116 lines
4.1 KiB
Go
116 lines
4.1 KiB
Go
// Copyright (C) 2014-2015 Docker Inc & Go Authors. All rights reserved.
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// Copyright (C) 2017 SUSE LLC. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package securejoin is an implementation of the hopefully-soon-to-be-included
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// SecureJoin helper that is meant to be part of the "path/filepath" package.
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// The purpose of this project is to provide a PoC implementation to make the
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// SecureJoin proposal (https://github.com/golang/go/issues/20126) more
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// tangible.
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package securejoin
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import (
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"bytes"
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"errors"
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"os"
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"path/filepath"
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"strings"
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"syscall"
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)
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// IsNotExist tells you if err is an error that implies that either the path
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// accessed does not exist (or path components don't exist). This is
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// effectively a more broad version of os.IsNotExist.
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func IsNotExist(err error) bool {
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// Check that it's not actually an ENOTDIR, which in some cases is a more
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// convoluted case of ENOENT (usually involving weird paths).
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return errors.Is(err, os.ErrNotExist) || errors.Is(err, syscall.ENOTDIR) || errors.Is(err, syscall.ENOENT)
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}
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// SecureJoinVFS joins the two given path components (similar to Join) except
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// that the returned path is guaranteed to be scoped inside the provided root
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// path (when evaluated). Any symbolic links in the path are evaluated with the
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// given root treated as the root of the filesystem, similar to a chroot. The
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// filesystem state is evaluated through the given VFS interface (if nil, the
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// standard os.* family of functions are used).
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//
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// Note that the guarantees provided by this function only apply if the path
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// components in the returned string are not modified (in other words are not
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// replaced with symlinks on the filesystem) after this function has returned.
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// Such a symlink race is necessarily out-of-scope of SecureJoin.
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func SecureJoinVFS(root, unsafePath string, vfs VFS) (string, error) {
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// Use the os.* VFS implementation if none was specified.
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if vfs == nil {
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vfs = osVFS{}
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}
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var path bytes.Buffer
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n := 0
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for unsafePath != "" {
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if n > 255 {
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return "", &os.PathError{Op: "SecureJoin", Path: root + "/" + unsafePath, Err: syscall.ELOOP}
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}
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// Next path component, p.
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i := strings.IndexRune(unsafePath, filepath.Separator)
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var p string
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if i == -1 {
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p, unsafePath = unsafePath, ""
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} else {
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p, unsafePath = unsafePath[:i], unsafePath[i+1:]
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}
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// Create a cleaned path, using the lexical semantics of /../a, to
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// create a "scoped" path component which can safely be joined to fullP
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// for evaluation. At this point, path.String() doesn't contain any
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// symlink components.
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cleanP := filepath.Clean(string(filepath.Separator) + path.String() + p)
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if cleanP == string(filepath.Separator) {
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path.Reset()
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continue
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}
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fullP := filepath.Clean(root + cleanP)
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// Figure out whether the path is a symlink.
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fi, err := vfs.Lstat(fullP)
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if err != nil && !IsNotExist(err) {
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return "", err
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}
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// Treat non-existent path components the same as non-symlinks (we
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// can't do any better here).
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if IsNotExist(err) || fi.Mode()&os.ModeSymlink == 0 {
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path.WriteString(p)
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path.WriteRune(filepath.Separator)
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continue
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}
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// Only increment when we actually dereference a link.
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n++
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// It's a symlink, expand it by prepending it to the yet-unparsed path.
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dest, err := vfs.Readlink(fullP)
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if err != nil {
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return "", err
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}
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// Absolute symlinks reset any work we've already done.
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if filepath.IsAbs(dest) {
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path.Reset()
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}
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unsafePath = dest + string(filepath.Separator) + unsafePath
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}
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// We have to clean path.String() here because it may contain '..'
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// components that are entirely lexical, but would be misleading otherwise.
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// And finally do a final clean to ensure that root is also lexically
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// clean.
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fullP := filepath.Clean(string(filepath.Separator) + path.String())
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return filepath.Clean(root + fullP), nil
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}
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// SecureJoin is a wrapper around SecureJoinVFS that just uses the os.* library
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// of functions as the VFS. If in doubt, use this function over SecureJoinVFS.
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func SecureJoin(root, unsafePath string) (string, error) {
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return SecureJoinVFS(root, unsafePath, nil)
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}
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