Reject proxy requests to 0.0.0.0 as well

[anguslees]

What type of PR is this?

/kind bug

What this PR does / why we need it:

Various IP addresses are known to be invalid/unsafe as destination addresses for apiserver proxy queries. The apiserver will reject attempts to proxy to these destination addresses, if a pod/service/node somehow reports as having one of these addresses.

Previously this check included loopback, link-local, and multicast CIDRs. This PR extends the checks to also reject all-zero. (All checks include both IPv4 and IPv6 equivalents.)

As an implementation detail, the change is implemented by switching to golang's net.IP.IsGlobalUnicast. I invite the reader to review the source of that function and satisfy themselves that the resulting checks are equivalent.

Does this PR introduce a user-facing change?

apiserver will now reject connection attempts to 0.0.0.0/:: when handling a proxy subresource request

[k8s-ci-robot]

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[aojea]

I don't know if it is possible for a service, node, ... to return those addresses, but tightening the validation here sgtm
/lgtm
/assign @thockin

[anguslees]

I don't know if it is possible for a service, node, ... to return those addresses, but tightening the validation here sgtm

To be clear: I don't know of a case where this can happen either, except through 'legitimate' RBAC write access to Service/Pod/Node, which is why I'm not tagging this as a 'security' issue at this time. This is just a defence-in-depth improvement from my pov.

[thockin]

Thanks!

/lgtm
/approve

[k8s-ci-robot]

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This pull-request has been approved by: anguslees, thockin

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