Re: [RFC PATCH v7 01/25] vhost: Track descriptor chain in private at SVQ

[Jason Wang]

在 2022/4/14 00:31, Eugenio Pérez 写道:

Only the first one of them were properly enqueued back.

I wonder if it's better to use two patches:

1) using private chain

2) fix the chain issue

Patch looks good itself.

Thanks

While we're at it, harden SVQ: The device could have access to modify
them, and it definitely have access when we implement packed vq. Harden
SVQ maintaining a private copy of the descriptor chain. Other fields
like buffer addresses are already maintained sepparatedly.

Fixes: 100890f7ca ("vhost: Shadow virtqueue buffers forwarding")

Signed-off-by: Eugenio Pérez <epere...@redhat.com>
---
  hw/virtio/vhost-shadow-virtqueue.h |  6 ++++++
  hw/virtio/vhost-shadow-virtqueue.c | 27 +++++++++++++++++++++------
  2 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/hw/virtio/vhost-shadow-virtqueue.h 
b/hw/virtio/vhost-shadow-virtqueue.h
index e5e24c536d..c132c994e9 100644
--- a/hw/virtio/vhost-shadow-virtqueue.h
+++ b/hw/virtio/vhost-shadow-virtqueue.h
@@ -53,6 +53,12 @@ typedef struct VhostShadowVirtqueue {
      /* Next VirtQueue element that guest made available */
      VirtQueueElement *next_guest_avail_elem;

+ /*

+     * Backup next field for each descriptor so we can recover securely, not
+     * needing to trust the device access.
+     */
+    uint16_t *desc_next;
+
      /* Next head to expose to the device */
      uint16_t shadow_avail_idx;

diff --git a/hw/virtio/vhost-shadow-virtqueue.c b/hw/virtio/vhost-shadow-virtqueue.c

index b232803d1b..a2531d5874 100644
--- a/hw/virtio/vhost-shadow-virtqueue.c
+++ b/hw/virtio/vhost-shadow-virtqueue.c
@@ -138,6 +138,7 @@ static void vhost_vring_write_descs(VhostShadowVirtqueue 
*svq, hwaddr *sg,
      for (n = 0; n < num; n++) {
          if (more_descs || (n + 1 < num)) {
              descs[i].flags = flags | cpu_to_le16(VRING_DESC_F_NEXT);
+            descs[i].next = cpu_to_le16(svq->desc_next[i]);
          } else {
              descs[i].flags = flags;
          }
@@ -145,10 +146,10 @@ static void vhost_vring_write_descs(VhostShadowVirtqueue 
*svq, hwaddr *sg,
          descs[i].len = cpu_to_le32(iovec[n].iov_len);

last = i;

-        i = cpu_to_le16(descs[i].next);
+        i = cpu_to_le16(svq->desc_next[i]);
      }

- svq->free_head = le16_to_cpu(descs[last].next);

+    svq->free_head = le16_to_cpu(svq->desc_next[last]);
  }

static bool vhost_svq_add_split(VhostShadowVirtqueue *svq,

@@ -333,13 +334,22 @@ static void 
vhost_svq_disable_notification(VhostShadowVirtqueue *svq)
      svq->vring.avail->flags |= cpu_to_le16(VRING_AVAIL_F_NO_INTERRUPT);
  }

+static uint16_t vhost_svq_last_desc_of_chain(const VhostShadowVirtqueue *svq,

+                                             uint16_t num, uint16_t i)
+{
+    for (uint16_t j = 0; j < num; ++j) {
+        i = le16_to_cpu(svq->desc_next[i]);
+    }
+
+    return i;
+}
+
  static VirtQueueElement *vhost_svq_get_buf(VhostShadowVirtqueue *svq,
                                             uint32_t *len)
  {
-    vring_desc_t *descs = svq->vring.desc;
      const vring_used_t *used = svq->vring.used;
      vring_used_elem_t used_elem;
-    uint16_t last_used;
+    uint16_t last_used, last_used_chain, num;

if (!vhost_svq_more_used(svq)) {

          return NULL;
@@ -365,7 +375,10 @@ static VirtQueueElement 
*vhost_svq_get_buf(VhostShadowVirtqueue *svq,
          return NULL;
      }

- descs[used_elem.id].next = svq->free_head;

+    num = svq->ring_id_maps[used_elem.id]->in_num +
+          svq->ring_id_maps[used_elem.id]->out_num;
+    last_used_chain = vhost_svq_last_desc_of_chain(svq, num, used_elem.id);
+    svq->desc_next[last_used_chain] = svq->free_head;
      svq->free_head = used_elem.id;

*len = used_elem.len;

@@ -540,8 +553,9 @@ void vhost_svq_start(VhostShadowVirtqueue *svq, 
VirtIODevice *vdev,
      svq->vring.used = qemu_memalign(qemu_real_host_page_size, device_size);
      memset(svq->vring.used, 0, device_size);
      svq->ring_id_maps = g_new0(VirtQueueElement *, svq->vring.num);
+    svq->desc_next = g_new0(uint16_t, svq->vring.num);
      for (unsigned i = 0; i < svq->vring.num - 1; i++) {
-        svq->vring.desc[i].next = cpu_to_le16(i + 1);
+        svq->desc_next[i] = cpu_to_le16(i + 1);
      }
  }

@@ -574,6 +588,7 @@ void vhost_svq_stop(VhostShadowVirtqueue *svq)

          virtqueue_detach_element(svq->vq, next_avail_elem, 0);
      }
      svq->vq = NULL;
+    g_free(svq->desc_next);
      g_free(svq->ring_id_maps);
      qemu_vfree(svq->vring.desc);
      qemu_vfree(svq->vring.used);