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[Xen-devel] [PATCH] RFC XSM/evtchn: Never pretend to have successfully created a Xen event channel

This is RFC because explicitly changes the logic introduced by c/s b34f2c375
"xsm: label xen-consumer event channels", and is only compile tested.

Xen event channels are not internal resources.  They still have one end in a
domain, and are created at the request of privileged domains.  This logic
which "successfully" creates a Xen event channel opens up undesirable failure
cases with ill-specified XSM policies.

If a domain is permitted to create ioreq servers or memevent listeners, but
not to create event channels, the ioreq/memevent creation will succeed but
attempting to bind the returned event channel will fail without any indication
of a permission error.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Tim Deegan <tim@xxxxxxx>
CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
CC: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
 xen/common/event_channel.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index cfe4978..89a7d99 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1160,11 +1160,13 @@ int alloc_unbound_xen_event_channel(
     chn = evtchn_from_port(d, port);
     rc = xsm_evtchn_unbound(XSM_TARGET, d, chn, remote_domid);
+    if ( rc )
+        goto out;
     chn->state = ECS_UNBOUND;
     chn->xen_consumer = get_xen_consumer(notification_fn);
     chn->notify_vcpu_id = local_vcpu->vcpu_id;
-    chn->u.unbound.remote_domid = !rc ? remote_domid : DOMID_INVALID;
+    chn->u.unbound.remote_domid = remote_domid;

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