pvrdma: Ensure correct input on ring init (CVE-2021-3607)

Check the guest passed a non zero page count
for pvrdma device ring buffers.

Fixes: CVE-2021-3607
Reported-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Reviewed-by: VictorV (Kunlun Lab) <vv474172261@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Message-Id: <20210630114634.2168872-1-marcel@redhat.com>
Reviewed-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Tested-by: Yuval Shaia <yuval.shaia.ml@gmail.com>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>

hw/rdma/vmw/pvrdma_main.c

@@ -92,6 +92,11 @@ static int init_dev_ring(PvrdmaRing *ring, PvrdmaRingState **ring_state,
     uint64_t *dir, *tbl;
     int rc = 0;
 
+    if (!num_pages) {
+        rdma_error_report("Ring pages count must be strictly positive");
+        return -EINVAL;
+    }
+
     dir = rdma_pci_dma_map(pci_dev, dir_addr, TARGET_PAGE_SIZE);
     if (!dir) {
         rdma_error_report("Failed to map to page directory (ring %s)", name);