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From 1f177df6a47fb1e2961067a50e005efad52595cc Mon Sep 17 00:00:00 2001
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From: Ladi Prosek <lprosek@redhat.com>
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Date: Wed, 5 Oct 2016 17:22:26 +0200
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Subject: [PATCH 4/8] balloon: fix segfault and harden the stats queue
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RH-Author: Ladi Prosek <lprosek@redhat.com>
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Message-id: <1475666548-9186-5-git-send-email-lprosek@redhat.com>
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Patchwork-id: 72483
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O-Subject: [RHEL-7.4 qemu-kvm v2 PATCH 4/6] balloon: fix segfault and harden the stats queue
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Bugzilla: 1377968
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RH-Acked-by: Paolo Bonzini <pbonzini@redhat.com>
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RH-Acked-by: Michael S. Tsirkin <mst@redhat.com>
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RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
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The segfault here is triggered by the driver notifying the stats queue
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twice after adding a buffer to it. This effectively resets stats_vq_elem
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back to NULL and QEMU crashes on the next stats timer tick in
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balloon_stats_poll_cb.
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This is a regression introduced in 51b19ebe4320f3dc, although admittedly
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the device assumed too much about the stats queue protocol even before
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that commit. This commit adds a few more checks and ensures that the one
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stats buffer gets deallocated on device reset.
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Cc: qemu-stable@nongnu.org
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Signed-off-by: Ladi Prosek <lprosek@redhat.com>
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Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
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Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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(cherry picked from commit 4eae2a657d1ff5ada56eb9b4966eae0eff333b0b)
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Signed-off-by: Ladi Prosek <lprosek@redhat.com>
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Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
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Conflicts:
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  * 1.5.3 does not return pointers from virtqueue_pop so only the
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    "harden the stats queue" part of the upstream patch description
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    applies
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  * a new field stats_vq_elem_pending is introduced to keep track
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    of the state of stats_vq_elem in lieu of its nullness upstream
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  * virtio_balloon_device_reset only resets stats_vq_elem_pending
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    because there is nothing to free
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---
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 hw/virtio/virtio-balloon.c         | 27 +++++++++++++++++++++++----
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 include/hw/virtio/virtio-balloon.h |  1 +
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 2 files changed, 24 insertions(+), 4 deletions(-)
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diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
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index 016dc60..17b3029 100644
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--- a/hw/virtio/virtio-balloon.c
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+++ b/hw/virtio/virtio-balloon.c
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@@ -95,13 +95,14 @@ static void balloon_stats_poll_cb(void *opaque)
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     VirtIOBalloon *s = opaque;
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     VirtIODevice *vdev = VIRTIO_DEVICE(s);
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-    if (!balloon_stats_supported(s)) {
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+    if (!s->stats_vq_elem_pending || !balloon_stats_supported(s)) {
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         /* re-schedule */
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         balloon_stats_change_timer(s, s->stats_poll_interval);
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         return;
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     }
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     virtqueue_push(s->svq, &s->stats_vq_elem, s->stats_vq_offset);
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+    s->stats_vq_elem_pending = false;
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     virtio_notify(vdev, s->svq);
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 }
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@@ -220,14 +221,22 @@ static void virtio_balloon_handle_output(VirtIODevice *vdev, VirtQueue *vq)
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 static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
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 {
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     VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
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-    VirtQueueElement *elem = &s->stats_vq_elem;
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+    VirtQueueElement elem;
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     VirtIOBalloonStat stat;
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     size_t offset = 0;
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     qemu_timeval tv;
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-    if (!virtqueue_pop(vq, elem)) {
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+    if (!virtqueue_pop(vq, &elem)) {
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         goto out;
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     }
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+    if (s->stats_vq_elem_pending) {
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+        /* This should never happen if the driver follows the spec. */
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+        virtqueue_push(vq, &s->stats_vq_elem, 0);
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+        virtio_notify(vdev, vq);
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+    }
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+
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+    s->stats_vq_elem = elem;
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+    s->stats_vq_elem_pending = true;
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     /* Initialize the stats to get rid of any stale values.  This is only
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      * needed to handle the case where a guest supports fewer stats than it
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@@ -235,7 +244,7 @@ static void virtio_balloon_receive_stats(VirtIODevice *vdev, VirtQueue *vq)
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      */
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     reset_stats(s);
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-    while (iov_to_buf(elem->out_sg, elem->out_num, offset, &stat, sizeof(stat))
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+    while (iov_to_buf(elem.out_sg, elem.out_num, offset, &stat, sizeof(stat))
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            == sizeof(stat)) {
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         uint16_t tag = tswap16(stat.tag);
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         uint64_t val = tswap64(stat.val);
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@@ -384,6 +393,15 @@ static void virtio_balloon_device_exit(VirtIODevice *vdev)
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     virtio_cleanup(vdev);
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 }
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+static void virtio_balloon_device_reset(VirtIODevice *vdev)
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+{
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+    VirtIOBalloon *s = VIRTIO_BALLOON(vdev);
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+
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+    if (s->stats_vq_elem_pending) {
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+        s->stats_vq_elem_pending = false;
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+    }
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+}
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+
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 static Property virtio_balloon_properties[] = {
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     DEFINE_PROP_END_OF_LIST(),
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 };
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@@ -396,6 +414,7 @@ static void virtio_balloon_class_init(ObjectClass *klass, void *data)
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     set_bit(DEVICE_CATEGORY_MISC, dc->categories);
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     vdc->init = virtio_balloon_device_init;
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     vdc->exit = virtio_balloon_device_exit;
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+    vdc->reset = virtio_balloon_device_reset;
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     vdc->get_config = virtio_balloon_get_config;
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     vdc->set_config = virtio_balloon_set_config;
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     vdc->get_features = virtio_balloon_get_features;
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diff --git a/include/hw/virtio/virtio-balloon.h b/include/hw/virtio/virtio-balloon.h
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index f863bfe..a84736b 100644
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--- a/include/hw/virtio/virtio-balloon.h
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+++ b/include/hw/virtio/virtio-balloon.h
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@@ -63,6 +63,7 @@ typedef struct VirtIOBalloon {
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     uint32_t actual;
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     uint64_t stats[VIRTIO_BALLOON_S_NR];
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     VirtQueueElement stats_vq_elem;
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+    bool stats_vq_elem_pending;
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     size_t stats_vq_offset;
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     QEMUTimer *stats_timer;
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     int64_t stats_last_update;
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-- 
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1.8.3.1
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