From 75255574498fad12727529c4ecbd4ccdabe86839 Mon Sep 17 00:00:00 2001

From: Ladi Prosek <lprosek@redhat.com>

Date: Wed, 5 Oct 2016 17:22:26 +0200

Subject: [PATCH 4/8] balloon: fix segfault and harden the stats queue

RH-Author: Ladi Prosek <lprosek@redhat.com>

Message-id: <1475666548-9186-5-git-send-email-lprosek@redhat.com>

Patchwork-id: 72483

O-Subject: [RHEL-7.4 qemu-kvm v2 PATCH 4/6] balloon: fix segfault and harden the stats queue

Bugzilla: 1393484

RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>

RH-Acked-by: Michael S. Tsirkin <mst@redhat.com>

RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

The segfault here is triggered by the driver notifying the stats queue

twice after adding a buffer to it. This effectively resets stats_vq_elem

back to NULL and QEMU crashes on the next stats timer tick in

balloon_stats_poll_cb.

This is a regression introduced in 51b19ebe4320f3dc, although admittedly

the device assumed too much about the stats queue protocol even before

that commit. This commit adds a few more checks and ensures that the one

stats buffer gets deallocated on device reset.

Cc: qemu-stable@nongnu.org

Signed-off-by: Ladi Prosek <lprosek@redhat.com>

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

(cherry picked from commit 4eae2a657d1ff5ada56eb9b4966eae0eff333b0b)

Signed-off-by: Ladi Prosek <lprosek@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

Conflicts:

  * 1.5.3 does not return pointers from virtqueue_pop so only the

    "harden the stats queue" part of the upstream patch description

    applies

  * a new field stats_vq_elem_pending is introduced to keep track

    of the state of stats_vq_elem in lieu of its nullness upstream

  * virtio_balloon_device_reset only resets stats_vq_elem_pending

    because there is nothing to free

---

 hw/virtio/virtio-balloon.c         | 27 +++++++++++++++++++++++----

 include/hw/virtio/virtio-balloon.h |  1 +

 2 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c

index 016dc60..17b3029 100644

--- a/hw/virtio/virtio-balloon.c

+++ b/hw/virtio/virtio-balloon.c

@@ -95,13 +95,14 @@ static void balloon_stats_poll_cb(void *opaque)

     VirtIOBalloon *s = opaque;

     VirtIODevice *vdev = VIRTIO_DEVICE(s);

-    if (!balloon_stats_supported(s)) {

+    if (!s->stats_vq_elem_pending || !balloon_stats_supported(s)) {

         /* re-schedule */

         balloon_stats_change_timer(s, s->stats_poll_interval);

         return;

     }

     virtqueue_push(s->svq, &s->stats_vq_elem, s->stats_vq_offset);

+    s->stats_vq_elem_pending = false;

     virtio_notify(vdev, s->svq);

 }

@@ -220,14 +221,22 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)

 static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)

 {

     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);

-    VirtQueueElement *elem = &s->stats_vq_elem;

+    VirtQueueElement elem;

     VirtIOBalloonStat stat;

     size_t offset = 0;

     qemu_timeval tv;

-    if (!virtqueue_pop(vq, elem)) {

+    if (!virtqueue_pop(vq, &elem)) {

         goto out;

     }

+    if (s->stats_vq_elem_pending) {

+        /* This should never happen if the driver follows the spec. */

+        virtqueue_push(vq, &s->stats_vq_elem, 0);

+        virtio_notify(vdev, vq);

+    }

+

+    s->stats_vq_elem = elem;

+    s->stats_vq_elem_pending = true;

     /* Initialize the stats to get rid of any stale values.  This is only

      * needed to handle the case where a guest supports fewer stats than it

@@ -235,7 +244,7 @@ static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)

      */

     reset_stats(s);

-    while (iov_to_buf(elem->out_sg, elem->out_num, offset, &stat, sizeof(stat))

+    while (iov_to_buf(elem.out_sg, elem.out_num, offset, &stat, sizeof(stat))

            == sizeof(stat)) {

         uint16_t tag = tswap16(stat.tag);

         uint64_t val = tswap64(stat.val);

@@ -384,6 +393,15 @@ static void virtio_balloon_device_exit(VirtIODevice *vdev)

     virtio_cleanup(vdev);

 }

+static void virtio_balloon_device_reset(VirtIODevice *vdev)

+{

+    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);

+

+    if (s->stats_vq_elem_pending) {

+        s->stats_vq_elem_pending = false;

+    }

+}

+

 static Property virtio_balloon_properties[] = {

     DEFINE_PROP_END_OF_LIST(),

 };

@@ -396,6 +414,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, void *data)

     set_bit(DEVICE_CATEGORY_MISC, dc->categories);

     vdc->init = virtio_balloon_device_init;

     vdc->exit = virtio_balloon_device_exit;

+    vdc->reset = virtio_balloon_device_reset;

     vdc->get_config = virtio_balloon_get_config;

     vdc->set_config = virtio_balloon_set_config;

     vdc->get_features = virtio_balloon_get_features;

diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h

index f863bfe..a84736b 100644

--- a/include/hw/virtio/virtio-balloon.h

+++ b/include/hw/virtio/virtio-balloon.h

@@ -63,6 +63,7 @@ typedef struct VirtIOBalloon {

     uint32_t actual;

     uint64_t stats[VIRTIO_BALLOON_S_NR];

     VirtQueueElement stats_vq_elem;

+    bool stats_vq_elem_pending;

     size_t stats_vq_offset;

     QEMUTimer *stats_timer;

     int64_t stats_last_update;

-- 

1.8.3.1