crypto/x509: reject empty GeneralSubtrees sequences

RFC 5280 defines GeneralSubtrees as a sequence containing at least one
GeneralSubtree. We currently reject a NameConstraints extension only
when both permittedSubtrees and excludedSubtrees are empty, accepting an
empty sequence when the other field contains a constraint.

Reject either field when it is present with an empty sequence.
The spec language is clear, and BoringSSL's pki verifier already does
this so I don't think there is a significant interop. risk.

This removes the x509-limbo allowances and adds explicit parser coverage
for both cases. Also adjusts two existing test certificates so their
intended conditions are not masked by an empty excludedSubtrees field.

Change-Id: Iea0640d4180dfa8acbf233b82a801ef79283cc30
Reviewed-on: https://go-review.googlesource.com/c/go/+/827347
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
4 files changed
tree: c7e9ddf8f8e43f2b62958f17b68fbb739037e526
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. codereview.cfg
  11. CONTRIBUTING.md
  12. go.env
  13. LICENSE
  14. PATENTS
  15. README.md
  16. SECURITY.md
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