helm: introduce customized chart loaders

This introduces our own `secureloader` package, with a directory
loader that's capable of following symlinks while validating they stay
within a certain root boundary.

Signed-off-by: Hidde Beydals <[email protected]>

Author: hiddeco

internal/helm/chart/secureloader/directory.go

@@ -0,0 +1,208 @@
+/*
+Copyright The Helm Authors.
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+
+This file has been derived from
+https://github.com/helm/helm/blob/v3.8.1/pkg/chart/loader/directory.go.
+
+It has been modified to not blindly accept any resolved symlink path, but
+instead check it against the configured root before allowing it to be included.
+It also allows for capping the size of any file loaded into the chart.
+*/
+
+package secureloader
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	securejoin "github.com/cyphar/filepath-securejoin"
+	"helm.sh/helm/v3/pkg/chart"
+	"helm.sh/helm/v3/pkg/chart/loader"
+
+	"github.com/fluxcd/source-controller/internal/helm/chart/secureloader/ignore"
+	"github.com/fluxcd/source-controller/internal/helm/chart/secureloader/sympath"
+)
+
+var (
+	// DefaultMaxFileSize is the default maximum file size of any chart file
+	// loaded.
+	DefaultMaxFileSize = 16 << 20 // 16MiB
+
+	utf8bom = []byte{0xEF, 0xBB, 0xBF}
+)
+
+// SecureDirLoader securely loads a chart from a directory while resolving
+// symlinks without including files outside root.
+type SecureDirLoader struct {
+	root    string
+	dir     string
+	maxSize int
+}
+
+// NewSecureDirLoader returns a new SecureDirLoader, configured to the scope of the
+// root and provided dir. Max size configures the maximum size a file must not
+// exceed to be loaded. If 0 it defaults to defaultMaxFileSize, it can be
+// disabled using a negative integer.
+func NewSecureDirLoader(root string, dir string, maxSize int) SecureDirLoader {
+	if maxSize == 0 {
+		maxSize = DefaultMaxFileSize
+	}
+	return SecureDirLoader{
+		root:    root,
+		dir:     dir,
+		maxSize: maxSize,
+	}
+}
+
+// Load loads and returns the chart.Chart, or an error.
+func (l SecureDirLoader) Load() (*chart.Chart, error) {
+	return SecureLoadDir(l.root, l.dir, l.maxSize)
+}
+
+// SecureLoadDir securely loads from a directory, without going outside root.
+func SecureLoadDir(root, dir string, maxSize int) (*chart.Chart, error) {
+	root, err := filepath.Abs(root)
+	if err != nil {
+		return nil, err
+	}
+
+	topDir, err := filepath.Abs(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	// Confirm topDir is actually relative to root
+	if _, err = isSecureSymlinkPath(root, topDir); err != nil {
+		return nil, fmt.Errorf("cannot load chart from dir: %w", err)
+	}
+
+	// Just used for errors
+	c := &chart.Chart{}
+
+	// Get the absolute location of the .helmignore file
+	relDirPath, err := filepath.Rel(root, topDir)
+	if err != nil {
+		// We are not expected to be returning this error, as the above call to
+		// isSecureSymlinkPath already does the same. However, especially
+		// because we are dealing with security aspects here, we check it
+		// anyway in case this assumption changes.
+		return nil, err
+	}
+	iFile, err := securejoin.SecureJoin(root, filepath.Join(relDirPath, ignore.HelmIgnore))
+
+	// Load the .helmignore rules
+	rules := ignore.Empty()
+	if _, err = os.Stat(iFile); err == nil {
+		r, err := ignore.ParseFile(iFile)
+		if err != nil {
+			return c, err
+		}
+		rules = r
+	}
+	rules.AddDefaults()
+
+	var files []*loader.BufferedFile
+	topDir += string(filepath.Separator)
+
+	walk := func(name, absoluteName string, fi os.FileInfo, err error) error {
+		n := strings.TrimPrefix(name, topDir)
+		if n == "" {
+			// No need to process top level. Avoid bug with helmignore .* matching
+			// empty names. See issue 1779.
+			return nil
+		}
+
+		// Normalize to / since it will also work on Windows
+		n = filepath.ToSlash(n)
+
+		if err != nil {
+			return err
+		}
+		if fi.IsDir() {
+			// Directory-based ignore rules should involve skipping the entire
+			// contents of that directory.
+			if rules.Ignore(n, fi) {
+				return filepath.SkipDir
+			}
+			// Check after excluding ignores to provide the user with an option
+			// to opt-out from including certain paths.
+			if _, err := isSecureSymlinkPath(root, absoluteName); err != nil {
+				return fmt.Errorf("cannot load '%s' directory: %w", n, err)
+			}
+			return nil
+		}
+
+		// If a .helmignore file matches, skip this file.
+		if rules.Ignore(n, fi) {
+			return nil
+		}
+
+		// Check after excluding ignores to provide the user with an option
+		// to opt-out from including certain paths.
+		if _, err := isSecureSymlinkPath(root, absoluteName); err != nil {
+			return fmt.Errorf("cannot load '%s' file: %w", n, err)
+		}
+
+		// Irregular files include devices, sockets, and other uses of files that
+		// are not regular files. In Go they have a file mode type bit set.
+		// See https://golang.org/pkg/os/#FileMode for examples.
+		if !fi.Mode().IsRegular() {
+			return fmt.Errorf("cannot load irregular file %s as it has file mode type bits set", n)
+		}
+
+		if fileSize := fi.Size(); maxSize > 0 && fileSize > int64(maxSize) {
+			return fmt.Errorf("cannot load file %s as file size (%d) exceeds limit (%d)", n, fileSize, maxSize)
+		}
+
+		data, err := os.ReadFile(name)
+		if err != nil {
+			return fmt.Errorf("error reading %s: %w", n, err)
+		}
+		data = bytes.TrimPrefix(data, utf8bom)
+
+		files = append(files, &loader.BufferedFile{Name: n, Data: data})
+		return nil
+	}
+	if err = sympath.Walk(topDir, walk); err != nil {
+		return c, err
+	}
+	return loader.LoadFiles(files)
+}
+
+// isSecureSymlinkPath attempts to make the given absolute path relative to
+// root and securely joins this with root. If the result equals absolute path,
+// it is safe to use.
+func isSecureSymlinkPath(root, absPath string) (bool, error) {
+	root, absPath = filepath.Clean(root), filepath.Clean(absPath)
+	if root == "/" {
+		return true, nil
+	}
+	unsafePath, err := filepath.Rel(root, absPath)
+	if err != nil {
+		return false, fmt.Errorf("cannot calculate path relative to root for resolved symlink")
+	}
+	safePath, err := securejoin.SecureJoin(root, unsafePath)
+	if err != nil {
+		return false, fmt.Errorf("cannot securely join root with resolved relative symlink path")
+	}
+	if safePath != absPath {
+		return false, fmt.Errorf("symlink traverses outside root boundary: relative path to root %s", unsafePath)
+	}
+	return true, nil
+}

