fix: reject dangling symlinks escaping sandbox in FsSandbox::resolve - #31
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Pull request overview
This PR hardens FsSandbox::resolve() against sandbox escapes by ensuring symlinks in the “non-existing tail” of a path are detected (including dangling symlinks) and rejected when their targets escape the sandbox root.
Changes:
- Add per-component
symlink_metadata()checks while reconstructing the non-existing tail, rejecting symlinks whose (lexically normalized) targets are outsideself.root. - Add Unix-only regression tests for dangling symlink escape attempts, valid internal symlinks, dangling-but-in-root symlinks, and a symlink chain case.
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| // Catch symlinks in the non-existing tail — including dangling ones. | ||
| if let Ok(meta) = std::fs::symlink_metadata(&out) { | ||
| if meta.file_type().is_symlink() { | ||
| let target = std::fs::read_link(&out)?; | ||
| let abs = if target.is_absolute() { | ||
| target | ||
| } else { | ||
| out.parent().unwrap_or(&self.root).join(&target) | ||
| }; | ||
| // Normalise the absolute target (resolve .. etc) without | ||
| // touching the filesystem, then check the prefix. | ||
| let mut norm = std::path::PathBuf::new(); | ||
| for c in abs.components() { | ||
| match c { | ||
| std::path::Component::ParentDir => { | ||
| norm.pop(); | ||
| } | ||
| std::path::Component::CurDir => {} | ||
| c => norm.push(c), | ||
| } | ||
| } | ||
| if !norm.starts_with(&self.root) { | ||
| return Err(anyhow!( | ||
| "symlink target escapes mount root: {:?}", | ||
| guest_path | ||
| )); | ||
| } | ||
| } |
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| // Create a dangling symlink whose target is outside root. | ||
| symlink("/tmp/nonexistent-outside-root", root.join("bad_link")).unwrap(); |
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| fn test_resolve_dangling_symlink_escape() { | ||
| use std::os::unix::fs::symlink; | ||
| let root = tmpdir("dangling-escape"); | ||
| // Create a dangling symlink whose target is outside root. | ||
| symlink("/tmp/nonexistent-outside-root", root.join("bad_link")).unwrap(); | ||
| let fs_sb = FsSandbox::new(&root).unwrap(); | ||
| let err = fs_sb.resolve("bad_link").unwrap_err().to_string(); | ||
| assert!( | ||
| err.contains("escapes mount root"), | ||
| "expected escape error, got: {err}" | ||
| ); | ||
| } | ||
|
|
||
| #[cfg(unix)] | ||
| #[test] | ||
| fn test_resolve_valid_internal_symlink() { | ||
| use std::os::unix::fs::symlink; |
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| fn test_resolve_symlink_chain_escape() { | ||
| use std::os::unix::fs::symlink; | ||
| let root = tmpdir("chain-escape"); | ||
| let outside = tmpdir("chain-outside"); | ||
| // symlink B -> outside (dangling or not, target is outside root) | ||
| symlink(&outside, root.join("link_b")).unwrap(); | ||
| // symlink A -> link_b | ||
| symlink(root.join("link_b"), root.join("link_a")).unwrap(); | ||
| let fs_sb = FsSandbox::new(&root).unwrap(); | ||
| // Resolving link_a should fail because it chains through link_b | ||
| // which points outside root. | ||
| let err = fs_sb.resolve("link_a").unwrap_err().to_string(); |
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Linux Benchmarks
Details
| Benchmark suite | Current: 095a075 | Previous: f5eb506 | Ratio |
|---|---|---|---|
hello_world (median) |
20 ms |
20 ms |
1 |
pandas (median) |
110 ms |
110 ms |
1 |
density (per VM) |
7 MB |
7 MB |
1 |
snapshot (disk) |
385 MiB |
385 MiB |
1 |
This comment was automatically generated by workflow using github-action-benchmark.
Signed-off-by: danbugs <danilochiarlone@gmail.com>
…verage Signed-off-by: danbugs <danilochiarlone@gmail.com>
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Windows Benchmarks
Details
| Benchmark suite | Current: 095a075 | Previous: f5eb506 | Ratio |
|---|---|---|---|
hello_world (median) |
291 ms |
221 ms |
1.32 |
pandas (median) |
917 ms |
697 ms |
1.32 |
density (per VM) |
6 MB |
6 MB |
1 |
snapshot (disk) |
393 MiB |
393 MiB |
1 |
This comment was automatically generated by workflow using github-action-benchmark.
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Summary
FsSandbox::resolve()failed to detect symlinks whose target does not exist(dangling symlinks). The ancestor-walk loop uses
Path::exists(), which followssymlinks — for a dangling symlink,
exists()returns false, so the symlink namewas appended to the output path without ever being canonicalized or checked. A
subsequent
fs_writethrough that symlink would create files outside the sandboxroot.
checked with
symlink_metadata()(which does not follow symlinks)the sandbox root
(allowed), and symlink chain escape
Test plan
test_resolve_dangling_symlink_escape— dangling symlink to outside root → Errtest_resolve_valid_internal_symlink— symlink to file inside root → Oktest_resolve_dangling_symlink_inside_root— dangling symlink inside root → Oktest_resolve_symlink_chain_escape— chained symlinks to outside → Err