fix: certify generated symlink state

scripts/build-web.mjs

@@ -1,12 +1,12 @@
 #!/usr/bin/env node
 
 import { spawn } from 'node:child_process';
-import { randomUUID } from 'node:crypto';
-import { mkdir, readFile, rename, rm, writeFile } from 'node:fs/promises';
+import { createHash, randomUUID } from 'node:crypto';
+import { lstat, mkdir, readFile, rename, rm, writeFile } from 'node:fs/promises';
 import { fileURLToPath } from 'node:url';
 import path from 'node:path';
 
-import { sourceFingerprint } from './preflight.mjs';
+import { generatedSymlinkManifest, sourceFingerprint } from './preflight.mjs';
 
 const scriptRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '..');
 
@@ -26,6 +26,18 @@ function run(command, args, options) {
 
 const delay = (milliseconds) => new Promise((resolve) => setTimeout(resolve, milliseconds));
 
+async function requireRealDirectory(target, { allowMissing = false } = {}) {
+  try {
+    const stats = await lstat(target);
+    if (!stats.isDirectory() || stats.isSymbolicLink()) {
+      throw new Error(`${target} must be a real directory, not a symbolic link.`);
+    }
+  } catch (error) {
+    if (allowMissing && error.code === 'ENOENT') return;
+    throw error;
+  }
+}
+
 async function acquireBuildLock(root) {
   const workRoot = path.join(root, '.mosaic-test-work');
   const lock = path.join(workRoot, 'web-build.lock');
@@ -89,12 +101,19 @@ export async function buildWeb({
   try {
     const webDir = path.join(root, 'apps', 'web');
     const nextDir = path.join(webDir, '.next');
-    const marker = path.join(nextDir, '.mosaic-source-hash');
-    const temporary = `${marker}.${randomUUID()}.tmp`;
+    const certificationMarker = path.join(nextDir, '.mosaic-source-hash');
+    const symlinkManifest = path.join(nextDir, '.mosaic-symlink-manifest');
+    const certificationTemporary = `${certificationMarker}.${randomUUID()}.tmp`;
+    const manifestTemporary = `${symlinkManifest}.${randomUUID()}.tmp`;
     const before = await fingerprint(root);
 
-    await rm(marker, { force: true });
+    await requireRealDirectory(nextDir, { allowMissing: true });
+    await Promise.all([
+      rm(certificationMarker, { force: true }),
+      rm(symlinkManifest, { force: true }),
+    ]);
     await runBuild(webDir);
+    await requireRealDirectory(nextDir);
 
     const after = await fingerprint(root);
     if (after !== before) {
@@ -103,9 +122,20 @@ export async function buildWeb({
       );
     }
 
-    await mkdir(nextDir, { recursive: true });
-    await writeFile(temporary, `${before}\n`, { mode: 0o600 });
-    await rename(temporary, marker);
+    const manifestContents = await generatedSymlinkManifest(nextDir);
+    const certificationContents = `${JSON.stringify({
+      version: 1,
+      sourceFingerprint: before,
+      symlinkManifestHash: createHash('sha256').update(manifestContents).digest('hex'),
+    })}\n`;
+    await Promise.all([
+      writeFile(certificationTemporary, certificationContents, { mode: 0o600 }),
+      writeFile(manifestTemporary, manifestContents, { mode: 0o600 }),
+    ]);
+    // The certification marker is the commit point. Publishing the manifest first
+    // leaves interrupted builds untrusted because the marker remains absent.
+    await rename(manifestTemporary, symlinkManifest);
+    await rename(certificationTemporary, certificationMarker);
   } finally {
     await releaseLock();
   }

scripts/build-web.test.mjs

@@ -1,5 +1,5 @@
 import assert from 'node:assert/strict';
-import { access, mkdir, readFile, rm, writeFile } from 'node:fs/promises';
+import { access, mkdir, readFile, rm, symlink, writeFile } from 'node:fs/promises';
 import path from 'node:path';
 import test from 'node:test';
 
@@ -26,19 +26,27 @@ test.after(async () => {
   await rm(fixtureRoot, { recursive: true, force: true });
 });
 
-test('a successful web build atomically publishes its source fingerprint', async () => {
+test('a successful web build atomically publishes its source and symlink certification', async () => {
   const root = await fixture('success');
   const marker = path.join(root, 'apps', 'web', '.next', '.mosaic-source-hash');
+  const manifest = path.join(root, 'apps', 'web', '.next', '.mosaic-symlink-manifest');
 
   await buildWeb({ root, fingerprint: async () => 'certified', runBuild: async () => {} });
 
