mirror of https://github.com/containers/podman.git
104 lines
4.0 KiB
Go
104 lines
4.0 KiB
Go
//go:build linux
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// Copyright (C) 2024 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
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import (
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"fmt"
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"os"
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"strconv"
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"golang.org/x/sys/unix"
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)
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// OpenatInRoot is equivalent to [OpenInRoot], except that the root is provided
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// using an *[os.File] handle, to ensure that the correct root directory is used.
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func OpenatInRoot(root *os.File, unsafePath string) (*os.File, error) {
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handle, err := completeLookupInRoot(root, unsafePath)
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if err != nil {
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return nil, &os.PathError{Op: "securejoin.OpenInRoot", Path: unsafePath, Err: err}
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}
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return handle, nil
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}
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// OpenInRoot safely opens the provided unsafePath within the root.
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// Effectively, OpenInRoot(root, unsafePath) is equivalent to
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//
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// path, _ := securejoin.SecureJoin(root, unsafePath)
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// handle, err := os.OpenFile(path, unix.O_PATH|unix.O_CLOEXEC)
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//
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// But is much safer. The above implementation is unsafe because if an attacker
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// can modify the filesystem tree between [SecureJoin] and [os.OpenFile], it is
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// possible for the returned file to be outside of the root.
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//
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// Note that the returned handle is an O_PATH handle, meaning that only a very
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// limited set of operations will work on the handle. This is done to avoid
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// accidentally opening an untrusted file that could cause issues (such as a
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// disconnected TTY that could cause a DoS, or some other issue). In order to
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// use the returned handle, you can "upgrade" it to a proper handle using
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// [Reopen].
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func OpenInRoot(root, unsafePath string) (*os.File, error) {
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rootDir, err := os.OpenFile(root, unix.O_PATH|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
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if err != nil {
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return nil, err
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}
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defer rootDir.Close()
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return OpenatInRoot(rootDir, unsafePath)
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}
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// Reopen takes an *[os.File] handle and re-opens it through /proc/self/fd.
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// Reopen(file, flags) is effectively equivalent to
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//
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// fdPath := fmt.Sprintf("/proc/self/fd/%d", file.Fd())
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// os.OpenFile(fdPath, flags|unix.O_CLOEXEC)
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//
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// But with some extra hardenings to ensure that we are not tricked by a
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// maliciously-configured /proc mount. While this attack scenario is not
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// common, in container runtimes it is possible for higher-level runtimes to be
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// tricked into configuring an unsafe /proc that can be used to attack file
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// operations. See [CVE-2019-19921] for more details.
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//
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// [CVE-2019-19921]: https://github.com/advisories/GHSA-fh74-hm69-rqjw
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func Reopen(handle *os.File, flags int) (*os.File, error) {
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procRoot, err := getProcRoot()
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if err != nil {
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return nil, err
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}
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// We can't operate on /proc/thread-self/fd/$n directly when doing a
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// re-open, so we need to open /proc/thread-self/fd and then open a single
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// final component.
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procFdDir, closer, err := procThreadSelf(procRoot, "fd/")
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if err != nil {
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return nil, fmt.Errorf("get safe /proc/thread-self/fd handle: %w", err)
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}
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defer procFdDir.Close()
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defer closer()
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// Try to detect if there is a mount on top of the magic-link we are about
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// to open. If we are using unsafeHostProcRoot(), this could change after
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// we check it (and there's nothing we can do about that) but for
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// privateProcRoot() this should be guaranteed to be safe (at least since
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// Linux 5.12[1], when anonymous mount namespaces were completely isolated
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// from external mounts including mount propagation events).
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//
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// [1]: Linux commit ee2e3f50629f ("mount: fix mounting of detached mounts
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// onto targets that reside on shared mounts").
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fdStr := strconv.Itoa(int(handle.Fd()))
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if err := checkSymlinkOvermount(procRoot, procFdDir, fdStr); err != nil {
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return nil, fmt.Errorf("check safety of /proc/thread-self/fd/%s magiclink: %w", fdStr, err)
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}
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flags |= unix.O_CLOEXEC
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// Rather than just wrapping openatFile, open-code it so we can copy
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// handle.Name().
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reopenFd, err := unix.Openat(int(procFdDir.Fd()), fdStr, flags, 0)
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if err != nil {
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return nil, fmt.Errorf("reopen fd %d: %w", handle.Fd(), err)
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}
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return os.NewFile(uintptr(reopenFd), handle.Name()), nil
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}
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