virtio: bugfix: check the value of caches before accessing it

Vring caches may be NULL in check_vring_avail_num() if
virtio_reset() is called at the same time, such as when
the virtual machine starts.
So check it before accessing it in vring_avail_idx().

Signed-off-by: Jinhua Cao <caojinhua1@huawei.com>

virtio-bugfix-check-the-value-of-caches-before-acces.patch

@@ -0,0 +1,42 @@
+From 74ab61b4317f12b231fb2cbcd54a333a07efd678 Mon Sep 17 00:00:00 2001
+From: Jinhua Cao <caojinhua1@huawei.com>
+Date: Thu, 10 Feb 2022 14:37:52 +0800
+Subject: [PATCH] virtio: bugfix: check the value of caches before accessing it
+
+Vring caches may be NULL in check_vring_avail_num() if
+virtio_reset() is called at the same time, such as when
+the virtual machine starts.
+So check it before accessing it in vring_avail_idx().
+
+Signed-off-by: Jinhua Cao <caojinhua1@huawei.com>
+---
+ hw/virtio/virtio.c | 11 +++++++++++
+ 1 file changed, 11 insertions(+)
+
+diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
+index 9a2a83d507..b08fff9419 100644
+--- a/hw/virtio/virtio.c
++++ b/hw/virtio/virtio.c
+@@ -2863,8 +2863,19 @@ static const VMStateDescription vmstate_virtio = {
+ static void check_vring_avail_num(VirtIODevice *vdev, int index)
+ {
+     uint16_t nheads;
++    VRingMemoryRegionCaches *caches;
+ 
+     rcu_read_lock();
++    caches = qatomic_rcu_read(&vdev->vq[index].vring.caches);
++    if (caches == NULL) {
++        /*
++         * caches may be NULL if virtio_reset is called at the same time,
++         * such as when the virtual machine starts.
++         */
++        rcu_read_unlock();
++        return;
++    }
++
+     /* Check it isn't doing strange things with descriptor numbers. */
+     nheads = vring_avail_idx(&vdev->vq[index]) - vdev->vq[index].last_avail_idx;
+     if (nheads > vdev->vq[index].vring.num) {
+-- 
+2.27.0
+