Add BPF LSM ptrace_access_check hook for cross-branch ptrace denial

Signed-off-by: Cong Wang <cwang@multikernel.io>

Author: congwang-mk

src/branching/process/bpf/branch_tracker.bpf.c

@@ -2,9 +2,10 @@
 /*
  * branch_tracker.bpf.c — BPF LSM program for branch process tracking.
  *
- * Hooks task_alloc, task_free, and task_kill to inescapably track all
- * descendants of registered branch root PIDs.  Supports atomic
- * fork-denial during branch teardown and cross-branch signal isolation.
+ * Hooks task_alloc, task_free, task_kill, and ptrace_access_check to
+ * inescapably track all descendants of registered branch root PIDs.
+ * Supports atomic fork-denial during branch teardown, cross-branch
+ * signal isolation, and cross-branch ptrace denial.
  *
  * Compile:
  *   clang -g -O2 -target bpf -D__TARGET_ARCH_x86 \
@@ -99,6 +100,42 @@ int BPF_PROG(branch_task_kill, struct task_struct *target,
 	return 0;
 }
 
+/*
+ * LSM hook: ptrace_access_check — fires on ptrace attach/peek/poke.
+ *
+ * Same boundary logic as task_kill: a tracked process can only ptrace
+ * processes within its own branch.  Untracked processes (the parent
+ * orchestrator) are unrestricted.
+ *
+ * Without this, a malicious child could ptrace the parent or a sibling
+ * branch process and read/write its memory, bypassing all other
+ * isolation.
+ */
+SEC("lsm/ptrace_access_check")
+int BPF_PROG(branch_ptrace_access_check, struct task_struct *child,
+	     unsigned int mode)
+{
+	__u32 tracer_pid = bpf_get_current_pid_tgid() >> 32;
+	__u64 *tracer_bid = bpf_map_lookup_elem(&branch_pids, &tracer_pid);
+
+	/* Untracked tracer (parent/orchestrator) — always allow. */
+	if (!tracer_bid)
+		return 0;
+
+	__u32 target_pid = BPF_CORE_READ(child, tgid);
+	__u64 *target_bid = bpf_map_lookup_elem(&branch_pids, &target_pid);
+
+	/* Target is not tracked — deny (protects parent). */
+	if (!target_bid)
+		return -1; /* -EPERM */
+
+	/* Both tracked — allow only within the same branch. */
+	if (*tracer_bid != *target_bid)
+		return -1; /* -EPERM */
+
+	return 0;
+}
+
 /*
  * LSM hook: task_free — fires when a process exits.
  *