vsock: vhost_user: vfio: Fix potential host memory overflow

The vsock packets that we're building are resolving guest addresses to
host ones and use the latter as raw pointers.
If the corresponding guest mapped buffer spans across several regions in
the guest, they will do so in the host as well. Since we have no
guarantees that host regions are contiguous, it may lead the VMM into
trying to access memory outside of its memory space.

For now we fix that by ensuring that the guest buffers do not span
across several regions. If they do, we error out.
Ideally, we should enhance the rust-vmm memory model to support safe
acces across host regions.

Fixes CVE-2019-18960

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

vm-virtio/src/queue.rs

@@ -63,7 +63,7 @@ impl<'a> Iterator for DescIter<'a> {
 /// A virtio descriptor constraints with C representive.
 #[repr(C)]
 #[derive(Default, Clone, Copy)]
-struct Descriptor {
+pub struct Descriptor {
     addr: u64,
     len: u32,
     flags: u16,