On 01/24/2016 12:06 PM, One Thousand Gnomes wrote:

quoted

quoted

That changes some of the goals the memory management subsystem has,
from "use all the resources effectively" to "use as few resources as
necessary, in case the host needs the memory for something else".

Also "and take guidance/provide telemetry" - because you want to tune the
VM behaviours based upon policy and to learn from them for when you re-run
that container.

quoted

Beyond memory consumption, I would be interested whether we can harden the kernel by the paravirt interfaces for memory protection in VMs (if any). For example, the hypervisor could write-protect part of the page tables or kernel data structures in VMs, and does it help?

There are four behaviours I can think of, some of which you see in
various hypervisors and security hardening systems

- die on write (a write here causes a security trap and termination after
  the guest has marked the page range die on write, and it cannot be
  unmarked). The guest OS at boot can for example mark all it's code as
  die-on-write.
- irrevocably read only (VM never allows page to be rewritten by guest
  after the guest marks the page range irrevocably r/o)

For these we get the question "how do we make it harder for the
guest to remap the page tables to point at read/write memory,
and modify that instead of the read-only memory?"

On "smaller" guests (less than 1TB in size), it may be enough to
ensure that the kernel PUD pointer points to the (read-only) kernel
PUD at context switch time, placing the main kernel page tables,
kernel text, and some other things in read-only memory.

- asynchronous faulting (pages the guest thinks are in it's memory but
  are in fact on the hosts swap cause a subscribable fault in the guest
  so that it can (where possible) be context switched

KVM (and s390) already do the asynchronous page fault trick.

- free if needed - marking pages as freed up and either you get a page
  back as it was or a fault and a zeroed page

People have worked on this for KVM. I do not remember what
happened to the code.

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