Meltdown doesn't _directly_ allow modification of code. However, a guest VM could run userspace code that accesses ring 0 / kernel memory of the hypervisor / physical machine.

This would mean that security certificates, passwords, etc of the hypervisor could be exposed, which could then allow compromise.

I would also assume this means exploits on a machine provisioned with multiple VMs from different customers would allow a malicious customer to leak data from another customer's VM located on the same machine.