[PATCH 1/3] epoll: Keep a reference on files added to the check list

[Tim Gardner]

From: Marc Zyngier <maz at kernel.org>

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 at vger.kernel.org
Signed-off-by: Marc Zyngier <maz at kernel.org>
Signed-off-by: Al Viro <viro at zeniv.linux.org.uk>
(cherry picked from commit a9ed4a6560b8562b7e2e2bed9527e88001f7b682)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 fs/eventpoll.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/fs/eventpoll.c b/fs/eventpoll.c
index f5a481089893..abf217835907 100644
--- a/fs/eventpoll.c
+++ b/fs/eventpoll.c
@@ -1972,9 +1972,11 @@ 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);
@@ -2018,6 +2020,7 @@ 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);
 }
@@ -2182,13 +2185,17 @@ 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)) {
-- 
2.17.1