crypto/x509: limit number of signature checks for each verification

That number grows quadratically with the number of intermediate
certificates in certain pathological cases (for example if they all have
the same Subject) leading to a CPU DoS. Set a fixed budget that should
fit all real world chains, given we only look at intermediates provided
by the peer.

The algorithm can be improved, but that's left for follow-up CLs:

    * the cache logic should be reviewed for correctness, as it seems to
      override the entire chain with the cached one
    * the equality check should compare Subject and public key, not the
      whole certificate
    * certificates with the right SKID but the wrong Subject should not
      be considered, and in particular should not take priority over
      certificates with the right Subject

Fixes #29233

Change-Id: Ib257c12cd5563df7723f9c81231d82b882854213
Reviewed-on: https://team-review.git.corp.google.com/c/370475Reviewed-by: Andrew Bonventre <andybons@google.com>
Reviewed-on: https://go-review.googlesource.com/c/154105Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>

src/crypto/x509/cert_pool.go

@@ -65,32 +65,16 @@ func SystemCertPool() (*CertPool, error) {
 	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
-// rejected.
-func (s *CertPool) findVerifiedParents(cert *Certificate) (parents []int, errCert *Certificate, err error) {
+// findPotentialParents returns the indexes of certificates in s which might
+// have signed cert. The caller must not modify the returned slice.
+func (s *CertPool) findPotentialParents(cert *Certificate) []int {
 	if s == nil {
-		return
+		return nil
 	}
-	var candidates []int
-
 	if len(cert.AuthorityKeyId) > 0 {
-		candidates = s.bySubjectKeyId[string(cert.AuthorityKeyId)]
-	}
-	if len(candidates) == 0 {
-		candidates = s.byName[string(cert.RawIssuer)]
+		return s.bySubjectKeyId[string(cert.AuthorityKeyId)]
 	}
-
-	for _, c := range candidates {
-		if err = cert.CheckSignatureFrom(s.certs[c]); err == nil {
-			parents = append(parents, c)
-		} else {
-			errCert = s.certs[c]
-		}
-	}
-
-	return
+	return s.byName[string(cert.RawIssuer)]
 }
 
 func (s *CertPool) contains(cert *Certificate) bool {

src/crypto/x509/verify.go

@@ -763,7 +763,7 @@ func (c *Certificate) Verify(opts VerifyOptions) (chains [][]*Certificate, err e
 	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 {
+		if candidateChains, err = c.buildChains(nil, []*Certificate{c}, nil, &opts); err != nil {
 			return nil, err
 		}
 	}
@@ -800,58 +800,74 @@ func appendToFreshChain(chain []*Certificate, cert *Certificate) []*Certificate
 	return n
 }
 
-func (c *Certificate) buildChains(cache map[int][][]*Certificate, currentChain []*Certificate, opts *VerifyOptions) (chains [][]*Certificate, err error) {
-	possibleRoots, failedRoot, rootErr := opts.Roots.findVerifiedParents(c)
-nextRoot:
-	for _, rootNum := range possibleRoots {
-		root := opts.Roots.certs[rootNum]
+// maxChainSignatureChecks is the maximum number of CheckSignatureFrom calls
+// that an invocation of buildChains will (tranistively) make. Most chains are
+// less than 15 certificates long, so this leaves space for multiple chains and
+// for failed checks due to different intermediates having the same Subject.
+const maxChainSignatureChecks = 100
 
+func (c *Certificate) buildChains(cache map[*Certificate][][]*Certificate, currentChain []*Certificate, sigChecks *int, opts *VerifyOptions) (chains [][]*Certificate, err error) {
+	var (
+		hintErr  error
+		hintCert *Certificate
+	)
+
+	considerCandidate := func(certType int, candidate *Certificate) {
 		for _, cert := range currentChain {
-			if cert.Equal(root) {
-				continue nextRoot
+			if cert.Equal(candidate) {
+				return
 			}
 		}
 
-		err = root.isValid(rootCertificate, currentChain, opts)
-		if err != nil {
-			continue
+		if sigChecks == nil {
+			sigChecks = new(int)
+		}
+		*sigChecks++
+		if *sigChecks > maxChainSignatureChecks {
+			err = errors.New("x509: signature check attempts limit reached while verifying certificate chain")
+			return
 		}
-		chains = append(chains, appendToFreshChain(currentChain, root))
-	}
 
