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