epoll: Keep a reference on files added to the check list

When adding a new fd to an epoll, and that this new fd is an
epoll fd itself, we recursively scan the fds attached to it
to detect cycles, and add non-epool files to a "check list"
that gets subsequently parsed.

However, this check list isn't completely safe when deletions
can happen concurrently. To sidestep the issue, make sure that
a struct file placed on the check list sees its f_count increased,
ensuring that a concurrent deletion won't result in the file
disapearing from under our feet.

Cc: stable@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 12eebcd..196003d 100644 (file)
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1994,9 +1994,11 @@ static int ep_loop_check_proc(void *priv, void *cookie, int call_nests)
                         * not already there, and calling reverse_path_check()
                         * during ep_insert().
                         */
-                       if (list_empty(&epi->ffd.file->f_tfile_llink))
+                       if (list_empty(&epi->ffd.file->f_tfile_llink)) {
+                               get_file(epi->ffd.file);
                                list_add(&epi->ffd.file->f_tfile_llink,
                                         &tfile_check_list);
+                       }
                }
        }
        mutex_unlock(&ep->mtx);
@@ -2040,6 +2042,7 @@ static void clear_tfile_check_list(void)
                file = list_first_entry(&tfile_check_list, struct file,
                                        f_tfile_llink);
                list_del_init(&file->f_tfile_llink);
+               fput(file);
        }
        INIT_LIST_HEAD(&tfile_check_list);
 }
@@ -2204,13 +2207,17 @@ int do_epoll_ctl(int epfd, int op, int fd, struct epoll_event *epds,
                                        clear_tfile_check_list();
                                        goto error_tgt_fput;
                                }
-                       } else
+                       } else {
+                               get_file(tf.file);
                                list_add(&tf.file->f_tfile_llink,
                                                        &tfile_check_list);
+                       }
                        error = epoll_mutex_lock(&ep->mtx, 0, nonblock);
                        if (error) {
 out_del:
                                list_del(&tf.file->f_tfile_llink);
+                               if (!is_file_epoll(tf.file))
+                                       fput(tf.file);
                                goto error_tgt_fput;
                        }
                        if (is_file_epoll(tf.file)) {