fix(sandbox): keep a derived ACL target inside the write root it came from - #1040
fix(sandbox): keep a derived ACL target inside the write root it came from#1040Vasanthdev2004 wants to merge 5 commits into
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… from FILE_FLAG_OPEN_REPARSE_POINT refuses a reparse point at the final path component and resolves every component above it, which is right for a path the operator named and not enough for one this package derived. The write-root carveouts are derived: <root>/.git/hooks and <root>/.git/config are constructed from a root, and <root>/.git is a name an unprivileged workspace writer can create before setup runs. mklink /J needs no privilege, so a junction there had the apply open <junction-target>/hooks, an ordinary directory with nothing wrong about its final component, and zero sandbox setup wrote a deny ACE on it as Administrator, outside the workspace. A derived entry now carries the root it came from, and the apply requires the object it finally holds to still live under it. The check is on the handle rather than the name: GetFinalPathNameByHandle answers where the open object actually is, and both sides go through it so the spelling normalizes the same way. Materialization is checked before it creates, since os.MkdirAll follows the same reparse points. Deliberately not a refusal of every reparse point on the path. Above the write root the path is the operator's, who may keep a workspace under a junction or a mapped directory; only the tail the sandbox appended is held strict. Closes #1024
The pre-create check asked where the deepest existing ancestor lives, and a junction answers with its own path: the open does not follow a final-component reparse point, so <root>/.git came back as <root>/.git and matched. os.MkdirAll does follow it. What disqualifies that ancestor is that it IS a reparse point, not where it reports living. Found by reverting the check and watching the test still pass, which said the guard was doing nothing rather than that the test was weak. Pinned against the function now, because through the whole apply the create is made and then removed on the failure path, so the filesystem afterwards looks identical either way. The plan wiring gets its own test for the same reason: every apply-level case here hands the group an anchor directly, so none of them would notice the builder never setting one.
Greptile SummaryThe PR adds write-root anchors to derived Windows ACL targets and compares opened targets against handle-resolved anchor paths to reject junction redirects.
Confidence Score: 3/5This PR should not merge until materialization is performed through traversal-resistant retained handles so an attacker cannot redirect the elevated creation after the containment check. The final ACL mutation is checked against the opened object, but the preceding materialization still traverses an attacker-mutable pathname after authorization and can create outside the write root. Files Needing Attention: internal/sandbox/windows_acl_apply_windows.go, internal/sandbox/windows_acl_containment_windows.go
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| Filename | Overview |
|---|---|
| internal/sandbox/windows_acl.go | Adds optional anchors to derived ACL entries and wires the originating write root into plan construction. |
| internal/sandbox/windows_acl_apply_windows.go | Enforces handle containment before ACL mutation, but pathname-based materialization remains separated from its containment check by an exploitable race. |
| internal/sandbox/windows_acl_containment_windows.go | Implements handle-resolved containment checks correctly for opened objects, while its pre-create ancestor check cannot secure a later pathname traversal. |
| internal/sandbox/windows_acl_containment_windows_test.go | Covers static junction redirects and plan wiring, but does not close or exercise the acknowledged component-swap race. |
Sequence Diagram
sequenceDiagram
participant W as Workspace writer
participant S as Elevated setup
participant F as Filesystem
S->>F: Verify existing derived tail
F-->>S: Existing ancestor is contained
W->>F: Replace checked component with junction
S->>F: os.MkdirAll(absolute path)
F-->>S: Create target outside write root
S->>F: Open and resolve created target
F-->>S: Outside-root final path
S-->>S: Refuse ACL after creation
Reviews (1): Last reviewed commit: "fix(sandbox): reject the reparse ancesto..." | Re-trigger Greptile
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| if err := verifyWindowsACLPathUnderAnchor(group.Anchor, path); err != nil { | ||
| return windowsACLSnapshot{}, false, err | ||
| } | ||
| if err := os.MkdirAll(path, 0o700); err != nil { |
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When a workspace writer replaces a checked component with a junction after verifyWindowsACLPathUnderAnchor returns, os.MkdirAll traverses the mutable absolute path and creates the target outside the write root before the later handle check rejects it. Materialization needs to be performed relative to retained, traversal-resistant handles. How this was verified: The containment function releases its ancestor handle before the separate pathname-based os.MkdirAll call.
Context Used: AGENTS.md (source)
Zero automated PR reviewVerdict: No blockers found Blockers
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ChangesWindows ACL junction containment
Priority: ⬆️ High Estimated code review effort: 4 (Complex) | ~45 minutes Severity of issue fixed: High Merge Risk: ⚪ Minimal · up to The change preserves native-path test coverage for anchored in-root paths while retaining unanchored behavior for operator-selected out-of-root paths. No current merge-readiness risk is identified. Sequence Diagram(s)sequenceDiagram
participant ACLPlan
participant ACLApply
participant Containment
participant WindowsFilesystem
ACLPlan->>ACLApply: provide derived path with Anchor
ACLApply->>Containment: validate target before materialization
Containment->>WindowsFilesystem: inspect ancestors and resolve handles
WindowsFilesystem-->>Containment: containment result
Containment-->>ACLApply: allow or reject
ACLApply->>Containment: verify opened target beneath Anchor
Containment-->>ACLApply: allow ACL update or cleanup failure
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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In `@internal/sandbox/windows_acl_containment_windows_test.go`:
- Line 89: Update both successful-apply tests at
internal/sandbox/windows_acl_containment_windows_test.go lines 89-89 and 160-160
to retain the snapshot returned by applyWindowsACLPathGroup instead of
discarding it, and register t.Cleanup handlers that call
rollbackWindowsACLSnapshots when applied is true. Apply the same rollback
pattern at both sites so the deny-write DACL is restored before
temporary-directory cleanup.
