x86: fix ordering of operations in destroy_irq()

The fix for XSA-36, switching the default of vector map management to
be per-device, exposed more readily a problem with the cleanup of these
vector maps: dynamic_irq_cleanup() clearing desc->arch.used_vectors
keeps the subsequently invoked clear_irq_vector() from clearing the
bits for both the in-use and a possibly still outstanding old vector.

Fix this by folding dynamic_irq_cleanup() into destroy_irq(), which was
its only caller, deferring the clearing of the vector map pointer until
after clear_irq_vector().

Once at it, also defer resetting of desc->handler until after the loop
around smp_mb() checking for IRQ_INPROGRESS to be clear, fixing a
(mostly theoretical) issue with the intercation with do_IRQ(): If we
don't defer the pointer reset, do_IRQ() could, for non-guest IRQs, call
->ack() and ->end() with different ->handler pointers, potentially
leading to an IRQ remaining un-acked. The issue is mostly theoretical
because non-guest IRQs are subject to destroy_irq() only on (boot time)
error paths.

As to the changed locking: Invoking clear_irq_vector() with desc->lock
held is okay because vector_lock already nests inside desc->lock (proven
by set_desc_affinity(), which takes vector_lock and gets called from
various desc->handler->ack implementations, getting invoked with
desc->lock held).

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -197,12 +197,24 @@ int create_irq(int node)
     return irq;
 }
 
-static void dynamic_irq_cleanup(unsigned int irq)
+void destroy_irq(unsigned int irq)
 {
     struct irq_desc *desc = irq_to_desc(irq);
     unsigned long flags;
     struct irqaction *action;
 
+    BUG_ON(!MSI_IRQ(irq));
+
+    if ( dom0 )
+    {
+        int err = irq_deny_access(dom0, irq);
+
+        if ( err )
+            printk(XENLOG_G_ERR
+                   "Could not revoke Dom0 access to IRQ%u (error %d)\n",
+                   irq, err);
+    }
+
     spin_lock_irqsave(&desc->lock, flags);
     desc->status  |= IRQ_DISABLED;
     desc->status  &= ~IRQ_GUEST;
@@ -210,16 +222,19 @@ static void dynamic_irq_cleanup(unsigned
     action = desc->action;
     desc->action  = NULL;
     desc->msi_desc = NULL;
-    desc->handler = &no_irq_type;
-    desc->arch.used_vectors = NULL;
     cpumask_setall(desc->affinity);
     spin_unlock_irqrestore(&desc->lock, flags);
 
     /* Wait to make sure it's not being used on another CPU */
     do { smp_mb(); } while ( desc->status & IRQ_INPROGRESS );
 
-    if (action)
-        xfree(action);
+    spin_lock_irqsave(&desc->lock, flags);
+    desc->handler = &no_irq_type;
+    clear_irq_vector(irq);
+    desc->arch.used_vectors = NULL;
+    spin_unlock_irqrestore(&desc->lock, flags);
+
+    xfree(action);
 }
 
 static void __clear_irq_vector(int irq)
@@ -286,24 +301,6 @@ void clear_irq_vector(int irq)
     spin_unlock_irqrestore(&vector_lock, flags);
 }
 
-void destroy_irq(unsigned int irq)
-{
-    BUG_ON(!MSI_IRQ(irq));
-
-    if ( dom0 )
-    {
-        int err = irq_deny_access(dom0, irq);
-
-        if ( err )
-            printk(XENLOG_G_ERR
-                   "Could not revoke Dom0 access to IRQ%u (error %d)\n",
-                   irq, err);
-    }
-
-    dynamic_irq_cleanup(irq);
-    clear_irq_vector(irq);
-}
-
 int irq_to_vector(int irq)
 {
     int vector = -1;