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

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: 9796d0ac8f ("virtio: use address_space_map/unmap to access descriptors")
Signed-off-by: Carlos López <clopez@suse.de>
Message-Id: <20230302100358.3613-1-clopez@suse.de>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
master
Carlos López 2023-03-02 11:03:59 +01:00 committed by Michael S. Tsirkin
parent 90e31232cf
commit bbc1c327d7
1 changed files with 5 additions and 6 deletions
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11
hw/virtio/virtio.c
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@ -1069,7 +1069,7 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
VRingMemoryRegionCaches *caches)
{
VirtIODevice *vdev = vq->vdev;
unsigned int max, idx;
unsigned int idx;
unsigned int total_bufs, in_total, out_total;
MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
int64_t len = 0;
@ -1078,13 +1078,12 @@ 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;
VRingDesc desc;
unsigned int i;
unsigned int max = vq->vring.num;
num_bufs = total_bufs;
@ -1206,7 +1205,7 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq,
VRingMemoryRegionCaches *caches)
{
VirtIODevice *vdev = vq->vdev;
unsigned int max, idx;
unsigned int idx;
unsigned int total_bufs, in_total, out_total;
MemoryRegionCache *desc_cache;
MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
@ -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;
unsigned int max = vq->vring.num;
desc_cache = &caches->desc;
vring_packed_desc_read(vdev, &desc, desc_cache, idx, true);
if (!is_desc_avail(desc.flags, wrap_counter)) {
break;