internal/helm/chart/secureloader/directory_test.go

@@ -0,0 +1,82 @@
+/*
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package secureloader
+
+import (
+	"testing"
+
+	. "github.com/onsi/gomega"
+)
+
+func Test_isSecureSymlinkPath(t *testing.T) {
+	tests := []struct {
+		name    string
+		root    string
+		absPath string
+		safe    bool
+		wantErr string
+	}{
+		{
+			name:    "absolute path in root",
+			root:    "/",
+			absPath: "/bar/",
+			safe:    true,
+		},
+
+		{
+			name:    "abs path not relative to root",
+			root:    "/working/dir",
+			absPath: "/working/in/another/dir",
+			safe:    false,
+			wantErr: "symlink traverses outside root boundary",
+		},
+		{
+			name:    "abs path relative to root",
+			root:    "/working/dir/",
+			absPath: "/working/dir/path",
+			safe:    true,
+		},
+		{
+			name:    "illegal abs path",
+			root:    "/working/dir",
+			absPath: "/working/dir/../but/not/really",
+			safe:    false,
+			wantErr: "symlink traverses outside root boundary",
+		},
+		{
+			name:    "illegal root",
+			root:    "working/dir/",
+			absPath: "/working/dir",
+			safe:    false,
+			wantErr: "cannot calculate path relative to root for resolved symlink",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			g := NewWithT(t)
+
+			got, err := isSecureSymlinkPath(tt.root, tt.absPath)
+			g.Expect(got).To(Equal(tt.safe))
+			if tt.wantErr != "" {
+				g.Expect(err).To(HaveOccurred())
+				g.Expect(err.Error()).To(ContainSubstring(tt.wantErr))
+				return
+			}
+			g.Expect(err).ToNot(HaveOccurred())
+		})
+	}
+}

internal/helm/chart/secureloader/file.go

@@ -0,0 +1,47 @@
+/*
+Copyright The Helm Authors.
+Copyright 2022 The Flux authors
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package secureloader
+
+import (
+	"io"
+
+	"helm.sh/helm/v3/pkg/chart"
+	"helm.sh/helm/v3/pkg/chart/loader"
+)
+
+// FileLoader is equal to Helm's.
+// Redeclared to avoid having to deal with multiple package imports,
+// possibly resulting in using the non-secure directory loader.
+type FileLoader = loader.FileLoader
+
+// LoadFile loads from an archive file.
+func LoadFile(name string) (*chart.Chart, error) {
+	return loader.LoadFile(name)
+}
+
+// LoadArchiveFiles reads in files out of an archive into memory. This function
+// performs important path security checks and should always be used before
+// expanding a tarball
+func LoadArchiveFiles(in io.Reader) ([]*loader.BufferedFile, error) {
+	return loader.LoadArchiveFiles(in)
+}
+
+// LoadArchive loads from a reader containing a compressed tar archive.
+func LoadArchive(in io.Reader) (*chart.Chart, error) {
+	return loader.LoadArchive(in)
+}