server: config entry replication now correctly uses namespaces in comparisons

[rboyer]

Summary

An operator with service:write ACL permissions in a Consul Enterprise cluster can write a malicious config entry that causes infinite raft writes due to issues with the namespace replication logic. This can lead to an operator with access to one namespace to be able to temporarily delete a doppelgänger configuration in another namespace they should not have access to modify.

This is CVE-2020-25201

Bug

Previously config entries sharing a kind & name but in different
namespaces could occasionally cause "stuck states" in replication
because the namespace fields were ignored during the differential
comparison phase.

Example:

Two config entries written to the primary:

    kind=A,name=web,namespace=bar
    kind=A,name=web,namespace=foo

Under the covers these both get saved to memdb, so they are sorted by
all 3 components (kind,name,namespace) during natural iteration. This
means that before the replication code does it's own incomplete sort,
the underlying data IS sorted by namespace ascending (bar comes before
foo).

After one pass of replication the primary and secondary datacenters have
the same set of config entries present. If
"kind=A,name=web,namespace=bar" were to be deleted, then things get
weird. Before replication the two sides look like:

primary: [
    kind=A,name=web,namespace=foo
]
secondary: [
    kind=A,name=web,namespace=bar
    kind=A,name=web,namespace=foo
]

The differential comparison phase walks these two lists in sorted order
and first compares "kind=A,name=web,namespace=foo" vs
"kind=A,name=web,namespace=bar" and falsely determines they are the SAME
and are thus cause an update of "kind=A,name=web,namespace=foo". Then it
compares "<nothing>" with "kind=A,name=web,namespace=foo" and falsely
determines that the latter should be DELETED.

During reconciliation the deletes are processed before updates, and so
for a brief moment in the secondary "kind=A,name=web,namespace=foo" is
erroneously deleted and then immediately restored.

Unfortunately after this replication phase the final state is identical
to the initial state, so when it loops around again (rate limited) it
repeats the same set of operations indefinitely.

[hashicorp-ci]

🍒✅ Cherry pick of commit 58387fe onto release/1.8.x succeeded!

[hashicorp-ci]

🍒✅ Cherry pick of commit 58387fe onto release/1.7.x succeeded!