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mkcomposefs: Document external sandboxing #362

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mkcomposefs: Document external sandboxing #362

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21 changes: 21 additions & 0 deletions man/mkcomposefs.md
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Expand Up @@ -104,6 +104,27 @@ Format version history:
The default if no version arguments are specified is version 0 and max
version 1.

# SANDBOXING RECOMMENDATIONS

This tool essentially just processes an input directory or
text file and writes a file. It does not require any privileges
at all. If you are invoking this as part of an otherwise privileged
process (such as a container runtime) we recommend dropping privileges
before invoking `mkcomposefs`.

Especially if you are using `--from-file`, you can run this
as an unprivileged uid that has no writable filesystem access at
all except to a passed file descriptor. For example, you can pass
a writable file descriptor for the desired target file as fd 3,
and run `mkcomposefs --from-file - /proc/self/fd/3`
to effectively use `mkcomposefs` as part of a pipeline.

An example simple sandboxing starting from root is `setpriv --nnp --reuid nobody -- mkcomposefs ...`.
Another is to use `systemd-run -P DynamicUser=yes -P ProtectSystem=strict`.
Yet another (especially if your code is already part of a container runtime)
is to use that runtime's existing functionality (seccomp, unsharing user namespace,
mounting a restricted subset of the rootfs, etc.

# SEE ALSO
**composefs-info(1)**, **mount.composefs(1)**, **composefs-dump(5)**

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