bpf: Fix an error around PTR_UNTRUSTED

Per discussion with Alexei, the PTR_UNTRUSTED flag should not been
cleared when we start to walk a new struct, because the struct in
question may be a struct nested in a union. We should also check and set
this flag before we walk its each member, in case itself is a union.
We will clear this flag if the field is BTF_TYPE_SAFE_RCU_OR_NULL.

Fixes: 6fcd486 ("bpf: Refactor RCU enforcement in the verifier.")
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20230713025642.27477-2-laoar.shao@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/btf.c

@@ -6133,7 +6133,6 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 	const char *tname, *mname, *tag_value;
 	u32 vlen, elem_id, mid;
 
-	*flag = 0;
 again:
 	if (btf_type_is_modifier(t))
 		t = btf_type_skip_modifiers(btf, t->type, NULL);
@@ -6144,6 +6143,14 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 	}
 
 	vlen = btf_type_vlen(t);
+	if (BTF_INFO_KIND(t->info) == BTF_KIND_UNION && vlen != 1 && !(*flag & PTR_UNTRUSTED))
+		/*
+		 * walking unions yields untrusted pointers
+		 * with exception of __bpf_md_ptr and other
+		 * unions with a single member
+		 */
+		*flag |= PTR_UNTRUSTED;
+
 	if (off + size > t->size) {
 		/* If the last element is a variable size array, we may
 		 * need to relax the rule.
@@ -6304,15 +6311,6 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
 		 * of this field or inside of this struct
 		 */
 		if (btf_type_is_struct(mtype)) {
-			if (BTF_INFO_KIND(mtype->info) == BTF_KIND_UNION &&
-			    btf_type_vlen(mtype) != 1)
-				/*
-				 * walking unions yields untrusted pointers
-				 * with exception of __bpf_md_ptr and other
-				 * unions with a single member
-				 */
-				*flag |= PTR_UNTRUSTED;
-
 			/* our field must be inside that union or struct */
 			t = mtype;
 
@@ -6478,7 +6476,7 @@ bool btf_struct_ids_match(struct bpf_verifier_log *log,
 			  bool strict)
 {
 	const struct btf_type *type;
-	enum bpf_type_flag flag;
+	enum bpf_type_flag flag = 0;
 	int err;
 
 	/* Are we already done? */

kernel/bpf/verifier.c

@@ -6067,6 +6067,11 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
 				   type_is_rcu_or_null(env, reg, field_name, btf_id)) {
 				/* __rcu tagged pointers can be NULL */
 				flag |= MEM_RCU | PTR_MAYBE_NULL;
+
+				/* We always trust them */
+				if (type_is_rcu_or_null(env, reg, field_name, btf_id) &&
+				    flag & PTR_UNTRUSTED)
+					flag &= ~PTR_UNTRUSTED;
 			} else if (flag & (MEM_PERCPU | MEM_USER)) {
 				/* keep as-is */
 			} else {