epoll: Keep a reference on files added to the check list
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@vger.kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 9bbd20326fefd709dc6e7cbf7442ea640bb5f601)
diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index e76d0c3..eaae764 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1749,9 +1749,11 @@
* 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);
@@ -1795,6 +1797,7 @@
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);
}
@@ -1949,9 +1952,11 @@
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;