crypto/x509: add SystemCertPool, refactor system cert pool loading

This exports the system cert pool.

The system cert loading was refactored to let it be run multiple times
(so callers get a copy, and can't mutate global state), and also to
not discard errors.

SystemCertPool returns an error on Windows. Maybe it's fixable later,
but so far we haven't used it, since the system verifies TLS.

Fixes #13335

Change-Id: I3dfb4656a373f241bae8529076d24c5f532f113c
Reviewed-on: https://go-review.googlesource.com/21293
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Andrew Gerrand <[email protected]>

src/crypto/x509/cert_pool.go

@@ -6,6 +6,8 @@ package x509
 
 import (
 	"encoding/pem"
+	"errors"
+	"runtime"
 )
 
 // CertPool is a set of certificates.
@@ -18,12 +20,22 @@ type CertPool struct {
 // NewCertPool returns a new, empty CertPool.
 func NewCertPool() *CertPool {
 	return &CertPool{
-		make(map[string][]int),
-		make(map[string][]int),
-		nil,
+		bySubjectKeyId: make(map[string][]int),
+		byName:         make(map[string][]int),
 	}
 }
 
+// SystemCertPool returns a copy of the system cert pool.
+//
+// Any mutations to the returned pool are not written to disk and do
+// not affect any other pool.
+func SystemCertPool() (*CertPool, error) {
+	if runtime.GOOS == "windows" {
+		return nil, errors.New("crypto/x509: system root pool is not available on Windows")
+	}
+	return loadSystemRoots()
+}
+
 // findVerifiedParents attempts to find certificates in s which have signed the
 // given certificate. If any candidates were rejected then errCert will be set
 // to one of them, arbitrarily, and err will contain the reason that it was
@@ -107,10 +119,10 @@ func (s *CertPool) AppendCertsFromPEM(pemCerts []byte) (ok bool) {
 
 // Subjects returns a list of the DER-encoded subjects of
 // all of the certificates in the pool.
-func (s *CertPool) Subjects() (res [][]byte) {
-	res = make([][]byte, len(s.certs))
+func (s *CertPool) Subjects() [][]byte {
+	res := make([][]byte, len(s.certs))
 	for i, c := range s.certs {
 		res[i] = c.RawSubject
 	}
-	return
+	return res
 }

src/crypto/x509/root.go

@@ -7,11 +7,16 @@ package x509
 import "sync"
 
 var (
-	once        sync.Once
-	systemRoots *CertPool
+	once           sync.Once
+	systemRoots    *CertPool
+	systemRootsErr error
 )
 
 func systemRootsPool() *CertPool {
 	once.Do(initSystemRoots)
 	return systemRoots
 }
+
+func initSystemRoots() {
+	systemRoots, systemRootsErr = loadSystemRoots()
+}

src/crypto/x509/root_cgo_darwin.go

@@ -61,19 +61,23 @@ int FetchPEMRoots(CFDataRef *pemRoots) {
 }
 */
 import "C"
-import "unsafe"
+import (
+	"errors"
+	"unsafe"
+)
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
 
 	var data C.CFDataRef = nil
 	err := C.FetchPEMRoots(&data)
 	if err == -1 {
-		return
+		// TODO: better error message
+		return nil, errors.New("crypto/x509: failed to load darwin system roots with cgo")
 	}
 
 	defer C.CFRelease(C.CFTypeRef(data))
 	buf := C.GoBytes(unsafe.Pointer(C.CFDataGetBytePtr(data)), C.int(C.CFDataGetLength(data)))
 	roots.AppendCertsFromPEM(buf)
-	systemRoots = roots
+	return roots, nil
 }

src/crypto/x509/root_darwin_arm_gen.go

@@ -184,8 +184,9 @@ const header = `
 
 package x509
 
-func initSystemRoots() {
-	systemRoots = NewCertPool()
-	systemRoots.AppendCertsFromPEM([]byte(systemRootsPEM))
+func loadSystemRoots() (*CertPool, error) {
+	p := NewCertPool()
+	p.AppendCertsFromPEM([]byte(systemRootsPEM))
+	return p
 }
 `

