fix(sandbox): address PR #309 review — TOCTOU direction, denial metadata, shared roots, docs

- fs-sandbox: delegate the mutation with the freshly re-canonicalized target
  (not the stale one), so the checked identity is the mutated identity — a
  symlink swapped in after resolve() can no longer escape workspace-write.
- tool-fs: map a denial to an FsError carrying FS_SANDBOX_DENIED (not a plain
  Error), so ToolRegistry keeps the structured code on result.error for
  retry/observers while the message stays the shared marker.
- sandbox-local: derive the Seatbelt writable set from the shared
  writableRoots() helper, so the profile and the fs fence cannot drift.
- gen-doc-graphs: ctx.sandboxPolicy is owned by dsh-sandbox-policy and read only
  by the sandboxed executor/provider (the tool layers use the pure fold).
- docs: bash-sandbox/bash/permission READMEs and bash.md reflect the relocated
  policy home and the sandbox/mode rename; drop the stale stdout.golden.jsonl.

packages/fs/fs-sandbox/src/index.ts

@@ -88,7 +88,7 @@ export class SandboxedFileSystem extends LocalFileSystem {
 
   /**
    * Fence the write by the per-call mode, then delegate to the inherited
-   * atomic write. See {@link assertWritable}.
+   * atomic write. See {@link checkedTarget}.
    * @param target - the resolved target to write.
    * @param content - the full new file content.
    * @param expected - the write intent guarding the write; omit for unconditional.
@@ -103,13 +103,12 @@ export class SandboxedFileSystem extends LocalFileSystem {
     signal?: AbortSignal,
     sandboxMode?: SandboxMode,
   ): Promise<FsWriteOutcome> {
-    await this.assertWritable(target, sandboxMode)
-    return super.writeText(target, content, expected, signal)
+    return super.writeText(await this.checkedTarget(target, sandboxMode), content, expected, signal)
   }
 
   /**
    * Fence the edit by the per-call mode, then delegate to the inherited
-   * atomic edit. See {@link assertWritable}.
+   * atomic edit. See {@link checkedTarget}.
    * @param target - the resolved target to edit.
    * @param edit - the literal search/replace request.
    * @param expected - the version guard; omit for an unconditional edit.
@@ -124,31 +123,34 @@ export class SandboxedFileSystem extends LocalFileSystem {
     signal?: AbortSignal,
     sandboxMode?: SandboxMode,
   ): Promise<FsEditOutcome> {
-    await this.assertWritable(target, sandboxMode)
-    return super.editText(target, edit, expected, signal)
+    return super.editText(await this.checkedTarget(target, sandboxMode), edit, expected, signal)
   }
 
   /**
-   * Enforce the per-call mode against `target` before delegating the mutation.
-   * `read-only` denies; `workspace-write` re-canonicalizes the target NOW
-   * (`resolve` realpaths the deepest existing ancestor, reflecting a
-   * concurrently swapped symlink) and requires containment under a writable
-   * root; `danger-full-access` allows. Throws the structured
-   * `FS_SANDBOX_DENIED` on refusal — the tool layer maps it to the model-facing
-   * `[sandbox: …]` marker and the escalation hint.
+   * Enforce the per-call mode against `target` and return the EXACT target the
+   * mutation must use, so the checked identity is the mutated one (no
+   * check-here-write-there TOCTOU). `read-only` denies; `workspace-write`
+   * re-canonicalizes NOW (`resolve` realpaths the deepest existing ancestor,
+   * reflecting a concurrently swapped symlink), requires containment under a
+   * writable root, and returns THAT fresh target; `danger-full-access` returns
+   * the caller's target unfenced. Throws the structured `FS_SANDBOX_DENIED` on
+   * refusal — the tool layer maps it to the model-facing `[sandbox: …]` marker
+   * and the escalation hint.
    */
-  private async assertWritable(target: FsTarget, sandboxMode?: SandboxMode): Promise<void> {
+  private async checkedTarget(target: FsTarget, sandboxMode?: SandboxMode): Promise<FsTarget> {
     const mode = sandboxMode ?? this.defaultMode
-    if (mode === 'danger-full-access') return
+    if (mode === 'danger-full-access') return target
     if (mode === 'read-only') {
       throw new FsError(`cannot write "${target.displayPath}": file access denied under read-only mode`, 'FS_SANDBOX_DENIED')
     }
     // workspace-write: containment on the FRESH canonical path (catches a
-    // symlink ancestor swapped since the tool resolved this target).
+    // symlink ancestor swapped since the tool resolved this target), and the
+    // mutation delegates with THIS fresh target — never the stale one.
     const fresh = await this.resolve(target.displayPath)
     if (!this.writableRoots.some(root => isUnder(fresh.targetKey, root))) {
       throw new FsError(`cannot write "${target.displayPath}": file access denied under workspace-write mode`, 'FS_SANDBOX_DENIED')
     }
+    return fresh
   }
 }
 

packages/fs/fs-sandbox/tests/fs-sandbox.spec.ts

@@ -14,7 +14,7 @@ import { existsSync } from 'node:fs'
 import { homedir, tmpdir } from 'node:os'
 import { join } from 'node:path'
 import { Context } from 'cordis'
-import { FsError } from '@deepseek-ai/dsh-fs'
+import { FsError, FsTargetKey } from '@deepseek-ai/dsh-fs'
 import type { FsTarget } from '@deepseek-ai/dsh-fs'
 import SandboxPolicyService from '@deepseek-ai/dsh-sandbox-policy'
 import type { SandboxMode } from '@deepseek-ai/dsh-sandbox'
@@ -146,6 +146,19 @@ describe('workspace-write containment', () => {
     expect(await readFile(path, 'utf8')).toBe('changed')
   })
 
+  it('mutates the freshly checked identity, not a stale outside targetKey (TOCTOU direction)', async () => {
+    // A target whose displayPath is inside the workspace but whose targetKey is
+    // a STALE outside path — as if an ancestor symlink pointed out at the tool's
+    // resolve() and was swapped in before the write. The fence re-resolves
+    // displayPath (now inside) AND delegates with that fresh target, so the byte
+    // lands inside and the stale outside path is never written.
+    const insidePath = join(workspace, 'landed.txt')
+    const staleTarget: FsTarget = { displayPath: insidePath, targetKey: FsTargetKey(join(outside, 'escaped.txt')) }
+    await fs.writeText(staleTarget, 'inside')
+    expect(await readFile(insidePath, 'utf8')).toBe('inside')
+    expect(existsSync(join(outside, 'escaped.txt'))).toBe(false)
+  })
+
   it('the workspace root itself passes the fence (path equal to a writable root), failing only on file type', async () => {
     // isUnder's path-equals-root branch: the fence allows the root, and the
     // write then fails because the root is a directory, not a regular file.