From df523969435b8945d939c7e2a849b50910ef4c25 Mon Sep 17 00:00:00 2001 From: Filippo Valsorda Date: Tue, 4 Dec 2018 22:23:22 -0500 Subject: [PATCH] [release-branch.go1.11-security] 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 Change-Id: Ib257c12cd5563df7723f9c81231d82b882854213 Reviewed-on: https://team-review.git.corp.google.com/c/370475 Reviewed-by: Andrew Bonventre (cherry picked from commit 09d57361bc99cbbfb9755ee30ddcb42ff5a9d7d6) Reviewed-on: https://team-review.git.corp.google.com/c/372858 Reviewed-by: Dmitri Shuralyov --- src/crypto/x509/cert_pool.go | 28 ++------ src/crypto/x509/verify.go | 86 ++++++++++++++---------- src/crypto/x509/verify_test.go | 119 +++++++++++++++++++++++++++++++++ 3 files changed, 176 insertions(+), 57 deletions(-) diff --git a/src/crypto/x509/cert_pool.go b/src/crypto/x509/cert_pool.go index a1646b98261de..61107f86b8e10 100644 --- a/src/crypto/x509/cert_pool.go +++ b/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 { diff --git a/src/crypto/x509/verify.go b/src/crypto/x509/verify.go index 0b75778a03840..d84272ecb4e8e 100644 --- a/src/crypto/x509/verify.go +++ b/src/crypto/x509/verify.go @@ -765,7 +765,7 @@ 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 } } @@ -802,58 +802,74 @@ 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} } diff --git a/src/crypto/x509/verify_test.go b/src/crypto/x509/verify_test.go index 0e24d3b5da3af..85f4703d4c3fd 100644 --- a/src/crypto/x509/verify_test.go +++ b/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)) +}