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169 lines
6.9 KiB
Markdown
169 lines
6.9 KiB
Markdown
## `filepath-securejoin` ##
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[](https://github.com/cyphar/filepath-securejoin/actions/workflows/ci.yml)
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### Old API ###
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This library was originally just an implementation of `SecureJoin` which was
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[intended to be included in the Go standard library][go#20126] as a safer
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`filepath.Join` that would restrict the path lookup to be inside a root
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directory.
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The implementation was based on code that existed in several container
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runtimes. Unfortunately, this API is **fundamentally unsafe** against attackers
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that can modify path components after `SecureJoin` returns and before the
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caller uses the path, allowing for some fairly trivial TOCTOU attacks.
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`SecureJoin` (and `SecureJoinVFS`) are still provided by this library to
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support legacy users, but new users are strongly suggested to avoid using
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`SecureJoin` and instead use the [new api](#new-api) or switch to
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[libpathrs][libpathrs].
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With the above limitations in mind, this library guarantees the following:
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* If no error is set, the resulting string **must** be a child path of
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`root` and will not contain any symlink path components (they will all be
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expanded).
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* When expanding symlinks, all symlink path components **must** be resolved
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relative to the provided root. In particular, this can be considered a
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userspace implementation of how `chroot(2)` operates on file paths. Note that
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these symlinks will **not** be expanded lexically (`filepath.Clean` is not
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called on the input before processing).
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* Non-existent path components are unaffected by `SecureJoin` (similar to
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`filepath.EvalSymlinks`'s semantics).
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* The returned path will always be `filepath.Clean`ed and thus not contain any
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`..` components.
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A (trivial) implementation of this function on GNU/Linux systems could be done
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with the following (note that this requires root privileges and is far more
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opaque than the implementation in this library, and also requires that
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`readlink` is inside the `root` path and is trustworthy):
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```go
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package securejoin
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import (
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"os/exec"
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"path/filepath"
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)
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func SecureJoin(root, unsafePath string) (string, error) {
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unsafePath = string(filepath.Separator) + unsafePath
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cmd := exec.Command("chroot", root,
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"readlink", "--canonicalize-missing", "--no-newline", unsafePath)
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output, err := cmd.CombinedOutput()
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if err != nil {
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return "", err
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}
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expanded := string(output)
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return filepath.Join(root, expanded), nil
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}
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```
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[libpathrs]: https://github.com/openSUSE/libpathrs
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[go#20126]: https://github.com/golang/go/issues/20126
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### New API ###
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While we recommend users switch to [libpathrs][libpathrs] as soon as it has a
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stable release, some methods implemented by libpathrs have been ported to this
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library to ease the transition. These APIs are only supported on Linux.
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These APIs are implemented such that `filepath-securejoin` will
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opportunistically use certain newer kernel APIs that make these operations far
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more secure. In particular:
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* All of the lookup operations will use [`openat2`][openat2.2] on new enough
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kernels (Linux 5.6 or later) to restrict lookups through magic-links and
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bind-mounts (for certain operations) and to make use of `RESOLVE_IN_ROOT` to
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efficiently resolve symlinks within a rootfs.
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* The APIs provide hardening against a malicious `/proc` mount to either detect
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or avoid being tricked by a `/proc` that is not legitimate. This is done
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using [`openat2`][openat2.2] for all users, and privileged users will also be
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further protected by using [`fsopen`][fsopen.2] and [`open_tree`][open_tree.2]
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(Linux 4.18 or later).
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[openat2.2]: https://www.man7.org/linux/man-pages/man2/openat2.2.html
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[fsopen.2]: https://github.com/brauner/man-pages-md/blob/main/fsopen.md
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[open_tree.2]: https://github.com/brauner/man-pages-md/blob/main/open_tree.md
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#### `OpenInRoot` ####
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```go
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func OpenInRoot(root, unsafePath string) (*os.File, error)
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func OpenatInRoot(root *os.File, unsafePath string) (*os.File, error)
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func Reopen(handle *os.File, flags int) (*os.File, error)
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```
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`OpenInRoot` is a much safer version of
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```go
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path, err := securejoin.SecureJoin(root, unsafePath)
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file, err := os.OpenFile(path, unix.O_PATH|unix.O_CLOEXEC)
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```
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that protects against various race attacks that could lead to serious security
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issues, depending on the application. Note that the returned `*os.File` is an
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`O_PATH` file descriptor, which is quite restricted. Callers will probably need
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to use `Reopen` to get a more usable handle (this split is done to provide
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useful features like PTY spawning and to avoid users accidentally opening bad
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inodes that could cause a DoS).
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Callers need to be careful in how they use the returned `*os.File`. Usually it
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is only safe to operate on the handle directly, and it is very easy to create a
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security issue. [libpathrs][libpathrs] provides far more helpers to make using
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these handles safer -- there is currently no plan to port them to
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`filepath-securejoin`.
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`OpenatInRoot` is like `OpenInRoot` except that the root is provided using an
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`*os.File`. This allows you to ensure that multiple `OpenatInRoot` (or
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`MkdirAllHandle`) calls are operating on the same rootfs.
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> **NOTE**: Unlike `SecureJoin`, `OpenInRoot` will error out as soon as it hits
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> a dangling symlink or non-existent path. This is in contrast to `SecureJoin`
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> which treated non-existent components as though they were real directories,
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> and would allow for partial resolution of dangling symlinks. These behaviours
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> are at odds with how Linux treats non-existent paths and dangling symlinks,
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> and so these are no longer allowed.
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#### `MkdirAll` ####
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```go
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func MkdirAll(root, unsafePath string, mode int) error
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func MkdirAllHandle(root *os.File, unsafePath string, mode int) (*os.File, error)
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```
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`MkdirAll` is a much safer version of
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```go
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path, err := securejoin.SecureJoin(root, unsafePath)
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err = os.MkdirAll(path, mode)
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```
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that protects against the same kinds of races that `OpenInRoot` protects
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against.
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`MkdirAllHandle` is like `MkdirAll` except that the root is provided using an
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`*os.File` (the reason for this is the same as with `OpenatInRoot`) and an
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`*os.File` of the final created directory is returned (this directory is
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guaranteed to be effectively identical to the directory created by
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`MkdirAllHandle`, which is not possible to ensure by just using `OpenatInRoot`
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after `MkdirAll`).
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> **NOTE**: Unlike `SecureJoin`, `MkdirAll` will error out as soon as it hits
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> a dangling symlink or non-existent path. This is in contrast to `SecureJoin`
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> which treated non-existent components as though they were real directories,
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> and would allow for partial resolution of dangling symlinks. These behaviours
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> are at odds with how Linux treats non-existent paths and dangling symlinks,
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> and so these are no longer allowed. This means that `MkdirAll` will not
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> create non-existent directories referenced by a dangling symlink.
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### License ###
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The license of this project is the same as Go, which is a BSD 3-clause license
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available in the `LICENSE` file.
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