The patch below does not apply to the 5.4-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@xxxxxxxxxxxxxxx>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
>From a9ed4a6560b8562b7e2e2bed9527e88001f7b682 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@xxxxxxxxxx>
Date: Wed, 19 Aug 2020 17:12:17 +0100
Subject: [PATCH] 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@xxxxxxxxxxxxxxx
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index 12eebcdea9c8..196003d9242c 100644
--- 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)) {
