fix(fleet): resolve-then-validate symlink guard + framework helper resolution (#1380) (#1383)

Co-authored-by: code-be-01 <[email protected]>

packages/mosaic/src/fleet/secure-file.ts

@@ -7,14 +7,25 @@ import {
   mkdirSync,
   openSync,
   readFileSync,
+  readlinkSync,
+  statSync,
 } from 'node:fs';
 import { platform } from 'node:os';
-import { dirname, isAbsolute, relative, resolve, sep } from 'node:path';
+import { basename, dirname, isAbsolute, relative, resolve, sep } from 'node:path';
 
 export interface SecureFileReadOptions {
   root: string;
   maxBytes?: number;
   executable?: boolean;
+  /**
+   * Additional roots a symlink component may resolve into (stack#1380).
+   * Default: only the managed root itself. Every symlink hop is validated —
+   * containment under the root or one of these roots, current-user ownership,
+   * no group/world-writable mode — and refusal stays the default for anything
+   * else. Callers that operate the split-home layout pass the brain home so
+   * the framework-created roster symlink resolves.
+   */
+  symlinkTargetRoots?: string[];
 }
 
 export interface SecureFileSnapshot {
@@ -88,7 +99,57 @@ function openDirectoryChain(absoluteDirectory: string): { fd: number; descriptor
   }
 }
 
-function openFileBeneathRoot(root: string, target: string): { fd: number; descriptors: number[] } {
+function containedUnder(root: string, target: string): boolean {
+  const rel = relative(resolve(root), resolve(target));
+  return rel !== '..' && !rel.startsWith(`..${sep}`) && !isAbsolute(rel) && rel !== '';
+}
+
+const MAX_SYMLINK_HOPS = 40;
+
+/**
+ * Resolve every symlink on `lexical` component-wise, validating each hop
+ * (stack#1380 resolve-then-validate): the hop target must stay under one of
+ * the sanctioned roots, must be owned by the current user (or root), and must
+ * not be group- or world-writable. Returns a symlink-free absolute path.
+ */
+function resolveRealPath(lexical: string, sanctionedRoots: string[]): string {
+  const hopTargets: string[] = [];
+  let current: string = sep;
+  for (const piece of resolve(lexical).split(sep).filter(Boolean)) {
+    current = resolve(current, piece);
+    for (let hops = 0; lstatSync(current).isSymbolicLink(); ) {
+      if (++hops > MAX_SYMLINK_HOPS) {
+        throw new Error(`symlink chain exceeds ${MAX_SYMLINK_HOPS} hops: ${lexical}`);
+      }
+      const linkTarget = readlinkSync(current);
+      const absolute = resolve(dirname(current), linkTarget);
+      if (!sanctionedRoots.some((root) => containedUnder(root, absolute))) {
+        throw new Error(
+          `symlink target escapes managed roots [${sanctionedRoots.join(', ')}]: ${absolute}`,
+        );
+      }
+      hopTargets.push(absolute);
+      current = absolute;
+    }
+  }
+  const uid = typeof process.getuid === 'function' ? process.getuid() : 0;
+  for (const hop of hopTargets) {
+    const stat = statSync(hop);
+    if (stat.uid !== uid && stat.uid !== 0) {
+      throw new Error(`symlink target is not owned by the current user: ${hop}`);
+    }
+    if (stat.mode & 0o022) {
+      throw new Error(`symlink target is group- or world-writable: ${hop}`);
+    }
+  }
+  return current;
+}
+
+function openFileBeneathRoot(
+  root: string,
+  target: string,
+  symlinkTargetRoots: string[] = [],
+): { fd: number; descriptors: number[] } {
   const canonicalRoot = resolve(root);
   const canonicalTarget = resolve(target);
   assertCanonicalContainment(canonicalRoot, canonicalTarget);
@@ -96,39 +157,52 @@ function openFileBeneathRoot(root: string, target: string): { fd: number; descri
   const fileName = components.pop();
   if (fileName === undefined) throw new Error('managed file path names the managed root');
 
-  const rootChain = openDirectoryChain(canonicalRoot);
+  // stack#1380: resolve-then-validate. The lexical path must name the managed
+  // root (above); symlink components are then resolved hop-by-hop under the
+  // sanctioned roots (validated per hop), and the descriptor traversal walks
+  // the symlink-free real path — keeping the O_NOFOLLOW chain as the race
+  // guard for anything substituted after resolution.
+  let realRoot: string;
+  try {
+    realRoot = resolveRealPath(canonicalRoot, [canonicalRoot]);
+  } catch (error) {
+    throw secureFilesystemError(
+      'secure descriptor traversal failed: symbolic link, unavailable, or not a directory',
+      error,
+    );
+  }
+  const sanctioned = [realRoot, ...symlinkTargetRoots.map((extra) => resolve(extra))];
+  let realTarget: string;
+  try {
+    realTarget = resolveRealPath(canonicalTarget, sanctioned);
+  } catch (error) {
+    if (error instanceof Error && !('code' in error)) throw error;
+    throw secureFilesystemError(
+      'secure descriptor traversal failed: symbolic link, unavailable, or not a directory',
+      error,
+    );
+  }
+  if (!sanctioned.some((sr) => containedUnder(sr, realTarget) || resolve(sr) === realTarget)) {
+    throw new Error(
+      `resolved path escapes managed roots [${sanctioned.join(', ')}]: ${realTarget}`,
+    );
+  }
+
+  const chain = openDirectoryChain(dirname(realTarget));
   try {
-    let parentFd = rootChain.fd;
-    for (const component of components) {
-      try {
-        parentFd = openSync(
-          procDescriptorPath(parentFd, component),
-          constants.O_RDONLY | constants.O_DIRECTORY | constants.O_NOFOLLOW,
-        );
-      } catch (error) {
-        throw secureFilesystemError(
-          'path ancestor is a symbolic link, unavailable, or not a directory',
-          error,
-        );
-      }
-      rootChain.descriptors.push(parentFd);
-      if (!fstatSync(parentFd).isDirectory()) {
-        throw new Error('path ancestor is a symbolic link or not a directory');
-      }
-    }
     let fd: number;
     try {
       fd = openSync(
-        procDescriptorPath(parentFd, fileName),
+        procDescriptorPath(chain.fd, basename(realTarget)),
         constants.O_RDONLY | constants.O_NONBLOCK | constants.O_NOFOLLOW,
       );
     } catch (error) {
       throw secureFilesystemError('file is a symbolic link or unavailable', error);
     }
-    rootChain.descriptors.push(fd);
-    return { fd, descriptors: rootChain.descriptors };
+    chain.descriptors.push(fd);
+    return { fd, descriptors: chain.descriptors };
   } catch (error) {
-    closeDescriptors(rootChain.descriptors);
+    closeDescriptors(chain.descriptors);
     if (error instanceof Error) throw error;
     throw new Error('secure managed file open failed');
   }
@@ -203,7 +277,7 @@ export function readRegularFileSecure(
   path: string,
   options: SecureFileReadOptions,
 ): SecureFileSnapshot {
-  const openedFile = openFileBeneathRoot(options.root, path);
+  const openedFile = openFileBeneathRoot(options.root, path, options.symlinkTargetRoots ?? []);
   try {
     const opened = fstatSync(openedFile.fd);
     if (!opened.isFile()) throw new Error('managed file is not a regular file');