-  assert.equal(await readFile(marker, 'utf8'), 'certified\n');
+  assert.deepEqual(JSON.parse(await readFile(marker, 'utf8')), {
+    version: 1,
+    sourceFingerprint: 'certified',
+    symlinkManifestHash: '8a5a375cea6a55d24bd5f875856da63feba33adbefb15a92a0007719b84bcf11',
+  });
+  assert.equal(await readFile(manifest, 'utf8'), '{"version":1,"links":[]}\n');
 });
 
 test('a failed web build leaves no certification marker', async () => {
   const root = await fixture('failure');
   const marker = path.join(root, 'apps', 'web', '.next', '.mosaic-source-hash');
+  const manifest = path.join(root, 'apps', 'web', '.next', '.mosaic-symlink-manifest');
   await writeFile(marker, 'stale\n');
+  await writeFile(manifest, 'stale\n');
 
   await assert.rejects(
     buildWeb({
@@ -52,12 +60,15 @@ test('a failed web build leaves no certification marker', async () => {
   );
 
   assert.equal(await exists(marker), false);
+  assert.equal(await exists(manifest), false);
 });
 
 test('overlapping web builds are serialized while the marker remains absent', async () => {
   const root = await fixture('overlap');
   const marker = path.join(root, 'apps', 'web', '.next', '.mosaic-source-hash');
+  const manifest = path.join(root, 'apps', 'web', '.next', '.mosaic-symlink-manifest');
   await writeFile(marker, 'stale\n');
+  await writeFile(manifest, 'stale\n');
   let releaseFirst;
   let secondEntered = false;
   const firstEntered = new Promise((resolve) => {
@@ -87,16 +98,41 @@ test('overlapping web builds are serialized while the marker remains absent', as
   await new Promise((resolve) => setTimeout(resolve, 75));
   assert.equal(secondEntered, false);
   assert.equal(await exists(marker), false);
+  assert.equal(await exists(manifest), false);
 
   releaseFirst();
   await Promise.all([first, second]);
   assert.equal(secondEntered, true);
-  assert.equal(await readFile(marker, 'utf8'), 'certified\n');
+  assert.equal(JSON.parse(await readFile(marker, 'utf8')).sourceFingerprint, 'certified');
+  assert.equal(await readFile(manifest, 'utf8'), '{"version":1,"links":[]}\n');
+});
+
+test('a build that replaces .next with a symbolic link cannot publish outside the checkout', async () => {
+  const root = await fixture('symbolic-next');
+  const nextDir = path.join(root, 'apps', 'web', '.next');
+  const outside = path.join(root, 'outside-generated');
+  await mkdir(outside);
+
+  await assert.rejects(
+    buildWeb({
+      root,
+      fingerprint: async () => 'certified',
+      runBuild: async () => {
+        await rm(nextDir, { recursive: true });
+        await symlink(outside, nextDir);
+      },
+    }),
+    /must be a real directory/,
+  );
+
+  assert.equal(await exists(path.join(outside, '.mosaic-source-hash')), false);
+  assert.equal(await exists(path.join(outside, '.mosaic-symlink-manifest')), false);
 });
 
 test('inputs changed during a web build are not certified', async () => {
   const root = await fixture('changed-inputs');
   const marker = path.join(root, 'apps', 'web', '.next', '.mosaic-source-hash');
+  const manifest = path.join(root, 'apps', 'web', '.next', '.mosaic-symlink-manifest');
   const fingerprints = ['before', 'after'];
 
   await assert.rejects(
@@ -109,4 +145,5 @@ test('inputs changed during a web build are not certified', async () => {
   );
 
   assert.equal(await exists(marker), false);
+  assert.equal(await exists(manifest), false);
 });

scripts/preflight.mjs

@@ -1,7 +1,7 @@
 #!/usr/bin/env node
 
 import { constants } from 'node:fs';
-import { access, lstat, readFile, readdir } from 'node:fs/promises';
+import { access, lstat, readFile, readdir, readlink } from 'node:fs/promises';
 import { createHash } from 'node:crypto';
 import { createRequire } from 'node:module';
 import { fileURLToPath } from 'node:url';
@@ -32,6 +32,19 @@ async function entries(root) {
   return result;
 }
 
