bpf: Fix race in btf_resolve_helper_id()

btf_resolve_helper_id() caching logic is a bit racy, since under root the
verifier can verify several programs in parallel. Fix it with READ/WRITE_ONCE.
Fix the type as well, since error is also recorded.

Fixes: a7658e1a ("bpf: Check types of arguments passed into helpers")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191114185720.1641606-15-ast@kernel.org

include/linux/bpf.h

@@ -248,7 +248,7 @@ struct bpf_func_proto {
 		};
 		enum bpf_arg_type arg_type[5];
 	};
-	u32 *btf_id; /* BTF ids of arguments */
+	int *btf_id; /* BTF ids of arguments */
 };
 
 /* bpf_context is intentionally undefined structure. Pointer to bpf_context is
@@ -881,7 +881,8 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		      const struct btf_type *t, int off, int size,
 		      enum bpf_access_type atype,
 		      u32 *next_btf_id);
-u32 btf_resolve_helper_id(struct bpf_verifier_log *log, void *, int);
+int btf_resolve_helper_id(struct bpf_verifier_log *log,
+			  const struct bpf_func_proto *fn, int);
 
 int btf_distill_func_proto(struct bpf_verifier_log *log,
 			   struct btf *btf,

kernel/bpf/btf.c

@@ -3721,7 +3721,8 @@ int btf_struct_access(struct bpf_verifier_log *log,
 	return -EINVAL;
 }
 
-u32 btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn, int arg)
+static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
+				   int arg)
 {
 	char fnname[KSYM_SYMBOL_LEN + 4] = "btf_";
 	const struct btf_param *args;
@@ -3789,6 +3790,29 @@ u32 btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn, int arg)
 	return btf_id;
 }
 
+int btf_resolve_helper_id(struct bpf_verifier_log *log,
+			  const struct bpf_func_proto *fn, int arg)
+{
+	int *btf_id = &fn->btf_id[arg];
+	int ret;
+
+	if (fn->arg_type[arg] != ARG_PTR_TO_BTF_ID)
+		return -EINVAL;
+
+	ret = READ_ONCE(*btf_id);
+	if (ret)
+		return ret;
+	/* ok to race the search. The result is the same */
+	ret = __btf_resolve_helper_id(log, fn->func, arg);
+	if (!ret) {
+		/* Function argument cannot be type 'void' */
+		bpf_log(log, "BTF resolution bug\n");
+		return -EFAULT;
+	}
+	WRITE_ONCE(*btf_id, ret);
+	return ret;
+}
+
 static int __get_type_size(struct btf *btf, u32 btf_id,
 			   const struct btf_type **bad_type)
 {

kernel/bpf/verifier.c

@@ -4147,11 +4147,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
 	meta.func_id = func_id;
 	/* check args */
 	for (i = 0; i < 5; i++) {
-		if (fn->arg_type[i] == ARG_PTR_TO_BTF_ID) {
-			if (!fn->btf_id[i])
-				fn->btf_id[i] = btf_resolve_helper_id(&env->log, fn->func, i);
-			meta.btf_id = fn->btf_id[i];
-		}
+		err = btf_resolve_helper_id(&env->log, fn, i);
+		if (err > 0)
+			meta.btf_id = err;
 		err = check_func_arg(env, BPF_REG_1 + i, fn->arg_type[i], &meta);
 		if (err)
 			return err;

net/core/filter.c

@@ -3816,7 +3816,7 @@ static const struct bpf_func_proto bpf_skb_event_output_proto = {
 	.arg5_type	= ARG_CONST_SIZE_OR_ZERO,
 };
 
-static u32 bpf_skb_output_btf_ids[5];
+static int bpf_skb_output_btf_ids[5];
 const struct bpf_func_proto bpf_skb_output_proto = {
 	.func		= bpf_skb_event_output,
 	.gpl_only	= true,