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 0a90a8404d5bdfb7cbe3165f99ed0bf9a7da3022..ab9e496696fd8b273653cd9a6bb16c5f1251cffc 100644 (file)
--- a/xen/common/event_fifo.c
+++ b/xen/common/event_fifo.c
@@ -227,6 +227,10 @@ 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;
 
@@ -452,6 +456,7 @@ 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;
 
@@ -462,10 +467,15 @@ 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;
 }