bpf: Fix struct_meta lookup for bpf_obj_free_fields kfunc call

Bugzilla: https://bugzilla.redhat.com/2177177

Conflicts: Change in data structure from the missing commit
07236ea ("bpf: factor out fetching basic kfunc metadata").
Unfortunately backporting 07236ea wouldn't be a clean backport
either unless several other patches would be backported too.

commit f6a6a5a
Author: Dave Marchevsky <davemarchevsky@fb.com>
Date:   Mon Apr 3 13:00:27 2023 -0700

    bpf: Fix struct_meta lookup for bpf_obj_free_fields kfunc call

    bpf_obj_drop_impl has a void return type. In check_kfunc_call, the "else
    if" which sets insn_aux->kptr_struct_meta for bpf_obj_drop_impl is
    surrounded by a larger if statement which checks btf_type_is_ptr. As a
    result:

      * The bpf_obj_drop_impl-specific code will never execute
      * The btf_struct_meta input to bpf_obj_drop is always NULL
      * __bpf_obj_drop_impl will always see a NULL btf_record when called
        from BPF program, and won't call bpf_obj_free_fields
      * program-allocated kptrs which have fields that should be cleaned up
        by bpf_obj_free_fields may instead leak resources

    This patch adds a btf_type_is_void branch to the larger if and moves
    special handling for bpf_obj_drop_impl there, fixing the issue.

    Fixes: ac9f060 ("bpf: Introduce bpf_obj_drop")
    Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
    Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
    Link: https://lore.kernel.org/r/20230403200027.2271029-1-davemarchevsky@fb.com
    Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/bpf/verifier.c

@@ -9238,10 +9238,6 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 				env->insn_aux_data[insn_idx].obj_new_size = ret_t->size;
 				env->insn_aux_data[insn_idx].kptr_struct_meta =
 					btf_find_struct_meta(ret_btf, ret_btf_id);
-			} else if (meta.func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
-				env->insn_aux_data[insn_idx].kptr_struct_meta =
-					btf_find_struct_meta(meta.arg_obj_drop.btf,
-							     meta.arg_obj_drop.btf_id);
 			} else if (meta.func_id == special_kfunc_list[KF_bpf_list_pop_front] ||
 				   meta.func_id == special_kfunc_list[KF_bpf_list_pop_back]) {
 				struct btf_field *field = meta.arg_list_head.field;
@@ -9319,7 +9315,15 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		}
 		if (reg_may_point_to_spin_lock(&regs[BPF_REG_0]) && !regs[BPF_REG_0].id)
 			regs[BPF_REG_0].id = ++env->id_gen;
-	} /* else { add_kfunc_call() ensures it is btf_type_is_void(t) } */
+	} else if (btf_type_is_void(t)) {
+		if (meta.btf == btf_vmlinux && btf_id_set_contains(&special_kfunc_set, meta.func_id)) {
+			if (meta.func_id == special_kfunc_list[KF_bpf_obj_drop_impl]) {
+				env->insn_aux_data[insn_idx].kptr_struct_meta =
+					btf_find_struct_meta(meta.arg_obj_drop.btf,
+							     meta.arg_obj_drop.btf_id);
+			}
+		}
+	}
 
 	nargs = btf_type_vlen(func_proto);
 	args = (const struct btf_param *)(func_proto + 1);