fix: restore bounded agent sandbox execution

hub/src/agent/security.ts

@@ -1,4 +1,3 @@
-import { createHash } from "node:crypto";
 import { chmod, lstat, mkdir, realpath } from "node:fs/promises";
 import { homedir } from "node:os";
 import { isAbsolute, join, relative, resolve } from "node:path";
@@ -30,7 +29,7 @@ const SANDBOX_HIDDEN_ENV_KEYS = [
 // Linux sockaddr_un.sun_path is 108 bytes including the terminator. Claude's
 // sandbox appends its own user directory and randomized bridge socket names,
 // so keep our prefix well below that hard limit.
-const MAX_AGENT_TMP_PREFIX_BYTES = 64;
+const MAX_AGENT_TMP_PREFIX_BYTES = 56;
 
 export interface AgentSecurityInput {
   readonly workspaceRoot: string;
@@ -78,7 +77,8 @@ export async function createAgentSecurityPolicy(input: AgentSecurityInput): Prom
   const agentCache = await ensureDirectoryTree(runtimeRoot, ["cache"]);
   const agentConfig = await ensureDirectoryTree(runtimeRoot, ["config"]);
   const agentState = await ensureDirectoryTree(runtimeRoot, ["state"]);
-  const agentTmp = await resolveShortAgentTemp(workspaceDir);
+  const agentTmp = await ensureDirectoryTree(cphRoot, ["t"]);
+  assertShortAgentTemp(agentTmp);
 
   const path = hostEnv["PATH"]?.trim();
   if (path === undefined || path === "") {
@@ -125,10 +125,7 @@ export async function createAgentSecurityPolicy(input: AgentSecurityInput): Prom
       autoAllowBashIfSandboxed: true,
       allowUnsandboxedCommands: false,
       filesystem: {
-        // The temp directory is service-owned, project-keyed, mode 0700, and
-        // contains only SDK sockets/scratch. User deliverables remain confined
-        // to workspaceDir.
-        allowWrite: [workspaceDir, agentTmp],
+        allowWrite: [workspaceDir],
         // Reject every write path by default, then re-open only the canonical
         // workspace. This prevents bubblewrap's ordinary temp exceptions from
         // turning an unauthorized path into a successful ephemeral write.
@@ -137,7 +134,7 @@ export async function createAgentSecurityPolicy(input: AgentSecurityInput): Prom
         // workspace plus the named system runtime needed to execute tools.
         // SDK allowRead takes precedence over matching denyRead paths.
         denyRead: ["/"],
-        allowRead: [workspaceDir, agentTmp, ...runtimeReadPaths],
+        allowRead: [workspaceDir, ...runtimeReadPaths],
       },
       credentials: {
         files: sensitiveReadPaths.map((path) => ({ path, mode: "deny" as const })),
@@ -147,22 +144,13 @@ export async function createAgentSecurityPolicy(input: AgentSecurityInput): Prom
   };
 }
 
-async function resolveShortAgentTemp(workspaceDir: string): Promise<string> {
-  // Do not use os.tmpdir() on macOS: its per-user path can itself exceed the
-  // Unix socket budget. /tmp canonicalizes to /private/tmp there and /tmp on
-  // Linux. The UID boundary prevents Organizations' service accounts sharing
-  // a writable runtime root; the workspace hash separates Projects.
-  const systemTmp = await realpath(process.platform === "win32" ? homedir() : "/tmp");
-  const uid = process.getuid?.() ?? "user";
-  const workspaceKey = createHash("sha256").update(workspaceDir).digest("hex").slice(0, 12);
-  const agentTmp = await ensureDirectoryTree(systemTmp, [`cph-agent-${uid}`, workspaceKey]);
+function assertShortAgentTemp(agentTmp: string): void {
   const prefixBytes = Buffer.byteLength(agentTmp);
   if (prefixBytes > MAX_AGENT_TMP_PREFIX_BYTES) {
     throw new Error(
       `Agent temp path is too long for sandbox bridge sockets (${prefixBytes} > ${MAX_AGENT_TMP_PREFIX_BYTES} bytes): ${agentTmp}`,
     );
   }
-  return agentTmp;
 }
 
 function hostRuntimeReadPaths(env: Readonly<Record<string, string | undefined>>): string[] {