bpf: Allow const char * from LSM hooks as kfunc const string arguments

Let the BPF verifier to recognize const char * arguments from LSM hooks
(and other BPF program types) as valid const string pointers that can be
passed to kfuncs expecting KF_ARG_PTR_TO_CONST_STR.

Previously, kfuncs with KF_ARG_PTR_TO_CONST_STR only accepted
PTR_TO_MAP_VALUE from readonly maps. This was limiting for LSM programs
that receive const char * arguments from hooks like sb_mount's dev_name.

Signed-off-by: Song Liu <[email protected]>

Author: liu-song-6

include/linux/btf.h

@@ -224,6 +224,7 @@ struct btf *btf_base_btf(const struct btf *btf);
 bool btf_type_is_i32(const struct btf_type *t);
 bool btf_type_is_i64(const struct btf_type *t);
 bool btf_type_is_primitive(const struct btf_type *t);
+bool btf_type_is_const_char_ptr(const struct btf *btf, const struct btf_type *t);
 bool btf_member_is_reg_int(const struct btf *btf, const struct btf_type *s,
 			   const struct btf_member *m,
 			   u32 expected_offset, u32 expected_size);

kernel/bpf/btf.c

@@ -897,6 +897,25 @@ bool btf_type_is_primitive(const struct btf_type *t)
 	       btf_is_any_enum(t);
 }
 
+bool btf_type_is_const_char_ptr(const struct btf *btf, const struct btf_type *t)
+{
+	const char *tname;
+
+	/* The type chain has to be PTR->CONST->CHAR */
+	if (BTF_INFO_KIND(t->info) != BTF_KIND_PTR)
+		return false;
+
+	t = btf_type_by_id(btf, t->type);
+	if (BTF_INFO_KIND(t->info) != BTF_KIND_CONST)
+		return false;
+
+	t = btf_type_by_id(btf, t->type);
+	tname = btf_name_by_offset(btf, t->name_off);
+	if (tname && strcmp(tname, "char") == 0)
+		return true;
+	return false;
+}
+
 /*
  * Check that given struct member is a regular int with expected
  * offset and size.
@@ -6746,6 +6765,20 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 			/* Default prog with MAX_BPF_FUNC_REG_ARGS args */
 			return true;
 		t = btf_type_by_id(btf, args[arg].type);
+
+		/*
+		 * For const string, we need to match "const char *"
+		 * exactly. Therefore, do the check before the skipping
+		 * modifiers.
+		 */
+		if (btf_type_is_const_char_ptr(btf, t)) {
+			info->reg_type = PTR_TO_BTF_ID;
+			if (prog_args_trusted(prog))
+				info->reg_type |= PTR_TRUSTED;
+			info->btf = btf;
+			info->btf_id = args[arg].type;
+			return true;
+		}
 	}
 
 	/* skip modifiers */

kernel/bpf/verifier.c

@@ -9598,18 +9598,19 @@ static enum bpf_dynptr_type dynptr_get_type(struct bpf_verifier_env *env,
 	return state->stack[spi].spilled_ptr.dynptr.type;
 }
 
-static int check_reg_const_str(struct bpf_verifier_env *env,
-			       struct bpf_reg_state *reg, u32 regno)
+/*
+ * Check for const string saved in a bpf map. The caller is responsible
+ * to check reg->type == PTR_TO_MAP_VALUE.
+ */
+static int check_reg_const_str_in_map(struct bpf_verifier_env *env,
+				      struct bpf_reg_state *reg, u32 regno)
 {
 	struct bpf_map *map = reg->map_ptr;
 	int err;
 	int map_off;
 	u64 map_addr;
 	char *str_ptr;
 
-	if (reg->type != PTR_TO_MAP_VALUE)
-		return -EINVAL;
-
 	if (!bpf_map_is_rdonly(map)) {
 		verbose(env, "R%d does not point to a readonly map'\n", regno);
 		return -EACCES;
@@ -9646,6 +9647,26 @@ static int check_reg_const_str(struct bpf_verifier_env *env,
 	return 0;
 }
 
+/* Check for const string passed in as input to the bpf program. */
+static int check_reg_const_str_arg(struct bpf_reg_state *reg)
+{
+	const struct btf *btf;
+	const struct btf_type *t;
+	const char *tname;
+
+	if (base_type(reg->type) != PTR_TO_BTF_ID)
+		return -EINVAL;
+
+	btf = reg->btf;
+	t = btf_type_by_id(btf, reg->btf_id);
+	if (!t)
+		return -EINVAL;
+
+	if (btf_type_is_const_char_ptr(btf, t))
+		return 0;
+	return -EINVAL;
+}
+
 /* Returns constant key value in `value` if possible, else negative error */
 static int get_constant_map_key(struct bpf_verifier_env *env,
 				struct bpf_reg_state *key,
@@ -9964,7 +9985,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
 		break;
 	case ARG_PTR_TO_CONST_STR:
 	{
-		err = check_reg_const_str(env, reg, regno);
+		if (reg->type != PTR_TO_MAP_VALUE)
+			return -EINVAL;
+		err = check_reg_const_str_in_map(env, reg, regno);
 		if (err)
 			return err;
 		break;
@@ -13626,13 +13649,17 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
 			meta->arg_btf_id = reg->btf_id;
 			break;
 		case KF_ARG_PTR_TO_CONST_STR:
-			if (reg->type != PTR_TO_MAP_VALUE) {
-				verbose(env, "arg#%d doesn't point to a const string\n", i);
-				return -EINVAL;
+			if (reg->type == PTR_TO_MAP_VALUE) {
+				ret = check_reg_const_str_in_map(env, reg, regno);
+				if (ret)
+					return ret;
+			} else {
+				ret = check_reg_const_str_arg(reg);
+				if (ret) {
+					verbose(env, "arg#%d doesn't point to a const string\n", i);
+					return ret;
+				}
 			}
-			ret = check_reg_const_str(env, reg, regno);
-			if (ret)
-				return ret;
 			break;
 		case KF_ARG_PTR_TO_WORKQUEUE:
 			if (reg->type != PTR_TO_MAP_VALUE) {