bpf: Harden register offset checks for release helpers and kfuncs

Let's ensure that the PTR_TO_BTF_ID reg being passed in to release BPF
helpers and kfuncs always has its offset set to 0. While not a real
problem now, there's a very real possibility this will become a problem
when more and more kfuncs are exposed, and more BPF helpers are added
which can release PTR_TO_BTF_ID.

Previous commits already protected against non-zero var_off. One of the
case we are concerned about now is when we have a type that can be
returned by e.g. an acquire kfunc:

struct foo {
	int a;
	int b;
	struct bar b;
};

... and struct bar is also a type that can be returned by another
acquire kfunc.

Then, doing the following sequence:

	struct foo *f = bpf_get_foo(); // acquire kfunc
	if (!f)
		return 0;
	bpf_put_bar(&f->b); // release kfunc

... would work with the current code, since the btf_struct_ids_match
takes reg->off into account for matching pointer type with release kfunc
argument type, but would obviously be incorrect, and most likely lead to
a kernel crash. A test has been included later to prevent regressions in
this area.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220304224645.3677453-5-memxor@gmail.com

include/linux/bpf_verifier.h

@@ -523,7 +523,8 @@ int check_ptr_off_reg(struct bpf_verifier_env *env,
 		      const struct bpf_reg_state *reg, int regno);
 int check_func_arg_reg_off(struct bpf_verifier_env *env,
 			   const struct bpf_reg_state *reg, int regno,
-			   enum bpf_arg_type arg_type);
+			   enum bpf_arg_type arg_type,
+			   bool is_release_func);
 int check_kfunc_mem_size_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,
 			     u32 regno);
 int check_mem_reg(struct bpf_verifier_env *env, struct bpf_reg_state *reg,

kernel/bpf/btf.c

@@ -5753,6 +5753,10 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 		return -EINVAL;
 	}
 
+	/* Only kfunc can be release func */
+	if (is_kfunc)
+		rel = btf_kfunc_id_set_contains(btf, resolve_prog_type(env->prog),
+						BTF_KFUNC_TYPE_RELEASE, func_id);
 	/* check that BTF function arguments match actual types that the
 	 * verifier sees.
 	 */
@@ -5777,7 +5781,7 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 		ref_t = btf_type_skip_modifiers(btf, t->type, &ref_id);
 		ref_tname = btf_name_by_offset(btf, ref_t->name_off);
 
-		ret = check_func_arg_reg_off(env, reg, regno, ARG_DONTCARE);
+		ret = check_func_arg_reg_off(env, reg, regno, ARG_DONTCARE, rel);
 		if (ret < 0)
 			return ret;
 
@@ -5809,7 +5813,11 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 			if (reg->type == PTR_TO_BTF_ID) {
 				reg_btf = reg->btf;
 				reg_ref_id = reg->btf_id;
-				/* Ensure only one argument is referenced PTR_TO_BTF_ID */
+				/* Ensure only one argument is referenced
+				 * PTR_TO_BTF_ID, check_func_arg_reg_off relies
+				 * on only one referenced register being allowed
+				 * for kfuncs.
+				 */
 				if (reg->ref_obj_id) {
 					if (ref_obj_id) {
 						bpf_log(log, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",
@@ -5891,18 +5899,15 @@ static int btf_check_func_arg_match(struct bpf_verifier_env *env,
 
 	/* Either both are set, or neither */
 	WARN_ON_ONCE((ref_obj_id && !ref_regno) || (!ref_obj_id && ref_regno));
-	if (is_kfunc) {
-		rel = btf_kfunc_id_set_contains(btf, resolve_prog_type(env->prog),
-						BTF_KFUNC_TYPE_RELEASE, func_id);
-		/* We already made sure ref_obj_id is set only for one argument */
-		if (rel && !ref_obj_id) {
-			bpf_log(log, "release kernel function %s expects refcounted PTR_TO_BTF_ID\n",
-				func_name);
-			return -EINVAL;
-		}
-		/* Allow (!rel && ref_obj_id), so that passing such referenced PTR_TO_BTF_ID to
-		 * other kfuncs works
-		 */
+	/* We already made sure ref_obj_id is set only for one argument. We do
+	 * allow (!rel && ref_obj_id), so that passing such referenced
+	 * PTR_TO_BTF_ID to other kfuncs works. Note that rel is only true when
+	 * is_kfunc is true.
+	 */
+	if (rel && !ref_obj_id) {
+		bpf_log(log, "release kernel function %s expects refcounted PTR_TO_BTF_ID\n",
+			func_name);
+		return -EINVAL;
 	}
 	/* returns argument register number > 0 in case of reference release kfunc */
 	return rel ? ref_regno : 0;

kernel/bpf/verifier.c

@@ -5367,10 +5367,11 @@ found:
 
 int check_func_arg_reg_off(struct bpf_verifier_env *env,
 			   const struct bpf_reg_state *reg, int regno,
-			   enum bpf_arg_type arg_type)
+			   enum bpf_arg_type arg_type,
+			   bool is_release_func)
 {
+	bool fixed_off_ok = false, release_reg;
 	enum bpf_reg_type type = reg->type;
-	bool fixed_off_ok = false;
 
 	switch ((u32)type) {
 	case SCALAR_VALUE:
@@ -5395,6 +5396,21 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env,
 	 * fixed offset.
 	 */
 	case PTR_TO_BTF_ID:
+		/* When referenced PTR_TO_BTF_ID is passed to release function,
+		 * it's fixed offset must be 0. We rely on the property that
+		 * only one referenced register can be passed to BPF helpers and
+		 * kfuncs. In the other cases, fixed offset can be non-zero.
+		 */
+		release_reg = is_release_func && reg->ref_obj_id;
+		if (release_reg && reg->off) {
+			verbose(env, "R%d must have zero offset when passed to release func\n",
+				regno);
+			return -EINVAL;
+		}
+		/* For release_reg == true, fixed_off_ok must be false, but we
+		 * already checked and rejected reg->off != 0 above, so set to
+		 * true to allow fixed offset for all other cases.
+		 */
 		fixed_off_ok = true;
 		break;
 	default:
@@ -5452,11 +5468,14 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 	if (err)
 		return err;
 
-	err = check_func_arg_reg_off(env, reg, regno, arg_type);
+	err = check_func_arg_reg_off(env, reg, regno, arg_type, is_release_function(meta->func_id));
 	if (err)
 		return err;
 
 skip_type_check:
+	/* check_func_arg_reg_off relies on only one referenced register being
+	 * allowed for BPF helpers.
+	 */
 	if (reg->ref_obj_id) {
 		if (meta->ref_obj_id) {
 			verbose(env, "verifier internal error: more than one arg with ref_obj_id R%d %u %u\n",