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// Copyright (C) 2014-2015 Docker Inc & Go Authors. All rights reserved.
// Copyright (C) 2017-2024 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 (
"errors"
"os"
"path/filepath"
"strings"
"syscall"
)
const maxSymlinkLimit = 255
// 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 {
// Check that it's not actually an ENOTDIR, which in some cases is a more
// convoluted case of ENOENT (usually involving weird paths).
return errors.Is(err, os.ErrNotExist) || errors.Is(err, syscall.ENOTDIR) || errors.Is(err, 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.
//
// NOTE: Due to the above limitation, Linux users are strongly encouraged to
// use OpenInRoot instead, which does safely protect against these kinds of
// attacks. There is no way to solve this problem with SecureJoinVFS because
// the API is fundamentally wrong (you cannot return a "safe" path string and
// guarantee it won't be modified afterwards).
//
// Volume names in unsafePath are always discarded, regardless if they are
// provided via direct input or when evaluating symlinks. Therefore:
//
// "C:\Temp" + "D:\path\to\file.txt" results in "C:\Temp\path\to\file.txt"
func SecureJoinVFS(root, unsafePath string, vfs VFS) (string, error) {
// Use the os.* VFS implementation if none was specified.
if vfs == nil {
vfs = osVFS{}
}
unsafePath = filepath.FromSlash(unsafePath)
var (
currentPath string
remainingPath = unsafePath
linksWalked int
)
for remainingPath != "" {
if v := filepath.VolumeName(remainingPath); v != "" {
remainingPath = remainingPath[len(v):]
}
// Get the next path component.
var part string
if i := strings.IndexRune(remainingPath, filepath.Separator); i == -1 {
part, remainingPath = remainingPath, ""
} else {
part, remainingPath = remainingPath[:i], remainingPath[i+1:]
}
// Apply the component lexically to the path we are building.
// currentPath does not contain any symlinks, and we are lexically
// dealing with a single component, so it's okay to do a filepath.Clean
// here.
nextPath := filepath.Join(string(filepath.Separator), currentPath, part)
if nextPath == string(filepath.Separator) {
currentPath = ""
continue
}
fullPath := root + string(filepath.Separator) + nextPath
// Figure out whether the path is a symlink.
fi, err := vfs.Lstat(fullPath)
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 {
currentPath = nextPath
continue
}
// It's a symlink, so get its contents and expand it by prepending it
// to the yet-unparsed path.
linksWalked++
if linksWalked > maxSymlinkLimit {
return "", &os.PathError{Op: "SecureJoin", Path: root + string(filepath.Separator) + unsafePath, Err: syscall.ELOOP}
}
dest, err := vfs.Readlink(fullPath)
if err != nil {
return "", err
}
remainingPath = dest + string(filepath.Separator) + remainingPath
// Absolute symlinks reset any work we've already done.
if filepath.IsAbs(dest) {
currentPath = ""
}
}
// There should be no lexical components like ".." left in the path here,
// but for safety clean up the path before joining it to the root.
finalPath := filepath.Join(string(filepath.Separator), currentPath)
return filepath.Join(root, finalPath), 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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