crypto/x509: return errors instead of panicking

Eliminate a panic in x509.CreateCertificate when passing templates with unknown ExtKeyUsage; return an error instead.

Fixes #41169

Change-Id: Ia229d3b0d4a1bdeef05928439d97dab228687b3c
Reviewed-on: https://go-review.googlesource.com/c/go/+/252557
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Filippo Valsorda <[email protected]>
Run-TryBot: Roland Shoemaker <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: tpaschalis

src/crypto/x509/x509.go

@@ -1689,7 +1689,8 @@ func buildExtensions(template *Certificate, subjectIsEmpty bool, authorityKeyId
 			if oid, ok := oidFromExtKeyUsage(u); ok {
 				oids = append(oids, oid)
 			} else {
-				panic("internal error")
+				err = errors.New("x509: unknown extended key usage")
+				return
 			}
 		}
 

src/crypto/x509/x509_test.go

@@ -2754,3 +2754,22 @@ func TestRSAPSAParameters(t *testing.T) {
 		}
 	}
 }
+
+func TestUnknownExtKey(t *testing.T) {
+	const errorContains = "unknown extended key usage"
+
+	template := &Certificate{
+		SerialNumber: big.NewInt(10),
+		DNSNames:     []string{"foo"},
+		ExtKeyUsage:  []ExtKeyUsage{ExtKeyUsage(-1)},
+	}
+	signer, err := rsa.GenerateKey(rand.Reader, 1024)
+	if err != nil {
+		t.Errorf("failed to generate key for TestUnknownExtKey")
+	}
+
+	_, err = CreateCertificate(rand.Reader, template, template, signer.Public(), signer)
+	if !strings.Contains(err.Error(), errorContains) {
+		t.Errorf("expected error containing %q, got %s", errorContains, err)
+	}
+}