[PATCH bpf-next v2 39/43] bpf: Refuse the prologue for an extension that cannot take it

@@ -858,6 +858,25 @@ int bpf_opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
 	return 0;
 }
 
+/*
+ * The prologue takes the context from R1, and it can clobber R2-R5. An
+ * extension gets the arguments of the function that it replaces.
+ */
+static bool ext_prog_takes_prologue(const struct bpf_verifier_env *env)
+{
+	const struct bpf_subprog_info *sub = &env->subprog_info[0];
+	int i, cnt = min_t(int, sub->arg_slot_cnt, MAX_BPF_FUNC_REG_ARGS);
+
+	if (!cnt || sub->args[0].arg_type != ARG_PTR_TO_CTX)
+		return false;
+
+	for (i = 1; i < cnt; i++)
+		if (sub->args[i].arg_type != ARG_SCALAR)
+			return false;
+
+	return true;
+}
+
 /* convert load instructions that access fields of a context type into a
  * sequence of instructions that access fields of the underlying structure:
  *     struct __sk_buff    -> struct sk_buff

@@ -909,6 +928,13 @@ int bpf_convert_ctx_accesses(struct bpf_verifier_env *env)
 			verifier_bug(env, "gen_prologue is null");
 			return -EFAULT;
 		}
+		if (env->seen_direct_write &&
+		    env->prog->type == BPF_PROG_TYPE_EXT &&
+		    !ext_prog_takes_prologue(env)) {
+			verbose(env,
+				"Extension writing packet data must take ctx and scalars only\n");
+			return -EINVAL;
+		}
 		cnt = ops->gen_prologue(insn_buf, env->seen_direct_write,
 					env->prog);
 		if (cnt >= INSN_BUF_SIZE) {