[PATCH] virtio: fix reachable assertion due to stale value of cached region size

[Carlos López]

In virtqueue_{split,packed}_get_avail_bytes() descriptors are read
in a loop via MemoryRegionCache regions and calls to
vring_{split,packed}_desc_read() - these take a region cache and the
index of the descriptor to be read.

For direct descriptors we use a cache provided by the caller, whose
size matches that of the virtqueue vring. We limit the number of
descriptors we can read by the size of that vring:

    max = vq->vring.num;
    ...
    MemoryRegionCache *desc_cache = &caches->desc;

For indirect descriptors, we initialize a new cache and limit the
number of descriptors by the size of the intermediate descriptor:

    len = address_space_cache_init(&indirect_desc_cache,
                                   vdev->dma_as,
                                   desc.addr, desc.len, false);
    desc_cache = &indirect_desc_cache;
    ...
    max = desc.len / sizeof(VRingDesc);

However, the first initialization of `max` is done outside the loop
where we process guest descriptors, while the second one is done
inside. This means that a sequence of an indirect descriptor followed
by a direct one will leave a stale value in `max`. If the second
descriptor's `next` field is smaller than the stale value, but
greater than the size of the virtqueue ring (and thus the cached
region), a failed assertion will be triggered in
address_space_read_cached() down the call chain.

Fix this by initializing `max` inside the loop in both functions.

Fixes: 9796d0ac8fb0 ("virtio: use address_space_map/unmap to access 
descriptors")
Signed-off-by: Carlos López <clopez@suse.de>
---
 hw/virtio/virtio.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index f35178f5fc..db70c4976e 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1071,6 +1071,7 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
     VirtIODevice *vdev = vq->vdev;
     unsigned int max, idx;
     unsigned int total_bufs, in_total, out_total;
+    MemoryRegionCache *desc_cache;
     MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
     int64_t len = 0;
     int rc;
@@ -1078,15 +1079,13 @@ static void virtqueue_split_get_avail_bytes(VirtQueue 
*vq,
     idx = vq->last_avail_idx;
     total_bufs = in_total = out_total = 0;
 
-    max = vq->vring.num;
-
     while ((rc = virtqueue_num_heads(vq, idx)) > 0) {
-        MemoryRegionCache *desc_cache = &caches->desc;
-        unsigned int num_bufs;
+        unsigned int num_bufs = total_bufs;
         VRingDesc desc;
         unsigned int i;
 
-        num_bufs = total_bufs;
+        desc_cache = &caches->desc;
+        max = vq->vring.num;
 
         if (!virtqueue_get_head(vq, idx++, &i)) {
             goto err;
@@ -1218,14 +1217,14 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue 
*vq,
     wrap_counter = vq->last_avail_wrap_counter;
     total_bufs = in_total = out_total = 0;
 
-    max = vq->vring.num;
-
     for (;;) {
         unsigned int num_bufs = total_bufs;
         unsigned int i = idx;
         int rc;
 
         desc_cache = &caches->desc;
+        max = vq->vring.num;
+
         vring_packed_desc_read(vdev, &desc, desc_cache, idx, true);
         if (!is_desc_avail(desc.flags, wrap_counter)) {
             break;
-- 
2.35.3