[xen staging] xen/events: allow setting of global virq handler only for unbound virqs

[patchbot@xxxxxxx]

commit 980822c5edd122ecb21080079f5ed801fef30f51
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Thu Mar 6 13:51:55 2025 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Mar 6 13:51:55 2025 +0100

    xen/events: allow setting of global virq handler only for unbound virqs
    
    XEN_DOMCTL_set_virq_handler will happily steal a global virq from the
    current domain having bound it and assign it to another domain. The
    former domain will just never receive any further events for that
    virq without knowing what happened.
    
    Change the behavior to allow XEN_DOMCTL_set_virq_handler only if the
    virq in question is not bound by the current domain allowed to use it.
    
    Currently the only user of XEN_DOMCTL_set_virq_handler in the Xen code
    base is init-xenstore-domain, so changing the behavior like above will
    not cause any problems.
    
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/common/event_channel.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index cd6f5a1211..4dba59efa2 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -991,7 +991,8 @@ void send_global_virq(uint32_t virq)
 
 int set_global_virq_handler(struct domain *d, uint32_t virq)
 {
-    struct domain *old;
+    struct domain *old, *hdl;
+    const struct vcpu *v;
     int rc = 0;
 
     if (virq >= NR_VIRQS)
@@ -1023,7 +1024,22 @@ int set_global_virq_handler(struct domain *d, uint32_t 
virq)
     else
     {
         old = global_virq_handlers[virq];
-        global_virq_handlers[virq] = d;
+        hdl = get_global_virq_handler(virq);
+        if ( hdl != d )
+        {
+            read_lock(&hdl->event_lock);
+
+            v = hdl->vcpu[0];
+            if ( v && read_atomic(&v->virq_to_evtchn[virq]) )
+            {
+                rc = -EBUSY;
+                old = d;
+            }
+            else
+                global_virq_handlers[virq] = d;
+
+            read_unlock(&hdl->event_lock);
+        }
     }
 
     spin_unlock(&global_virq_handlers_lock);
--
generated by git-patchbot for /home/xen/git/xen.git#staging