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merge #15 into cyphar/filepath-securejoin:main
Aleksa Sarai (6): procfs: make procSelfFdReadlink more generic with generics open: add Open(at)InRoot and Reopen open: add OpenInRoot and Reopen tests mkdirall: switch away from O_PATH for mkdir loop README: update to describe and strongly recommend new APIs LGTMs: cyphar
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,83 @@ | ||
| //go:build linux | ||
|
|
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| // Copyright (C) 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 | ||
|
|
||
| import ( | ||
| "fmt" | ||
| "os" | ||
|
|
||
| "golang.org/x/sys/unix" | ||
| ) | ||
|
|
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| // OpenatInRoot is equivalent to OpenInRoot, except that the root is provided | ||
| // using an *os.File handle, to ensure that the correct root directory is used. | ||
| func OpenatInRoot(root *os.File, unsafePath string) (*os.File, error) { | ||
| handle, remainingPath, err := partialLookupInRoot(root, unsafePath) | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
| if remainingPath != "" { | ||
| _ = handle.Close() | ||
| return nil, &os.PathError{Op: "securejoin.OpenInRoot", Path: unsafePath, Err: unix.ENOENT} | ||
| } | ||
| return handle, nil | ||
| } | ||
|
|
||
| // OpenInRoot safely opens the provided unsafePath within the root. | ||
| // Effectively, OpenInRoot(root, unsafePath) is equivalent to | ||
| // | ||
| // path, _ := securejoin.SecureJoin(root, unsafePath) | ||
| // handle, err := os.OpenFile(path, unix.O_PATH|unix.O_CLOEXEC) | ||
| // | ||
| // But is much safer. The above implementation is unsafe because if an attacker | ||
| // can modify the filesystem tree between SecureJoin and OpenFile, it is | ||
| // possible for the returned file to be outside of the root. | ||
| // | ||
| // Note that the returned handle is an O_PATH handle, meaning that only a very | ||
| // limited set of operations will work on the handle. This is done to avoid | ||
| // accidentally opening an untrusted file that could cause issues (such as a | ||
| // disconnected TTY that could cause a DoS, or some other issue). In order to | ||
| // use the returned handle, you can "upgrade" it to a proper handle using | ||
| // Reopen. | ||
| func OpenInRoot(root, unsafePath string) (*os.File, error) { | ||
| rootDir, err := os.OpenFile(root, unix.O_PATH|unix.O_DIRECTORY|unix.O_CLOEXEC, 0) | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
| defer rootDir.Close() | ||
| return OpenatInRoot(rootDir, unsafePath) | ||
| } | ||
|
|
||
| // Reopen takes an *os.File handle and re-opens it through /proc/self/fd. | ||
| // Reopen(file, flags) is effectively equivalent to | ||
| // | ||
| // fdPath := fmt.Sprintf("/proc/self/fd/%d", file.Fd()) | ||
| // os.OpenFile(fdPath, flags|unix.O_CLOEXEC) | ||
| // | ||
| // But with some extra hardenings to ensure that we are not tricked by a | ||
| // maliciously-configured /proc mount. While this attack scenario is not | ||
| // common, in container runtimes it is possible for higher-level runtimes to be | ||
| // tricked into configuring an unsafe /proc that can be used to attack file | ||
| // operations. See CVE-2019-19921 for more details. | ||
| func Reopen(handle *os.File, flags int) (*os.File, error) { | ||
| procRoot, err := getProcRoot() | ||
| if err != nil { | ||
| return nil, err | ||
| } | ||
|
|
||
| flags |= unix.O_CLOEXEC | ||
| fdPath := fmt.Sprintf("fd/%d", handle.Fd()) | ||
| return doProcSelfMagiclink(procRoot, fdPath, func(procDirHandle *os.File, base string) (*os.File, error) { | ||
| // Rather than just wrapping openatFile, open-code it so we can copy | ||
| // handle.Name(). | ||
| reopenFd, err := unix.Openat(int(procDirHandle.Fd()), base, flags, 0) | ||
| if err != nil { | ||
| return nil, fmt.Errorf("reopen fd %d: %w", handle.Fd(), err) | ||
| } | ||
| return os.NewFile(uintptr(reopenFd), handle.Name()), nil | ||
| }) | ||
| } |
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