crypto/x509: allow a leaf certificate to be specified directly as root.

In other systems, putting a leaf certificate in the root store works to
express that exactly that certificate is acceptable. This change makes
that work in Go too.

Fixes #16763.

Change-Id: I5c0a8dbc47aa631b23dd49061fb217ed8b0c719c
Reviewed-on: https://go-review.googlesource.com/27393
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/x509/cert_pool.go

@@ -5,6 +5,7 @@
 package x509
 
 import (
+	"bytes"
 	"encoding/pem"
 	"errors"
 	"runtime"
@@ -64,6 +65,21 @@ func (s *CertPool) findVerifiedParents(cert *Certificate) (parents []int, errCer
 	return
 }
 
+func (s *CertPool) contains(cert *Certificate) bool {
+	if s == nil {
+		return false
+	}
+
+	candidates := s.byName[string(cert.RawSubject)]
+	for _, c := range candidates {
+		if bytes.Equal(cert.Raw, s.certs[c].Raw) {
+			return true
+		}
+	}
+
+	return false
+}
+
 // AddCert adds a certificate to a pool.
 func (s *CertPool) AddCert(cert *Certificate) {
 	if cert == nil {

src/crypto/x509/verify.go

@@ -262,9 +262,13 @@ func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err e
 		}
 	}
 
-	candidateChains, err := c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts)
-	if err != nil {
-		return
+	var candidateChains [][]*Certificate
+	if opts.Roots.contains(c) {
+		candidateChains = append(candidateChains, []*Certificate{c})
+	} else {
+		if candidateChains, err = c.buildChains(make(map[int][][]*Certificate), []*Certificate{c}, &opts); err != nil {
+			return nil, err
+		}
 	}
 
 	keyUsages := opts.KeyUsages

src/crypto/x509/verify_test.go

@@ -235,6 +235,30 @@ var verifyTests = []verifyTest{
 			},
 		},
 	},
+	{
+		// Putting a certificate as a root directly should work as a
+		// way of saying “exactly this”.
+		leaf:        selfSigned,
+		roots:       []string{selfSigned},
+		currentTime: 1471624472,
+		dnsName:     "foo.example",
+		systemSkip:  true,
+
+		expectedChains: [][]string{
+			{"Acme Co"},
+		},
+	},
+	{
+		// Putting a certificate as a root directly should not skip
+		// other checks however.
+		leaf:        selfSigned,
+		roots:       []string{selfSigned},
+		currentTime: 1471624472,
+		dnsName:     "notfoo.example",
+		systemSkip:  true,
+
+		errorCallback: expectHostnameError,
+	},
 }
 
 func expectHostnameError(t *testing.T, i int, err error) (ok bool) {
@@ -1088,3 +1112,22 @@ Nr4TDea9Y355e6cJDUCrat2PisP29owaQgVR1EX1n6diIWgVIEM8med8vSTYqZEX
 c4g/VhsxOBi0cQ+azcgOno4uG+GMmIPLHzHxREzGBHNJdmAPx/i9F4BrLunMTA5a
 mnkPIAou1Z5jJh5VkpTYghdae9C8x49OhgQ=
 -----END CERTIFICATE-----`
+
+const selfSigned = `-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----`