In `@internal/sandbox/windows_acl.go`:
- Around line 54-55: Update BuildWindowsACLPlan and the
windowsWriteRootCapabilities flow so copied ReadOnlySubpaths are not assigned
Anchor: capability.Root unless they have been validated as root descendants; for
supported operator-named paths, leave Anchor empty instead. Preserve anchoring
only for paths proven to remain under the capability root, including
reparse-point safety.
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internal/sandbox/windows_acl.gointernal/sandbox/windows_acl_apply_windows.gointernal/sandbox/windows_acl_containment_windows.gointernal/sandbox/windows_acl_containment_windows_test.go
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…put the DACL back Two from review, both right. ReadOnlySubpaths is a profile field an operator can set to any path, and one placed outside the write root is a configuration that works today; anchoring it unconditionally turned that into a containment refusal. Only paths lexically under the root are anchored now, which is where the derived carveouts are anyway, and anything else keeps the final-component guard it always had. The successful-apply tests left their deny ACE in place, so t.TempDir could fail to remove the tree. Restoring the snapshot exposed the sharper half: the ACE denied the group the test runs as, which revoked its own WRITE_DAC and left the rollback unable to reopen the target. The tests deny a group this process is not a member of instead, which is what a capability SID is in production, and roll back afterwards.
…t case included The plan builder is cross-platform and its anchor tests were in a Windows-only file, so nothing checked the wiring on Linux or macOS. Moved beside the other BuildWindowsACLPlan tests, with the case review raised: an out-of-root ReadOnlySubpath stays unanchored, and an in-root one is still held to its write root.
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Both taken, at 0ddf21d and 832b73b. The anchor on Only paths lexically under the root are anchored now, via the existing The rollback in the successful-apply tests. Also right, and following it turned up the sharper half. Restoring the snapshot failed with The tests deny While there: the plan tests I added were in a Windows-only file even though the builder is cross-platform, so nothing checked the anchoring on Linux or macOS. Moved beside the other |
Moving these beside the other BuildWindowsACLPlan tests made them run on Linux and macOS, where their Windows path literals stop meaning what they say: pathWithinRoot is filepath.Rel underneath, a backslash is an ordinary character off Windows, and C:\workspace\.git\hooks is then one component that is not under C:\workspace, so every anchor came back empty. The neighbouring tests get away with such literals because they only compare strings they built the same way and never ask whether one contains another. Paths are built with filepath.Join from temp roots now, so the containment question is asked in the separator the running platform actually uses.
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Closes #1024.
FILE_FLAG_OPEN_REPARSE_POINTguards one component. The apply opens its target with it, which refuses a reparse point at the final component and resolves every component above it, as any absolute path has to. That is the whole guard for a path the operator named, and not enough for one this package derived.The write-root carveouts are derived:
<root>/.git/hooksand<root>/.git/configare constructed from a root, and<root>/.gitis a name an unprivileged workspace writer can create before setup runs.mklink /Jneeds no privilege. With a junction there, the apply opens<junction-target>/hooks, which is an ordinary directory with nothing wrong about its final component, andzero sandbox setupwrites a deny ACE on it as Administrator, outside the workspace.Probed rather than reasoned about, since Windows path semantics do not reward reasoning. Opening
<root>/.git/hooksthrough a junction and opening the outside path directly return the sameGetFinalPathNameByHandleanswer,\\?\...\002\target\hooks, while the anchor answers\\?\...\001. So the handle knows where it really is even when the name does not.What changed
A derived entry now records the write root it came from, and the apply requires the object it finally holds to still live under it. The comparison is between two
GetFinalPathNameByHandleanswers, so both sides normalize the same way (\\?\prefix, long names, drive letter) rather than being compared as written.Materialization is checked before it creates, because
os.MkdirAllfollows the same reparse points and would otherwise put the directory on the far side with only the after-the-fact check noticing.This is deliberately not a refusal of every reparse point on the path. Above the write root the path is the operator's, who may keep a workspace under a junction or a mapped directory, and refusing that would break setups this has nothing to say about. Only the tail the sandbox appended is held strict, which is the "owned intermediate" rule the issue asks for.
What is not fixed here
The window between the pre-create check and
os.MkdirAllis still open: closing it needs the components created relative to retained handles rather than by pathname, which is the rooted descent in #808. Same for theos.RemoveAllon the failure path. This PR is scoped to refusing the ACE, which is what #1024 describes, and stays out of #808's way.Tests
.git: the ACE is refused and the error names where it actually resolved.Junctions rather than symlinks throughout, so these run on an ordinary unelevated account, which is the account the attack needs.
Two of these exist because falsification caught me. Reverting the pre-create check left its test passing, which turned out to mean the check was doing nothing: it asked where the deepest existing ancestor lived, and a junction answers with its own path, because the open does not follow a final-component reparse point. What disqualifies that ancestor is that it IS a reparse point. Reverting the plan wiring also left everything passing, because every apply-level test hands the group an anchor directly and none of them would notice the builder never setting one. Both now fail by name.
Summary by CodeRabbit
Bug Fixes
Tests