From a9ed4a6560b8562b7e2e2bed9527e88001f7b682 Mon Sep 17 00:00:00 2001
From: Marc Zyngier <maz@kernel.org>
Date: Wed Aug 19 17:12:17 2020 +0100
Subject: [PATCH] epoll: Keep a reference on files added to the check list 
Git-commit: a9ed4a6560b8562b7e2e2bed9527e88001f7b682
References: bsc#1180031
Patch-mainline: v5.9-rc2


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>
Acked-by: Goldwyn Rodrigues <rgoldwyn@suse.com>

---
 fs/eventpoll.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1978,9 +1978,11 @@
 			 * 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);
@@ -2024,6 +2026,7 @@
 		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);
 }
@@ -2183,9 +2186,11 @@
 					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;