src/crypto/x509/root_darwin_armx.go

@@ -10,9 +10,10 @@
 
 package x509
 
-func initSystemRoots() {
-	systemRoots = NewCertPool()
-	systemRoots.AppendCertsFromPEM([]byte(systemRootsPEM))
+func loadSystemRoots() (*CertPool, error) {
+	p := NewCertPool()
+	p.AppendCertsFromPEM([]byte(systemRootsPEM))
+	return p
 }
 
 const systemRootsPEM = `

src/crypto/x509/root_nocgo_darwin.go

@@ -6,6 +6,6 @@
 
 package x509
 
-func initSystemRoots() {
-	systemRoots, _ = execSecurityRoots()
+func loadSystemRoots() (*CertPool, error) {
+	return execSecurityRoots()
 }

src/crypto/x509/root_plan9.go

@@ -6,7 +6,10 @@
 
 package x509
 
-import "io/ioutil"
+import (
+	"io/ioutil"
+	"os"
+)
 
 // Possible certificate files; stop after finding one.
 var certFiles = []string{
@@ -17,17 +20,18 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return nil, nil
 }
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
+	var bestErr error
 	for _, file := range certFiles {
 		data, err := ioutil.ReadFile(file)
 		if err == nil {
 			roots.AppendCertsFromPEM(data)
-			systemRoots = roots
-			return
+			return roots, nil
+		}
+		if bestErr == nil || (os.IsNotExist(bestErr) && !os.IsNotExist(err)) {
+			bestErr = err
 		}
 	}
-
-	// All of the files failed to load. systemRoots will be nil which will
-	// trigger a specific error at verification time.
+	return nil, bestErr
 }

src/crypto/x509/root_unix.go

@@ -6,7 +6,10 @@
 
 package x509
 
-import "io/ioutil"
+import (
+	"io/ioutil"
+	"os"
+)
 
 // Possible directories with certificate files; stop after successfully
 // reading at least one file from a directory.
@@ -19,20 +22,26 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return nil, nil
 }
 
-func initSystemRoots() {
+func loadSystemRoots() (*CertPool, error) {
 	roots := NewCertPool()
+	var firstErr error
 	for _, file := range certFiles {
 		data, err := ioutil.ReadFile(file)
 		if err == nil {
 			roots.AppendCertsFromPEM(data)
-			systemRoots = roots
-			return
+			return roots, nil
+		}
+		if firstErr == nil && !os.IsNotExist(err) {
+			firstErr = err
 		}
 	}
 
 	for _, directory := range certDirectories {
 		fis, err := ioutil.ReadDir(directory)
 		if err != nil {
+			if firstErr == nil && !os.IsNotExist(err) {
+				firstErr = err
+			}
 			continue
 		}
 		rootsAdded := false
@@ -43,11 +52,9 @@ func initSystemRoots() {
 			}
 		}
 		if rootsAdded {
-			systemRoots = roots
-			return
+			return roots, nil
 		}
 	}
 
-	// All of the files failed to load. systemRoots will be nil which will
-	// trigger a specific error at verification time.
+	return nil, firstErr
 }

src/crypto/x509/root_windows.go

@@ -225,5 +225,4 @@ func (c *Certificate) systemVerify(opts *VerifyOptions) (chains [][]*Certificate
 	return chains, nil
 }
 
-func initSystemRoots() {
-}
+func loadSystemRoots() (*CertPool, error) { return nil, nil }

src/crypto/x509/verify.go

@@ -117,10 +117,16 @@ func (e UnknownAuthorityError) Error() string {
 }
 
 // SystemRootsError results when we fail to load the system root certificates.
-type SystemRootsError struct{}
+type SystemRootsError struct {
+	Err error
+}
 
-func (SystemRootsError) Error() string {
-	return "x509: failed to load system roots and no roots provided"
+func (se SystemRootsError) Error() string {
+	msg := "x509: failed to load system roots and no roots provided"
+	if se.Err != nil {
+		return msg + "; " + se.Err.Error()
+	}
+	return msg
 }
 
 // errNotParsed is returned when a certificate without ASN.1 contents is
@@ -240,7 +246,7 @@ func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err e
 	if opts.Roots == nil {
 		opts.Roots = systemRootsPool()
 		if opts.Roots == nil {
-			return nil, SystemRootsError{}
+			return nil, SystemRootsError{systemRootsErr}
 		}
 	}