xen-netback: avoid race that can lead to NULL pointer dereference

In function xenvif_disconnect_queue(), the value of queue->rx_irq is
zeroed *before* queue->task is stopped. Unfortunately that task may call
notify_remote_via_irq(queue->rx_irq) and calling that function with a
zero value results in a NULL pointer dereference in evtchn_from_irq().

This patch simply re-orders things, stopping all tasks before zero-ing the
irq values, thereby avoiding the possibility of the race.

Fixes: 2ac061ce97f4 ("xen/netback: cleanup init and deinit code")
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-netback/interface.c
index 68dd7bb07ca66c634dfcc7aceb4f95d29487f647..f15ba3de6195a61d7762bf6cad530b79c6befc93 100644 (file)
--- a/drivers/net/xen-netback/interface.c
+++ b/drivers/net/xen-netback/interface.c
@@ -628,18 +628,6 @@ err:
 
 static void xenvif_disconnect_queue(struct xenvif_queue *queue)
 {
-       if (queue->tx_irq) {
-               unbind_from_irqhandler(queue->tx_irq, queue);
-               if (queue->tx_irq == queue->rx_irq)
-                       queue->rx_irq = 0;
-               queue->tx_irq = 0;
-       }
-
-       if (queue->rx_irq) {
-               unbind_from_irqhandler(queue->rx_irq, queue);
-               queue->rx_irq = 0;
-       }
-
        if (queue->task) {
                kthread_stop(queue->task);
                queue->task = NULL;
@@ -655,6 +643,18 @@ static void xenvif_disconnect_queue(struct xenvif_queue *queue)
                queue->napi.poll = NULL;
        }
 
+       if (queue->tx_irq) {
+               unbind_from_irqhandler(queue->tx_irq, queue);
+               if (queue->tx_irq == queue->rx_irq)
+                       queue->rx_irq = 0;
+               queue->tx_irq = 0;
+       }
+
+       if (queue->rx_irq) {
+               unbind_from_irqhandler(queue->rx_irq, queue);
+               queue->rx_irq = 0;
+       }
+
        xenvif_unmap_frontend_data_rings(queue);
 }