evtchn/FIFO: re-order and synchronize (with) map_control_block()

For evtchn_fifo_set_pending()'s check of the control block having been
set to be effective, ordering of respective reads and writes needs to be
ensured: The control block pointer needs to be recorded strictly after
the setting of all the queue heads, and it needs checking strictly
before any uses of them (this latter aspect was already guaranteed).

This is XSA-358 / CVE-2020-29570.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 3310dc00d7c6e278789501a31d8d07502987169d..2fb01b82db8445f64b4b106906fa6a3cfd4b1d64 100644 (file)
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -252,6 +252,10 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
      * Link the event if it unmasked and not already linked.
      */
     if ( !guest_test_bit(d, EVTCHN_FIFO_MASKED, word) &&
+         /*
+          * This also acts as the read counterpart of the smp_wmb() in
+          * map_control_block().
+          */
          !guest_test_and_set_bit(d, EVTCHN_FIFO_LINKED, word) )
     {
         /*
@@ -478,6 +482,7 @@ static int setup_control_block(struct vcpu *v)
 static int map_control_block(struct vcpu *v, uint64_t gfn, uint32_t offset)
 {
     void *virt;
+    struct evtchn_fifo_control_block *control_block;
     unsigned int i;
     int rc;
 
@@ -488,10 +493,15 @@ static int map_control_block(struct vcpu *v, uint64_t gfn, uint32_t offset)
     if ( rc < 0 )
         return rc;
 
-    v->evtchn_fifo->control_block = virt + offset;
+    control_block = virt + offset;
 
     for ( i = 0; i <= EVTCHN_FIFO_PRIORITY_MIN; i++ )
-        v->evtchn_fifo->queue[i].head = &v->evtchn_fifo->control_block->head[i];
+        v->evtchn_fifo->queue[i].head = &control_block->head[i];
+
+    /* All queue heads must have been set before setting the control block. */
+    smp_wmb();
+
+    v->evtchn_fifo->control_block = control_block;
 
     return 0;
 }