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80 lines
3 KiB
Markdown
80 lines
3 KiB
Markdown
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## `filepath-securejoin` ##
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[](https://github.com/cyphar/filepath-securejoin/actions/workflows/ci.yml)
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An implementation of `SecureJoin`, a [candidate for inclusion in the Go
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standard library][go#20126]. The purpose of this function is to be a "secure"
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alternative to `filepath.Join`, and in particular it provides certain
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guarantees that are not provided by `filepath.Join`.
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> **NOTE**: This code is *only* safe if you are not at risk of other processes
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> modifying path components after you've used `SecureJoin`. If it is possible
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> for a malicious process to modify path components of the resolved path, then
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> you will be vulnerable to some fairly trivial TOCTOU race conditions. [There
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> are some Linux kernel patches I'm working on which might allow for a better
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> solution.][lwn-obeneath]
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>
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> In addition, with a slightly modified API it might be possible to use
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> `O_PATH` and verify that the opened path is actually the resolved one -- but
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> I have not done that yet. I might add it in the future as a helper function
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> to help users verify the path (we can't just return `/proc/self/fd/<foo>`
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> because that doesn't always work transparently for all users).
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This is the function prototype:
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```go
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func SecureJoin(root, unsafePath string) (string, error)
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```
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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):
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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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[lwn-obeneath]: https://lwn.net/Articles/767547/
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[go#20126]: https://github.com/golang/go/issues/20126
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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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