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>
master commit: c5e63651fdc706954d920a2d98f74f4a21b46a7e
master date: 2020-12-15 13:46:37 +0100

diff --git a/xen/common/event_fifo.c b/xen/common/event_fifo.c
index 2f5e868b7ae188b081eee8f4fa0bf721d94d8c7f..742ca31449f547141a06027ecad07cf1c1828085 100644 (file)
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -228,6 +228,10 @@ static void evtchn_fifo_set_pending(struct vcpu *v, struct evtchn *evtchn)
             goto unlock;
         }
 
+        /*
+         * This also acts as the read counterpart of the smp_wmb() in
+         * map_control_block().
+         */
         if ( guest_test_and_set_bit(d, EVTCHN_FIFO_LINKED, word) )
             goto unlock;
 
@@ -453,6 +457,7 @@ 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;
 
@@ -463,10 +468,15 @@ 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;
 }