This is a note to let you know that I've just added the patch titled epoll: Keep a reference on files added to the check list to the 4.19-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: epoll-keep-a-reference-on-files-added-to-the-check-list.patch and it can be found in the queue-4.19 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From a9ed4a6560b8562b7e2e2bed9527e88001f7b682 Mon Sep 17 00:00:00 2001 From: Marc Zyngier <maz@xxxxxxxxxx> Date: Wed, 19 Aug 2020 17:12:17 +0100 Subject: epoll: Keep a reference on files added to the check list From: Marc Zyngier <maz@xxxxxxxxxx> commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682 upstream. 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@xxxxxxxxxxxxxxx Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- fs/eventpoll.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1890,9 +1890,11 @@ static int ep_loop_check_proc(void *priv * 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); @@ -1936,6 +1938,7 @@ 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); } @@ -2095,9 +2098,11 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, in clear_tfile_check_list(); goto error_tgt_fput; } - } else + } else { + get_file(tf.file); list_add(&tf.file->f_tfile_llink, &tfile_check_list); + } mutex_lock_nested(&ep->mtx, 0); if (is_file_epoll(tf.file)) { tep = tf.file->private_data; Patches currently in stable-queue which might be from maz@xxxxxxxxxx are queue-4.19/do_epoll_ctl-clean-the-failure-exits-up-a-bit.patch queue-4.19/epoll-keep-a-reference-on-files-added-to-the-check-list.patch