+export async function generatedSymlinkManifest(nextDir) {
+  const links = [];
+  for (const target of (await entries(nextDir)).sort()) {
+    const stats = await lstat(target);
+    if (!stats.isSymbolicLink()) continue;
+    links.push({
+      path: path.relative(nextDir, target).split(path.sep).join('/'),
+      target: await readlink(target),
+    });
+  }
+  return `${JSON.stringify({ version: 1, links })}\n`;
+}
+
 const webSourceRoots = (root) => [
   path.join(root, 'apps', 'web', 'src'),
   path.join(root, 'apps', 'web', 'public'),
@@ -150,7 +163,14 @@ export async function runPreflight({ root = process.cwd(), uid = process.getuid?
   const nextDir = path.join(root, 'apps', 'web', '.next');
   let generated = [];
   try {
-    await lstat(nextDir);
+    const nextStats = await lstat(nextDir);
+    if (nextStats.isSymbolicLink()) {
+      return {
+        code: GENERATED_STATE_EXIT,
+        message:
+          'MOSAIC_PREFLIGHT_GENERATED_STATE: apps/web/.next contains a symbolic link and is not trustworthy; run pnpm clean:generated, then rerun the gate',
+      };
+    }
     generated = [nextDir, ...(await entries(nextDir))];
   } catch (error) {
     if (error.code !== 'ENOENT') throw error;
@@ -158,25 +178,55 @@ export async function runPreflight({ root = process.cwd(), uid = process.getuid?
 
   if (generated.length > 0) {
     const foreign = [];
-    if (uid !== undefined) {
-      for (const target of generated) {
-        if ((await lstat(target)).uid !== uid) foreign.push(path.relative(root, target));
-      }
+    for (const target of generated) {
+      const stats = await lstat(target);
+      if (uid !== undefined && stats.uid !== uid) foreign.push(path.relative(root, target));
     }
 
-    let generatedFingerprint = null;
+    // Threat model: this detects accidental, independent, or stale generated-state
+    // mutation. It does NOT defend against an actor with same-UID write access to
+    // the generated tree: that actor can regenerate both the manifest and marker
+    // consistently (CWE-345). No local construction can without a trust anchor
+    // outside that actor's authority.
+    let certification = null;
+    let certifiedManifest = null;
     try {
-      generatedFingerprint = (
-        await readFile(path.join(nextDir, '.mosaic-source-hash'), 'utf8')
-      ).trim();
+      const [certificationContents, manifestContents] = await Promise.all([
+        readFile(path.join(nextDir, '.mosaic-source-hash'), 'utf8'),
+        readFile(path.join(nextDir, '.mosaic-symlink-manifest'), 'utf8'),
+      ]);
+      try {
+        const parsed = JSON.parse(certificationContents);
+        if (
+          parsed.version === 1 &&
+          typeof parsed.sourceFingerprint === 'string' &&
+          typeof parsed.symlinkManifestHash === 'string'
+        ) {
+          certification = parsed;
+          certifiedManifest = manifestContents;
+        }
+      } catch {
+        // Invalid certification is handled as untrusted generated state below.
+      }
     } catch (error) {
       if (error.code !== 'ENOENT') throw error;
     }
-    const stale = generatedFingerprint !== (await sourceFingerprint(root));
-    if (foreign.length > 0 || stale) {
+    const stale = certification?.sourceFingerprint !== (await sourceFingerprint(root));
+    const actualManifest = await generatedSymlinkManifest(nextDir);
+    const certifiedManifestHash =
+      certifiedManifest === null
+        ? null
+        : createHash('sha256').update(certifiedManifest).digest('hex');
+    const changedSymlinks =
+      certification?.symlinkManifestHash !== certifiedManifestHash ||
+      certifiedManifest !== actualManifest;
+    if (foreign.length > 0 || stale || changedSymlinks) {
       const reasons = [
         foreign.length > 0 ? `foreign-owned paths: ${foreign.slice(0, 3).join(', ')}` : '',
         stale ? 'generated source fingerprint does not match web source/configuration' : '',
+        changedSymlinks
+          ? 'generated symbolic-link manifest does not match the certified build'
+          : '',
       ].filter(Boolean);
       return {
         code: GENERATED_STATE_EXIT,

scripts/preflight.test.mjs

@@ -1,4 +1,5 @@
 import assert from 'node:assert/strict';
+import { createHash } from 'node:crypto';
 import { chmod, mkdir, rm, symlink, utimes, writeFile } from 'node:fs/promises';
 import path from 'node:path';
 import test from 'node:test';
@@ -28,6 +29,22 @@ async function installRequiredBins(root) {
   );
 }
 
+async function certifyGeneratedState(root, links = []) {
+  const nextDir = path.join(root, 'apps', 'web', '.next');
+  await mkdir(nextDir, { recursive: true });
+  const manifest = `${JSON.stringify({ version: 1, links })}\n`;
+  const manifestHash = createHash('sha256').update(manifest).digest('hex');
+  await writeFile(path.join(nextDir, '.mosaic-symlink-manifest'), manifest);
+  await writeFile(
+    path.join(nextDir, '.mosaic-source-hash'),
+    `${JSON.stringify({
+      version: 1,
+      sourceFingerprint: await sourceFingerprint(root),
+      symlinkManifestHash: manifestHash,
+    })}\n`,
+  );
+}
+
 test.after(async () => {
   await rm(fixtureRoot, { recursive: true, force: true });
 });
@@ -98,6 +115,103 @@ test('a generated marker mismatch is identified separately from source errors',
   assert.match(result.message, /pnpm clean:generated/);
 });
 