-	possibleIntermediates, failedIntermediate, intermediateErr := opts.Intermediates.findVerifiedParents(c)
-nextIntermediate:
-	for _, intermediateNum := range possibleIntermediates {
-		intermediate := opts.Intermediates.certs[intermediateNum]
-		for _, cert := range currentChain {
-			if cert.Equal(intermediate) {
-				continue nextIntermediate
+		if err := c.CheckSignatureFrom(candidate); err != nil {
+			if hintErr == nil {
+				hintErr = err
+				hintCert = candidate
 			}
+			return
 		}
-		err = intermediate.isValid(intermediateCertificate, currentChain, opts)
+
+		err = candidate.isValid(certType, currentChain, opts)
 		if err != nil {
-			continue
+			return
 		}
-		var childChains [][]*Certificate
-		childChains, ok := cache[intermediateNum]
-		if !ok {
-			childChains, err = intermediate.buildChains(cache, appendToFreshChain(currentChain, intermediate), opts)
-			cache[intermediateNum] = childChains
+
+		switch certType {
+		case rootCertificate:
+			chains = append(chains, appendToFreshChain(currentChain, candidate))
+		case intermediateCertificate:
+			if cache == nil {
+				cache = make(map[*Certificate][][]*Certificate)
+			}
+			childChains, ok := cache[candidate]
+			if !ok {
+				childChains, err = candidate.buildChains(cache, appendToFreshChain(currentChain, candidate), sigChecks, opts)
+				cache[candidate] = childChains
+			}
+			chains = append(chains, childChains...)
 		}
-		chains = append(chains, childChains...)
+	}
+
+	for _, rootNum := range opts.Roots.findPotentialParents(c) {
+		considerCandidate(rootCertificate, opts.Roots.certs[rootNum])
+	}
+	for _, intermediateNum := range opts.Intermediates.findPotentialParents(c) {
+		considerCandidate(intermediateCertificate, opts.Intermediates.certs[intermediateNum])
 	}
 
 	if len(chains) > 0 {
 		err = nil
 	}
-
 	if len(chains) == 0 && err == nil {
-		hintErr := rootErr
-		hintCert := failedRoot
-		if hintErr == nil {
-			hintErr = intermediateErr
-			hintCert = failedIntermediate
-		}
 		err = UnknownAuthorityError{c, hintErr, hintCert}
 	}
 

src/crypto/x509/verify_test.go

@@ -5,10 +5,15 @@
 package x509
 
 import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
 	"crypto/x509/pkix"
 	"encoding/pem"
 	"errors"
 	"fmt"
+	"math/big"
 	"runtime"
 	"strings"
 	"testing"
@@ -1889,3 +1894,117 @@ func TestValidHostname(t *testing.T) {
 		}
 	}
 }
+
+func generateCert(cn string, isCA bool, issuer *Certificate, issuerKey crypto.PrivateKey) (*Certificate, crypto.PrivateKey, error) {
+	priv, err := ecdsa.GenerateKey(elliptic.P256(), rand.Reader)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	serialNumberLimit := new(big.Int).Lsh(big.NewInt(1), 128)
+	serialNumber, _ := rand.Int(rand.Reader, serialNumberLimit)
+
+	template := &Certificate{
+		SerialNumber: serialNumber,
+		Subject:      pkix.Name{CommonName: cn},
+		NotBefore:    time.Now().Add(-1 * time.Hour),
+		NotAfter:     time.Now().Add(24 * time.Hour),
+
+		KeyUsage:              KeyUsageKeyEncipherment | KeyUsageDigitalSignature | KeyUsageCertSign,
+		ExtKeyUsage:           []ExtKeyUsage{ExtKeyUsageServerAuth},
+		BasicConstraintsValid: true,
+		IsCA:                  isCA,
+	}
+	if issuer == nil {
+		issuer = template
+		issuerKey = priv
+	}
+
+	derBytes, err := CreateCertificate(rand.Reader, template, issuer, priv.Public(), issuerKey)
+	if err != nil {
+		return nil, nil, err
+	}
+	cert, err := ParseCertificate(derBytes)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return cert, priv, nil
+}
+
+func TestPathologicalChain(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping generation of a long chain of certificates in short mode")
+	}
+
+	// Build a chain where all intermediates share the same subject, to hit the
+	// path building worst behavior.
+	roots, intermediates := NewCertPool(), NewCertPool()
+
+	parent, parentKey, err := generateCert("Root CA", true, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	roots.AddCert(parent)
+
+	for i := 1; i < 100; i++ {
+		parent, parentKey, err = generateCert("Intermediate CA", true, parent, parentKey)
+		if err != nil {
+			t.Fatal(err)
+		}
+		intermediates.AddCert(parent)
+	}
+
+	leaf, _, err := generateCert("Leaf", false, parent, parentKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	start := time.Now()
+	_, err = leaf.Verify(VerifyOptions{
+		Roots:         roots,
+		Intermediates: intermediates,
+	})
+	t.Logf("verification took %v", time.Since(start))
+
+	if err == nil || !strings.Contains(err.Error(), "signature check attempts limit") {
+		t.Errorf("expected verification to fail with a signature checks limit error; got %v", err)
+	}
+}
+
+func TestLongChain(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping generation of a long chain of certificates in short mode")
+	}
+
+	roots, intermediates := NewCertPool(), NewCertPool()
+
+	parent, parentKey, err := generateCert("Root CA", true, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	roots.AddCert(parent)
+
+	for i := 1; i < 15; i++ {
+		name := fmt.Sprintf("Intermediate CA #%d", i)
+		parent, parentKey, err = generateCert(name, true, parent, parentKey)
+		if err != nil {
+			t.Fatal(err)
+		}
+		intermediates.AddCert(parent)
+	}
+
+	leaf, _, err := generateCert("Leaf", false, parent, parentKey)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	start := time.Now()
+	if _, err := leaf.Verify(VerifyOptions{
+		Roots:         roots,
+		Intermediates: intermediates,
+	}); err != nil {
+		t.Error(err)
+	}
+	t.Logf("verification took %v", time.Since(start))
+}