because synchronize_srcu() cannot be called inside the
+	 * critical section, first collect all the evtchnfd objects
+	 * in an array as they are removed from evtchn_ports.
+	 */
+	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i)
+		n++;
+
+	all_evtchnfds = kmalloc_array(n, sizeof(struct evtchnfd *), GFP_KERNEL);
+	if (!all_evtchnfds) {
+		mutex_unlock(&kvm->lock);
+		return -ENOMEM;
+	}
+
+	n = 0;
 	idr_for_each_entry(&kvm->arch.xen.evtchn_ports, evtchnfd, i) {
+		all_evtchnfds[n++] = evtchnfd;
 		idr_remove(&kvm->arch.xen.evtchn_ports, evtchnfd->send_port);
-		synchronize_srcu(&kvm->srcu);
+	}
+	mutex_unlock(&kvm->lock);
+
+	synchronize_srcu(&kvm->srcu);
+
+	while (n--) {
+		evtchnfd = all_evtchnfds[n];
 		if (!evtchnfd->deliver.port.port)
 			eventfd_ctx_put(evtchnfd->deliver.eventfd.ctx);
 		kfree(evtchnfd);
 	}
-	mutex_unlock(&kvm->lock);
+	kfree(all_evtchnfds);
 
 	return 0;
 }
diff --git a/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c b/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
index 2a5727188c8d3..8383457e66990 100644
--- a/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
+++ b/tools/testing/selftests/kvm/x86_64/xen_shinfo_test.c
@@ -942,6 +942,12 @@ int main(int argc, char *argv[])
 	}
 
  done:
+	struct kvm_xen_hvm_attr evt_reset = {
+		.type = KVM_XEN_ATTR_TYPE_EVTCHN,
+		.u.evtchn.flags = KVM_XEN_EVTCHN_RESET,
+	};
+	vm_ioctl(vm, KVM_XEN_HVM_SET_ATTR, &evt_reset);
+
 	alarm(0);
 	clock_gettime(CLOCK_REALTIME, &max_ts);