+test('generated-state symbolic links are accepted only when exactly build-certified', async (t) => {
+  await t.test('apps/web/.next itself is rejected when it is a symbolic link', async () => {
+    const root = await fixture('symbolic-next-root');
+    await installRequiredBins(root);
+    await writeFile(path.join(root, 'outside-generated'), 'not a Next build\n');
+    await symlink(path.join(root, 'outside-generated'), path.join(root, 'apps', 'web', '.next'));
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /MOSAIC_PREFLIGHT_GENERATED_STATE/);
+    assert.match(result.message, /symbolic link/);
+  });
+
+  await t.test('an added descendant symlink is rejected', async () => {
+    const root = await fixture('symbolic-next-added');
+    await installRequiredBins(root);
+    await certifyGeneratedState(root);
+    await symlink('/etc/hosts', path.join(root, 'apps', 'web', '.next', 'reviewer-symlink'));
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /symbolic-link manifest/);
+  });
+
+  await t.test('a removed certified descendant symlink is rejected', async () => {
+    const root = await fixture('symbolic-next-removed');
+    await installRequiredBins(root);
+    const link = path.join(root, 'apps', 'web', '.next', 'dependency-link');
+    await mkdir(path.dirname(link), { recursive: true });
+    await symlink('../dependency-one', link);
+    await certifyGeneratedState(root, [{ path: 'dependency-link', target: '../dependency-one' }]);
+    await rm(link);
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /symbolic-link manifest/);
+  });
+
+  await t.test('a retargeted certified descendant symlink is rejected', async () => {
+    const root = await fixture('symbolic-next-retargeted');
+    await installRequiredBins(root);
+    const link = path.join(root, 'apps', 'web', '.next', 'dependency-link');
+    await mkdir(path.dirname(link), { recursive: true });
+    await symlink('../dependency-one', link);
+    await certifyGeneratedState(root, [{ path: 'dependency-link', target: '../dependency-one' }]);
+    await rm(link);
+    await symlink('../dependency-two', link);
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /symbolic-link manifest/);
+  });
+
+  await t.test('a manifest edited to whitelist a rogue symlink is rejected', async () => {
+    const root = await fixture('symbolic-next-tampered-manifest');
+    await installRequiredBins(root);
+    await certifyGeneratedState(root);
+    const nextDir = path.join(root, 'apps', 'web', '.next');
+    await symlink('/etc/hosts', path.join(nextDir, 'reviewer-symlink'));
+    await writeFile(
+      path.join(nextDir, '.mosaic-symlink-manifest'),
+      `${JSON.stringify({
+        version: 1,
+        links: [{ path: 'reviewer-symlink', target: '/etc/hosts' }],
+      })}\n`,
+    );
+
+    const result = await runPreflight({ root });
+    assert.equal(result.code, 43);
+    assert.match(result.message, /symbolic-link manifest/);
+  });
+
+  await t.test('unchanged canonical-style descendant symlinks are accepted', async () => {
+    const root = await fixture('symbolic-next-certified');
+    await installRequiredBins(root);
+    const link = path.join(
+      root,
+      'apps',
+      'web',
+      '.next',
+      'standalone',
+      'node_modules',
+      'dependency',
+    );
+    await mkdir(path.dirname(link), { recursive: true });
+    await symlink('../.pnpm/dependency', link);
+    await certifyGeneratedState(root, [
+      { path: 'standalone/node_modules/dependency', target: '../.pnpm/dependency' },
+    ]);
+
+    assert.deepEqual(await runPreflight({ root }), {
+      code: 0,
+      message: 'checkout preflight passed',
+    });
+  });
+});
+
 test('the source fingerprint includes inherited TypeScript configuration', async () => {
   const root = await fixture('inherited-typescript-config');
   const config = path.join(root, 'tsconfig.base.json');
@@ -143,10 +257,7 @@ test('a matching generation marker accepts incremental output with mixed mtimes'
   await utimes(generated, new Date('2020-01-01T00:00:00Z'), new Date('2020-01-01T00:00:00Z'));
   const fresh = path.join(root, 'apps', 'web', '.next', 'types', 'routes.ts');
   await writeFile(fresh, 'fresh generated output');
-  await writeFile(
-    path.join(root, 'apps', 'web', '.next', '.mosaic-source-hash'),
-    await sourceFingerprint(root),
-  );
+  await certifyGeneratedState(root);
 
   assert.deepEqual(await runPreflight({ root }), { code: 0, message: 'checkout preflight passed' });
 });